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How to use organic fertilizers and soil conditioners

ORGANIC FERTILIZERS vs. STRAIGHT SOIL CONDITIONERS

"Organic fertilizers" is a broad term and actually includes 3 categories:

• Straight soil conditioners such as rice hulls and sawdust.

• Straight organic fertilizers such as fish meal and wood ashes.

• Combination organic fertilizers-soil improvers such as compost, manure, and green manure crops.

We'll cover all of these and also deal with mulching and earthworms.

STRAIGHT SOIL CONDITIONERS AND THEIR USES

Coarse materials like rice hulls (husks), peanut shells, and sawdust have very low nutrient value but are very useful for loosening up clayey soils. As mentioned in Chapter 4 on seedbed preparation, rice hulls are also very useful for making a good nursery seedbox soil mix for raising transplants. These materials can also be used as a surface mulch. Contrary to popular belief, sawdust doesn't make the soil more acid.

Beware of N tie-up: Since these materials are very low in nitrogen, adding large amounts to the soil can cause a temporary N tie-up while they're decomposing, unless extra N is added in the form of organic or chemical fertilizers. Roughly 1 kg of actual N should be added to the soil per 100 kg of low N material; this equals about 2 kg of urea fertilizer (45% N) or 200 kg of compost or manure. You can also avoid N tie-up by first composting these coarse materials with high-N materials like manure and young green grass. (Using rice trolls, etc. as a surface mulch is unlikely to cause N tie-up unless they're worked into the soil.)

STRAIGHT ORGANIC FERTILIZERS AND THEIR USES

Some organics like blood meal, fish meal, and cottonseed meal have much higher nutrient contents than compost and manure. Because of this and the fact that some of them are expensive (they're in demand as livestock feed), they're applied at rates too low to improve soil physical condition.

TABLE 8-1 Nutrient Value of Some Straight Organic Fertilizers

 

N

P2O5

K2O

Bat guano

10%

4%

2%

Blood meal

12%

2%

1%

Bone meal, steamed$

2%

26-30%

0%

Coffee grounds

2%

0.3%

0.3%

Cottonseed meal

6%

3%

1%

Feathers

15%

1%

1%

Fish meal

10%

4%

1%

Hair

12-16%

1%

1%

Rock phosphate*

0%

31-33%

0%

Seaweed (kelp)

0.2%

0.1%

0.6%

Wood ashes

0

1-2%

5-10%

Much of the P in bone meal and raw rock phosphate is insoluble and only very slowly available.

Characteristics of Some Straight Organic Fertilizers

Bone Meal: Raw bone meal consists of cooked bones ground into a meal without removal of the gelatine or glue. Steamed bone meal has been steamed under pressure to remove some of the gelatine. Both types contain good levels of phosphorus and calcium, but much of the P is in a very slowly, available form which limits its immediate effectiveness. It works best when broadcast and worked into acid soils high in organic matter; it should be very finely ground to promote its reaction with the soil and the release of available P.

Rock Phosphate: It's considered an organic fertilizer, since it's mined from the earth and used either raw or after heat treatment, which improves its P availability. About 90% of the P in raw rock phosphate is insoluble and largely unavailable. As with bone meal, it works best on acid soils high in organic matter and should be broadcast in a finely-ground form. Heat-treated rock phosphate is discussed in Chapter 9 under phosphorus fertilizers. It is known the mycorrhizee fungi (see Chapter 1) help improve the P availability of rock phosphate.

Wood Ashes: Although an excellent source of K, they're also a very potent liming material. Even moderate applications have raised soil pH too high when done regularly. In fact, the Univ. of Connecticut experiment station now recommends applying no more than 100-150 grams (about 300-450 cc) per sq. meter per year. There's now evidence that trees in areas affected by industrial pollution (or vehicle exhaust) may accumulate heavy metals such as lead and cadmium which end up in the ash. Studies have shown that leafy or root crops such as lettuce and beets can absorb heavy metals from the soil. (Where soil lead levels are high, plant uptake can be effectively minimized by keeping the pH above 6.0 and the soil organic matter content above 25% - a very high level.)

Sea Cucumbers (Holuthurians): The PATS ag trade school on Ponape Island in Micronesia uses these marine animals as an effective fertilizer on vegetables. They are first fermented in a drum for 10-14 days and then applied as a 50-50 mix with water as a sidedressing to growing plants. Unfortunately, the fermentation produces objectionable odors; nonetheless, sea cucumbers are considered to be a promising fertilizer for the Pacific islands.

Seaweed is really too low strength an organic to rank in this group, but it's great as a mulch and soil improver. It's also an especially good nutrient source when applied at comparable rates to compost and manure, because it contains all known minerals. It should first be washed with fresh water or exposed to rainfall to avoid possible salt problems, although this causes some nutrient loss.

Diluted seawater is currently being studied as a possible source of micronutrients.

COMBINATION ORGANIC FERTILIZERS-SOIL CONDITIONERS:

Manure, Compost. and Green Manure Crops

MANURE

If properly stored and applied, manure is a great nutrient source and soil conditioner. There's also some evidence that it may contain other growth-promoting substances like natural hormones and B vitamins.

Fertilizer Value of Manure

Table 8-2 compares the approximate NPK contents of various animal manures. As we'll see, however, the actual nutrient content of a manure is highly variable, since it's influenced by other factors aside from animal species.

TABLE 8-2

Approximate Composition of Various Animal Manures

   

KG OF NUTRIENT PER1000 KG OF MANURE

Fresh manure with Bedding or Litter

Moisture Content

N*

P2O5*

K2O*

Chicken

70%

11

11

5

Sheep, Goat

70%

10

7.5

10.5

Cow

86%

5.5

2

5

Horse

80%

6.5

2.5

6.5

Pig

87%

5.5

3

4.5

Rabbit

60%

15

10

10

% N = nitrogen; P2O5 = phosphoric acid; K2O = potassium oxide (potash)

Manure varies greatly in nutrient value: Although Table 8-2 shows that rabbit, sheep, and poultry manures are richer in nutrients than cow manure, that's not always the case. In fact, the nutrient content can vary greatly, even among animals of the same species. The reason is that the fertilizer value of a manure is also greatly affected by diet, amount of bedding, storage, and application method.

• Diet: The N content of fresh manure is directly related to the amount of protein in the animal's diet. For example, pigs fed a low-protein diet of mainly maize will produce manure lower in N than those fed a higher-protein ration.

• Amount of bedding: Animal urine contains about 30-50% of the total N and 50-80% of the total K. It will be largely wasted unless animals are penned and bedding is used to soak it up. However, most bedding such as straw and sawdust is very low in nutrient value, and high amounts will greatly dilute the manure's fertilizer value. In fact, manure with excessive bedding may actually create a temporary N tie-up in the soil.

• Storage method: Outdoor storage without cover results in high nutrient losses (esp. N and K) due to leaching by rainfall or excessive drying out of the pile (the latter increases the loss of N as ammonia gas).

• Application method: Ideally, manure should be worked into the soil immediately after application. Fresh manure can lose up to 25% of its N in a day and 50% in 2 days. The same applies to manure that is left exposed to the elements before collection.

As a rough figure, 1000 kg of fresh manure with some bedding contains about 5 kg of N, 2.5 kg of P2O5, and 5 kg of K2O. This works out to a 0.5-0.25-0.5 fertilizer formula (see Chapter 9 on chemical fertilizers). However, only about 50% of the N and 20% of the P in fresh manure is actually available to the crop during the first few months. The rest is in the more slowly-released organic form which must first be mineralized by soil microbes. Manure can also be a good source of micronutrients, especially from animals fed balanced rations.

"Hot" vs. "Cold" Manures: Fresh pig, poultry, and sheep manures are often referred to as "hot" manures, since they are likely to "burn" (injure) plants or prevent seeds from sprouting if applied too heavily or too close to planting time. In fact, most fresh manures can be considered "hot" with the possible exception of rabbit manure. Aged (well-rotted) manure that has partially decomposed can be considered "cold"; it releases nutrients more slowly and is likely to injure crops.

Amount of Manure Produced


TABLE 8-3 Amount of Manure Produced Annually Per kg of Live Weight

As Table 8-3 shows, farm animals produce large amounts of manure; the problem is collecting it. Encouraging farmers to pen free-ranging animals at night will help increase the supply. Notice that the total amount of actual dry matter produced is very similar among the animals.

Storage and Application Guidelines for Manure Fresh vs. Aged (Composted) Manure

Manure can be applied in either form. Fresh manure is more likely to injure plants or seeds, since it can release harmful ammonia fumes. Fresh poultry, sheep, and pig manure are the most likely to cause injury. On the other hand, fresh manure usually provides more readily-available nitrogen. (The one exception is anerobically composted manure (see below), half of whose total N is in the readily-available ammonium form). Fresh manure can be safely used if applied a week or two in advance of planting and thoroughly worked into the topsoil to dilute it.

Aged (composted) manure will have shrunk to about 40-60% of its original volume. If protected from rain during rotting and storage, composted manure will be roughly twice as concentrated in nutrients, except for nitrogen, considerable amounts of which can be lost to the air as ammonia gas. Aged manure releases its N more slowly since much of it is changed into the slow-release organic form during composting. This is actually beneficial since it lessens leaching losses (organic N doesn't leach) and also provides N over a longer period.

How to Store Manure

If manure is not applied fresh, it can either be composted together with low-N residues like rice hulls and straw or be put in a pile by itself to rot. Manure piles should be protected from the elements. Rain will leach out soluble nutrients like potassium and nitrate N, while sunlight can cause excessive drying out of the pile which increases the loss of N as ammonia gas. According to how the pile is managed, it will undergo either an aerobic (with oxygen) or anaerobic composting (partial decomposition) process.

• Aerobic composting occurs in loose, semi-moiat manure piles where oxygen is plentiful. Losses of N due to ammonia volatilization are roughly 30-50%; most of the remaining N is in the slow-release, organic form. If the manure's N content is adequate, the fungi and bacteria responsible for composting will complete the process within 3-8 weeks and will generate considerable heat from their metabolic activities. Additional water may be needed to maintain the pile in a semi-moist condition.

• Anaerobic composting occurs in compact, wet manure piles where oxygen is excluded. It is a slower process, but N losses are much lower (as long as the pile in kept wet), and shrinkage of the pile is less. About half of the remaining N exists as readily-available ammonium, the remainder being in the slow-release, organic form. Unfortunately, such moist, anaerobic piles produce more odors and attract more flies than aerobic piles. Covering the pile with black plastic will minimize these problems and also help to maintain anaerobic conditions by reducing moisture loss and excluding air. Manure can also be used for methane production in anaerobic biogas digesters and the residue used as a fertilizer/soil conditioner.

• Biogas production: Manure can be mixed with water and anaerobically fermented in a digester tank to form biogas (65% methane, 35% carbon dioxide), which can be used for cooking, heating, lighting, and even for running engines. The digested residue (called sludge) consists of 90% liquid and 10% floating solids and is a soil conditioner and lowstrength fertilizer. However it must first be aged and aerated in a shallow pond for 2-4 weeks before application to dissipate plant-injurious substances like hydrogen sulfide.

NOTE: Biogas production is more complex than popularly believed, which has led to many failures. For more information, refer to the Biogas/Biofertilizer Business Handbook, Peace Corps/ICE Reprint R-8, 1982.

How to Apply Manure

• A good way to apply manure is to spread it evenly over the bed and work it thoroughly into the topsoil before planting or transplanting.

• If double-digging is done (see Chapter 4), manure (or some other kind of organic fertilizer/conditioner should also be worked into the subsoil.

• Fresh manure should ideally be applied 1-2 weeks in advance of planting or transplanting and thoroughly mixed with the the topsoil to avoid the possibility of plant injury.

• When crops like squash and melons are planted in "hills" (clusters of seeds spaced a meter or so apart), manure should be thoroughly worked into the hill area itself.

• If manure is scarce, it can be applied in strips or slots centered over the row, instead of covering the entire area.

• Whatever method you use, be sure to work the manure into the soil immediately to avoid loss of nitrogen as ammonia gas (up to 50 percent in just 2 days). Well-rotted manure won't release ammonia, but may have lost a considerable amount during the rotting process. (However, manure that has been anaerobically composted may be susceptible to some ammonia loss, because about half its N exists in the ammonium form.)

A warning about human. dog. cat. and Pig manures: All are likely to contain parasites and disease organisms that can be transmitted to humans. Most composting methods can't be relied upon to kill these bad guys. Pregnant women should be careful not to handle cat manure which can spread a disease called toxoplasmosis that can harm the fetus. Pig manure is OK to use on most crops except those like carrots and lettuce whose edible parts are in contact with the soil.

Don't use feedlot manure: Cattle fattened in feedlots are fed high levels of salt (sodium chloride). Feedlot manure can contain up to 10 times the salt of normal manure and i" especially likely to burn plants.

Watch out for Weeds!: Fresh manure from animals fed on pastures, hay, or wild vegetation can contain many weed seeds which can still germinate well after passing through the digestive system. Composting the manure first may generate enough heat to kill many of the weed seeds.

Suggested Application Rates for Manure

• Fresh or comported manure applied at 6-12 liters/sq. meter (a layer 6-12 mm or 0.25-0.5" thick) per crop planting should provide enough nutrients for good yields if the manure is of good quality and has been properly stored. This rate also supplies enough organic matter to achieve at least some improvement in soil physical condition.

NOTE: In soils very low in P, rates several times higher than this would be needed. In such cases, it's often better to supplement the manure application by applying a chemical fertilizer like superphosphate at planting or transplanting time. For long-term crops like vining (indeterminate) tomatoes, additional N sidedressings in the form of manure tea (to be discussed shortly) or chemical fertilizer may be needed.

Fresh poultry and sheep manures are especially "hot" and shouldn't be applied at more than 6 liters/sq. meter.

• If plentiful or of poor quality, manure can be applied at rates several times the above, except in the case of very fresh manures which can burn seeds or seedlings or even cause nitrate pollution of nearby water sources, especially shallow wells. Excessive nitrate levels in drinking water can cause potentially fatal "blue baby" disease (methemoglobinemia) in infants (particularly those on water-mixed formula) and are also toxic to livestock.

• If manure is scarce, it's usually better to apply a moderate rate over a larger area than a high rate over over a smaller area.

Some Handy Conversions for Manure Application

• One liter/sq. meter equals a layer one millimeter thick. Therefore, 6-12 liters/sq. meter equals a layer 6-12 me thick (about 0.25-0.5").

• One shovelful of manure contains about 3-4 liters. Therefore, 2-4 shovelfuls will supply the recommended "ballpark" rate of 6-12 liters/sq. meter.

• One cubic meter of manure contains 1000 liters and will supply enough nutrients for about 80-160 sq. meters of actual planted area (i.e. bed area alone, not counting alleyway area), if of good quality.

• One liter of fresh (wet) manure weighs roughly 1 kg. One liter of very dry manure weighs 0.3 kg (dairy and pig manures) or 0.6 kg (chicken manure).

Making and Using Manure Tea

Manure tea is a liquid form of fertilizer made by steeping a bag of manure in a drum of water for 2-4 weeks; the tea-like liquid can then be applied to growing crops or used as a starter solution during transplanting (refer to the vegetable section in Chapter 10). Recent research (summarized below) has shown that tea made from fresh manure contains good levels of most nutrients. As with manure, the tea probably contains some additional growthpromoting substances like hormones and B vitamins. Here's how to make and apply it:

NOTE: Manure tea can be made in smaller hatcher than used in this example.

• Fill a 50 kg burlap bag or other porous sack with fresh manure and tie it shut. (Fresh manure provides more readily-soluble nutrients than rotted manure, especially in the case of N.)

• Place the bag in a 55 gal (200 liter) drum and fill with water to the top. If necessary, weight the bag down with rocks to keep it submerged. Cover the drum to keep flies away and to prevent mosquitos from breeding. Let it stand for about 2 weeks.

• Given the many variables, application rates vary widely, and you'll have to experiment. Most recommendations say to further dilute the mixture with water to a weak brown color. Don't reuse the manure, but start with a fresh batch each time the drum is refilled. The leftover solids can be used as a soil conditioner but aren't likely to be a good nutrient source.

Recent Research on Manure Tea: Although manure tea has been used for centuries with good results, only recently has valid research been conducted with it. Below is a brief summary of trials evaluating the usefulness of chicken manure tea as a fertilizer carried out by a senior ag student at California Polytechnic University in San Luis Obispo. Note that the fresh chicken manure from well-fed birds used in this study has an especially high nutrient content compared to most other manures, and the same would be true of the resulting tea.

• Tea preparation: Fresh chicken manure was placed in a burlap bag at 3 rates (20, 35, 50 lbs.) and steeped in water in 35 gallon garbage cans for 4 weeks.

• Nitrogen content: Good levels of available N were obtained in the tea, almost entirely in the ammonium form (NH.+). With the 20 lb. bag, the tea attained about 85% of its maximum ammonium content (860 ppm in week 4) in the first week. For the 35 and 50 lb. bags,

only about 50% of the maximum ammonium content (1514 and 1424 ppm respectively in week 4) was obtained the first week. The reason that the 50 lb. bag yielded less available N than the 35 lb. bag was probably because the higher manure rate depressed bacterial activity.

• Overall nutrient content: The tea from the 20 lb. bag was diluted with water by 4-fold (1 part tea, 3 parts water) after 4 weeks, and its nutrient content was compared to that of a standard hydroponic growing solution (i.e. where all nutrients must be supplied by the solution itself). The diluted tea's N, P, K, and zinc levels compared favorably with the solution; other nutrients would need to be supplied by other sources (e.g. soil, manure, compost, chemical fertilizer).

• Plant growth: Greenhouse trials were conducted to compare the diluted tea and a hydroponic solution when applied to tomatoes grown on 3 different mediums: sand, redwood chips, and redwood sawdust. In all cases the tea-fed plants performed almost as well as those receiving the chemical solution, despite the supposed lack of several nutrients. It is likely that the missing nutrients were supplied by the growing medium. It was calculated that each tomato plant used about 4.5 gallons of tea (equal to 1.4 lbs. poultry manure during the 3 month trial period).

Compost Tea: Making a similar tea out of compost has been recommended; however, it wouldn't seem to be as effective since more of the nutrients in compost are in the insoluble, organic form and would take longer to leach into the tea. You may want to experiment with this, however.

COMPOST

WHAT IS COMPOST?

Compost is organic matter such as crop residues or manure that has been fairly well decomposed (usually in a pile or heap) and is well on its way to becoming the dark, crumbly stuff called humus.

WHAT IS COMPOST MADE OF?

It can be made from almost any waste organic matter such as:

Crop residues: Maize stalks, rice straw, leaves, etc.

Natural vegetation: Weeds, grass, tree leaves.

Manure

Some materials that shouldn't be used: Human feces (urine is OK), dog and cat manure are likely to contain parasites and disease organisms that can be passed to humans. Although a well-managed compost pile should generate enough heat to kill these bad guys, most piles fall short of this, mainly because of insufficient nitrogen and lack of periodic turning. Pig manure can be composted but probably shouldn't be used on crops whose edible parts are in contact with the soil. Likewise, vegetation showing signs of plant diseases should be avoided, except under ideal composting conditions. Meat scraps or dead animals are sure to attract flies and rodents.

HOW DOES COMPOST COMPARE WITH MANURE?

Like manure, compost is a low-strength, slow-release fertilizer. In fact, compared with fresh manure, its nitrogen is in a more stable form and not susceptible to loss as ammonia gas. As with well-rotted (composted) manure, compost won't "burn" seeds or seedlings.

Nutrient value: Like manure, the nutrient value of compost varies a lot and depends on what it's made from (see Table 8-4). Aside from N, P, and K, it also supplies varying amounts of secondary nutrients and micronutrients. As with manure, compost contains other growth-promoting substances such as B vitamins, natural hormones, and organic acids. Compost that has been made from a variety of materials is likely to provide the best spectrum of nutrients.

TABLE 8-4 Common NPK Ranges for Composts

Nutrient

Percentage

Kg of Nutrient per 1000 kg of Compost

N

0.75-1.5%

7.5-15 kg

P2O5

0.25-0.5%

2.5-5 kg

K2O

0.5-1.0%

5.0-10 kg

Composting First vs. Adding Fresh Materials Directly to the Soil

Adding fresh manure or plant residues directly to the soil usually ends up adding more nutrients and humus than composting these materials first. Losses of N as ammonia gas can be high during composting, especially when fresh manure is used. However, there may be some advantages to composting the materials first:

• Although some studies have shown that composted manure provides only about half as much readily-available N compared to adding the same manure in fresh form, it's less likely to burn plants and also supplies N over a longer period due to its slow-release nature.

• The composting process reduces the material's volume by about half, meaning there's less hauling to do.

• Fresh manure can be mixed and composted with low-N materials such as straw and rice hulls and will supply the needed N to break them down more quickly; such mixing will also reduce the loss of N as ammonia gas from the fresh manure during the composting process.

• A well-made and properly turned compost pile will generate enough heat to kill the weed seeds contained in the fresh manure from animals fed on pasture, hay, or wild vegetation.

SOME POSSIBLE LIMITATIONS OF COMPOSTING

Like manure, composting is more practical for small plots because:

• It requires a lot of hand labor.

• It requires a lot of water to keep the pile moist during the 1-4 months of composting, which may be a problem during the dry season.

• Most farmers won't have enough compostable materials to meet their needs. Crop residues like millet and sorghum stalks are often fed to livestock or used for for fencing or building materials. Also, it's usually much easier, and may be just as beneficial, to leave them on the soil surface in the field.

• Shrinkage of the pile, which averages about 50%, can be discouraging. Part of this is due to settling, but most of it results from the activities of fungi and bacteria that digest the material, converting much of its carbon to carbon dioxide gas.

Composting is usually best suited to a farmer's smaller plots and to communal garden projects. It's especially feasible for school garden projects where there's lots of labor available.

WHAT GOES ON IN A COMPOST PILE?

Compost results largely from the the activities of various kinds of fungi and bacteria that feed on the materials in the pile and gradually convert them into humus. Insects and earthworms are also found in compost piles, especially at the latter stages of decomposition, and aid in the process.

NOTE: Composting is ordinarily an aerobic process involving microbes that require oxygen; although it's also possible to make compost anaerobically (without oxygen), this section focuses on the more common aerobic process.

Heat buildup in the pile: A properly made pile will heat up to about 65-70C (150-160F) within 2-4 days due to the action of thermophilic (heat loving) bacteria. The pile will gradually cool down as other types of fungi and bacteria take over. During the first several weeks, the pile will usually reheat somewhat each time it's turned.

FACTORS INFLUENCING THE SPEED OF COMPOSTING

Under the most ideal conditions, you can make compost in just 10-15 days, but few limitedresource farmers or gardening projects will be able to do this. More likely, it will take 2-4 months. This is because it's very difficult to meet the 5 essential requirements for rapid composting. These are:

• Finely shredded material.

• An adequate carbon:nitrogen ratio

• Adequate moisture.

• Adequate aeration.

• Self-insulation

An understanding of these 5 essentials is very helpful in learning how to make compost successfully and will also give you an idea of of how long the process may take in your situation. Let's look at each requirement in detail.

FINELY-SHREDDED MATERIAL: Pieces 6 mm (1/4") or smaller are essential for rapid composting, because they provide much more surface area for the microbes and insects to work on. This isn't likely to be practical without a shredder. (Hand-cranked shredders are available in some countries but may produce larger pieces.) Rather than spending a lot of time hand-chopping, it's better to settle for larger pieces and a slower composting process. However, even coarsely-chopped material will make the pile much easier to turn.

AN ADEQUATE CARBON: NITROGEN RATIO: The fungi and bacteria need the right diet to function efficiently. They need carbon for energy, and they need nitrogen for growth and multiplication. (They convert the N into protein.) A C:N ratio of about 20-30:1 is ideal, but most materials contain more carbon and less nitrogen than this. Here are some useful guidelines for distinguishing between high-N materials (those with a narrow C:N ratio) and low-N materials (those with a wide C:N ratio):

LOW-N MATERIALS: Any old, brown or yellow, fibrous vegetation like maize-sorghummillet stalks, rice straw, old grass with seed heads, rice hulls, peanut hulls, old dry leaves, sawdust. The yellower or browner and the older the material is, the lower its likely N content.

Examples: Sawdust has a C:N ratio of about 500:1 and straw ranges from about 50:1 to 130:1; maize, sorghum, and millet stalks (at harvest time) have a C:N ratio of about 60:1.

HIGH-N MATERIALS: Young, soft, green vegetation, especially from legumes, is likely to be a good N source. Fresh manure, especially poultry manure, that has been protected from the elements and that doesn't contain too much strawy bedding is an excellent N source.

Examples: Fresh manure has a C:N ratio of about 15-20:1, strawy manure 50:1, fresh young grass 15:1, green legume leaves 12:1.

Some General Guidelines for Getting the Right C:N Ratio: There's no way to be exact, and luckily you don't have to be. However, piles with too little N will take longer to break down and will shrink more, too, as the microbes try to burn off the excess carbon (as carbon dioxide) to narrow the C:N ratio to their liking. Surprisingly, it's not a good idea to have too much N in the pile, either, because the microbes will convert the excess N they don't need into ammonia gas which is lost to the air. Your best bet is to "guestimate" an appropriate mixture of high-N and low-N materials, based on their likely C:N ratios. The more low-N materials used, the greater the need for high-N materials. Here are some approximate "recipes":

• 1/3 low-N materials (straw, peanut hulls, etc.) + 2/3 high-N vegetation (young and green)

• 1/2 low-N materials + 1/2 high quality manure

ADEQUATE MOISTURE: The compost microbes need moisture to thrive. An adequately moist pile should contain about 5060% moisture and should feel about as wet as a cloth towel that has been wrung out after being immersed in water. When lightly squeezed, the material should leave a film of moisture on your hand. However, excessive water will lower the pile's oxygen content or it may drain out the bottom, carrying nutrients with it. Since the pile's heat production speeds up moisture evaporation, it will need periodic rewetting, especially in the early stages. Compost made in covered pits (see below) loses less moisture.

ADEQUATE AERATION: The fungi and bacteria involved in the typical aerobic composting process require oxygen to thrive. Two practices are essential to avoid oxygen deprivation.

• Avoid over-compacting the pile while building it, especially if it's made from lots of green, succulent material or wet manure.

• Turning the pile periodically is the best way to maintain adequate oxygen. The more often the pile is turned, the more quickly it will fore compost. For rapid composting (10-15 days), you'd need to turn the pile at least twice a week (aside from using finely shredded material) which maybe too laborious. You can also try placing vertical poles in the pile during building and then withdrawing them to leave air channels. For piles that won't be turned, it's possible to introduce earthworms into the pile after the initial heating has subsided, (Don't turn the pile afterwards or it may reheat and kill them.) The earthworms usually multiply rapidly and will help aerate and mix the pile.

• SELF-INSULATION: The pile should be large enough to hold the heat in. A good minimum size (before shrinkage) is about 2 cubic meters (i.e. a square-shaped pile measuring about 1.25 meters on a side or a cone-shaped pile measuring 2 meters in diameter at the base and 2 meters tall).

HOW TO MAKE COMPOST

Now that you understand the basic principles of composting, let's talk about how to actually make compost. We'll cover several methods:

• Compost piles: above-ground stacks, below-ground pits.

• In-the-bed basket composting for gardens.

• Direct-composting and mulch-composting

Two Types of Compost Piles: Stacks and Pits

Stacks: Above-ground piles work well in the wet season or where there's enough readily available water for periodic rewetting. (See Figure 8-2.)

Pits: Below-ground pits reduce water evaporation losses and are well suited to the dry season or in cases where nearby water is scarce. They shouldn't be used where the water table is shallow or they may end up being flooded. Some other guidelines:

• If you dig 2 pits next to each other, the compost can be easily and effectively turned (aerated) by moving it from one pit to the other.

• With 3 pits, you can maintain 2 separate piles and also turn them. (See Figure 8-1.)

• Pits with sloping sides make turning an,d removal much easier.

• Building a shade structure over the pit or covering it with straw, palm leaves, or plastic will further reduce water losses.

• Aeration needs of PitS : Pits are more likely then stacks to run short on oxygen and usually require more frequent turning, especially if deeper than 75 cm.


FIGURE 8-1: Making compost using the pit method. With 3 pits it's possible to maintain 2 separate compost piles and turn then by alternately transferring them back and forth into pit B.


FIGURE 8-2: An above-ground compost pile (stack).

How to Make and Maintain Compost Piles (Stacks or Pits)

• An especially good location for a compost pile is under banana or other fruit trees. They'll provide shade (for people and the compost) and will reap any benefits from nutrients that leach out if excess water seeps from the pile.

• When using several different materials, it's a good idea to add each material as a separate layer to keep track of the relative amounts being added. Another way is to make separate stacks of the different materials in the approximate proportions needed and then combine them to build the pile.

• Add water (if needed) as the pile is being built up. The material is wet enough if it leaves moisture on your hand when lightly squeezed. (It shouldn't be so wet that you can actually squeeze water from it, however.) Too much water may cause poor aeration or leak out the bottom and carry away nutrients. Wet manure and fresh, green material may require little or no added water.

Mix up the pile well to combine the different materials. This is important so that low-N and high-N materials can be mixed together to provide a good carbon: nitrogen ratio for the bacteria and fungi. (Some garden references imply that the layers should remain intact, but this isn't correct).

Avoid over-compacting the pile since this reduces aeration.

To reduce water loss in dry season stacks, cover the outside of the pile with insulating material like millet stalks, straw, or plastic (see Fig. 8-2).

Where stacks are used during high-rainfall periods, excess water can be avoided by making them cone-shaped and/or covering the outside with stalks or plastic. (If using plastic, the pile may require more frequent turning to insure good aeration.)

How to turn a pile: Try to turn it "inside-out" so that the outer layer ends up on the inside and vice-versa. This will help assure a good kill of weed seeds and possible pathogens by exposing all the material to high temperatures in the pile's interior.

Some Other Guidelines for Compost Piles

Is soil needed?: Some sources suggest that soil should make up as much as one third of the pile in order to supply needed microbes. However, studies have proven that manure or fresh, green material will supply all the microbes needed. Soil adds a lot of unnecessary weight to the pile, too.

One appropriate use of soil: In cases where the pile has an excess of high-N materials, ammonia gas (82% N) will be given off. If this occurs, N losses can be reduced by covering the pile with 4-5 cm of soil or by mixing soil into the pile; this will help trap the ammonia gas and convert it to stable ammonium. However, adding more low-N materials like straw or sawdust to the pile is just as effective.

Use of lime or wood ashes: It used to be thought that this would help speed up the process and keep the pile from becoming too acid. Now it's known that lime is rarely needed unless lots of very acid materials like pine needles or oak leaves are used. Wood ashes should only be used in small amounts since they are a potent liming material and may end up raising the soil pH too high if large amounts of ash-laden compost are applied. A high pH in the pile itself will increase losses of nitrogen as ammonia gas during composting.

• Use of N fertilizer: It can be added to narrow the C:N ratio of low-N compost piles that lack enough high-N organic materials. However, this is an expensive use of chemical fertilizer, and you may be better off looking for organic sources or settling for a slower process. A ballpark rate for N fertilizer is the equivalent of 100150 cc of urea (45% N) per sq. meter of pile per 20 cm layer (500-750 cc of urea per cubic meter).

• Use of compost "starters": Despite advertising claims, these commercial liquids or powders containing microbes and nutrients have proven to be of no significant value in properly made piles. However, recent research in India has shown promising results from innoculating rice straw compost piles (very low in N) with fungi and N-fixing microbes; the treated straw composted 4 weeks faster and yielded 3 times as much humus (probably due to the carbon-saving effect of the N-fixing microbes).

Troubleshooting Faulty Compost Piles

Failure to heat up: The most likely cause is insufficient N or perhaps not enough moisture in the pile. Lack of oxygen is unlikely to be the problem, because piles will contain enough air for at least an initial heat buildup unless they have been overly compacted. Oxygen deficiency is more likely to be a problem in compost pits.

Ammonia smell: Caused by too much N in the pile. Mix in some low-N materials like straw or rice hulls or try covering the pile with a layer of moist soil to trap the ammonia.

Foul odors: Indicates anaerobic conditions (too little air). The remedy is to loosen up the pile by adding coarser materials and to turn it daily until the odor disappears.

When is Compost Ready for Use?

You don't have to wait until the pile has completely broken down into fine crumbly material. It's ready to use once it's reached a semi-rotted stage where the materials are no longer distinguishable. They'll continue to decompose in the soil.

How to Apply Compost

Compost is applied in much the same way as manure and rates are similar. (Refer to the previous section on manure.) It can also be used as a mulch when plentiful. Unlike fresh manure, compost can be left on the soil surface without losing nitrogen as ammonia gas. (However, anerobically composted material contains about half its N in the ammonium form and is prone to ammonia gas losses unless mixed into the soil.) As with manure, very high rates of may be needed to supply enough P on soil's low in this nutrient. In this case, it may be better to use compost in combination with a chemical fertilizer such as superphosphate (see Chapter 9).

SOME OTHER COMPOSTING METHODS

Basket Composting in Vegetable Beds

Basket composting is being used by vegetable gardeners in the Philippines where it was popularized by the Mindanao Baptist Rural Life Center. In this method, compostable material is placed in half-buried baskets located right in the garden beds where it forms compost (see Figure 8-3). Crops like peppers, okra, tomatoes, squash, and yardlong beans are planted around the baskets. All the watering is done through the baskets to move the nutrients out into the soil and encourage crop roots to enter the baskets. Where suited, basket composting is more likely to be adopted than usual composting methods for several reasons.

• It takes less work, because no turning is needed and materials can be added bit by bit.

• Instead taking 2-4 months, basket compost begins to be usable by plants as soon as the materials begin decomposing.

The method is best adapted for garden plots in areas where there's a good supply of green vegetation or manure, since such materials break down relatively quickly. You can modify this method to suit local conditions, but here are some general guidelines for making basket compost:

• The baskets act as containers for the composting material and should be at least 30 cm in diameter and 30 cm tall. Place them about 1 meter apart in holes so that about half their height is buried. If old baskets aren't available, place sticks in a circle and then weave palm thatch, etc. around this framework to form a basket.

• Place the most decomposed materials in the bottom of the baskets and the newer ones at the top; that way the crop roots will be able to utilize the nutrients more quickly and you can plant or transplant around the baskets right away.

• If the materials are fresh, wait 2-3 weeks before planting or setting plants around the baskets to allow time for some breakdown and nutrient release to occur.

• Apply all garden water through the baskets themselves to help move the nutrients into the soil and encourage plant roots to enter the buried portion of the baskets.

• After harvest, remove the baskets' contents and work them into the surrounding soil.


FIGURE 8-3: Basket composting.

Mulch -Composting

Some organic materials like ipil-ipil (leucaena) leaves and rice hulls make good mulches. They can be applied over the soil surface and eventually worked into the soil as they break down. If used by themselves, low-N materials such as rice hulls and sawdust may cause a temporary N deficiency if worked into the ground before they have rotted enough, unless additional N is supplied.

Direct-Composting (Sheet-Composting)

Don't forget that fresh organic wastes can also be added directly to the soil where they'll eventually become compost. However, you may find that adding a lot of fresh vegetation to the soil interferes with vegetable seedbed preparation and planting. Adding low-N materials alone may cause a temporary N tie-up unless an additional source of N is applied.

GREEN MANURING AND COVER-CROPPING

Green Manuring vs. Cover-cropping

These terms are often used interchangeably, but there are some distinctions.

Green Manuring: This is the practice of planting a crop for the purpose of improving the soil. Instead of being harvested, a green manure crop is incorporated into the soil (i.e. turned under), usually while still green and immature, where it decomposes. The crop is closely sown (usually broadcast), as the main aim is to produce the maximum amount of green material. Green manuring can add a lot of organic matter to the soil. If the crop is a legume like cowpeas, considerable nitrogen may be added, too. A green manure crop can also function as a cover crop during growth by protecting the soil from erosion by wind and rain.

Cover-Cropping: This is the practice of planting a closely-sown crop mainly to protect the soil between normal cropping periods. Cover crops can also be planted between trees in orchards. In some situations, it's also possible to saw a cover crop between the rows of a maturing crop such as maize. Cover crops are often eventually turned under as green manures; however, unlike normal green manuring where the crop is usually turned under 4090 days after planting, cover-cropping often takes place over a longer period, because its main purpose is soil protection. In fact, a cover crop nay even be eventually harvested or periodically cut or grazed over a number of months.

The Benefits of Green Manuring and Cover-Cropping

• Green manuring can produce a lot of valuable organic matter in a short period, often with little cost and labor.

• If a legume is used, green manuring can add appreciable N to the soil due to N fixation.

• In addition, other nutrients may be "mobilized" by being taken up by the green manure and released in a more available form when the crop is turned under; however, unlike in the case of nitrogen, there is no net addition to the soil.

• Unlike compost and manure, green manuring is also well suited to large plots, although a plow, harrow, or considerable hand labor are needed to turn under a green manure crop.

• Both practices protect the ground from water and wind erosion.

• They act as "holding tanks" by absorbing nutrients and preventing them from being lost from the root zone by leaching (especially a problem with N).

• They suppress weed growth through shading and competition.

• They also can provide some good-quality forage for livestock (especially when legumes like cowpeas or kudzu are used) or food for people.

• Some green manure or cover crops can be intercropped with cereal grains to help suppress weeds. (More on this below.)

• Two legume green manure/cover crops, Crotalaria spectabilis (showy crotalaria or rattlebox) and Indigofera hirsute (hairy indigo) have proven to be effective in reducing the populations of harmful soil nematodes that attack crop roots. (Note that the seeds of all crotalaria species, as well as the leaves of Crotalaria spectabilis and C. juncea, are poisonous to livestock.)

The Fertilizer Value of Green Manure Crops

Nitrogen: Only legume green manures will add new N to the soil, and the amount added varies greatly with the species, length of growing period, soil N content, and other factors:

• The amount of new N produced is often overestimated, because not all the N in a legume comes from N fixation; some comes from the soil itself. Research has shown that the proportion of N derived from fixation can range from 30-80% and is largely dependent on the soil's content of available N. The higher the level of available N (nitrate and ammonium) in the soil the less N the rhizobia bacteria will fix (they become "lazy"), and the less net N added to the soil.

• When grown as short-term green manures (40-50 days), legumes like cowpeas and mungbeans will return about 40-60 kg/ha of N. When grown as longer-term green manures (90 days), forage legumes like stylo and kudzu will return about 100-200 kg/ha of N. In both cases, the amount of net N added (i.e. that due to fixation) will be roughly half these amounts. This means that additional N may be needed in some cases for moderate to high yields of a non-legume crop that follows a legume green manure.

• Temperate-zone research has shown that roughly half of the legume's N becomes available to succeeding crops in the first year, but this figure is likely to be much higher in the tropics, given the more rapid breakdown of organic matter at higher temperatures.

Other Nutrients: Although legume green manures can add appreciable new N to the soil, it's important to realize that neither legume nor non-legume green manures will add new amounts of other nutrients when turned under "in place", since these nutrients are derived from the soil alone. However, if green manure crops are cut and carried to other plots, they will enrich that land with nutrients by transfer, especially if concentrated on a smaller area.

Are Green Manuring and Cover-Cropping Feasible for Third World Small Farmers?

Despite the apparent advantages of been manuring and cover-cropping, they're not always feasible for Third World small farmers. Compared to North America and Europe, there's been much less experience and research in this area in moat developing countries. Crop selection and timing can be very location-specific. Here are some other points to consider:

• Green manures and cover crops often fit in better with the cropping systems of the humid tropics where there's more of a year-round growing season. In the wet-dry or semiarid tropics with long dry spells, crop scheduling is often very tight. Farmers may not be able to fit in green manures or cover crops and still produce their normal ones. Even where irrigation is available, it's often more cost effective to use it for cash crops.

NOTE: A great opportunity for green manuring occurs during the wet season in dry season gardens which often aren't cropped during the rainy months.

• In drier areas, they may seriously deplete the soil moisture needed for a succeeding cash or staple crop.

• Under tropical conditions, the increase in soil humus from turning under a green manure crop may be short-lived, because organic matter breaks down much more quickly in warm temperatures. On the other hand, this means that the green manure's N will become more quickly available.

• Turning under a non-legume can sometimes cause a temporary deficiency of N, P, or sulfur, especially if done at an advanced stage of growth when the carbon: nitrogen ratio widens. Legume vegetation generally contains twice as much N as non-legume vegetation (particularly at later stages) and breaks down quickly to release these nutrients. (See Chapter 6 for an explanation of this type of tie-up.)

USING GREEN MANURE AND COVER CROPS

For the most relevant information for your specific ares, check with the local ag extension service. If there's an ag experiment station in your agro-climatic zone, it would be well worth a visit to see what work is being done in this area Here are some general guidelines:

Choosing a Green Manure or Cover Crop

Some common green manure and cover crops in temperate areas are Austrian winter peas, crimson clover, broad beans fava beans), winter vetch, oats, winter wheat, and annual ryegrass. However, they aren't adapted to tropical regions. Sudan grass, sorghum-sudan grass, and pearl millet are some non-legumes that are adapted to both the tropics and to summer production in temperate zones. Legume green manure and cover crops adapted to the tropics and subtropics are listed in Table 8-5 and described in more detail in Appendix F.

Legumes vs. Non-Legumes: Farmers are usually better off using a legume for a green manure, since it can add considerable new N to the soil. In addition, there's little likelihood of causing a temporary tie-up of N, P, or sulfur as can sometimes occur when a non-legume is turned under, especially at an advanced stage of growth. However, where there is a need for a long-term cover crop, grasses or grass-legume mixes can be very appropriate.

Using Weeds: In some cases, it may be advantageous in terms of seed cost, adaptation, and availability to use a naturally-occurring weed as a green manure, especially if it's a legume. In fact, some of the legumes listed in Table 8-5, such as crotalaria and coffeeweed, are also weeds. However, care must be taken in the selection and management of such plants to avoid their truly becoming weeds. Don't use species that propagate by hard-to-eradicate runners, and never allow the crop to produce seed. (In fact, many weeds have the ability to continue maturing their seeds, even when cut down at the early seed head stage.)

Here are some other important criteria to consider when selecting a green manure or cover crop:

• Adaptation to the area in terms of climate, soils, insects, diseases, and nematodes.

TABLE 8-5

SOME LEGUMES FOR GREEN MANURING OR COVER-CROPPING IN TROPICAL AND SUBTROPICAL REGIONS

(See Appendix F for a description of each legume and its adaptation).

I. Quick-Growing Legumes for Short-Term Use (40-80 days)

Scientific Name

Common Names

Glycine max

Soybean

Phaseolus aureus

Mungbean, green gram

Phaseolus acutifolius

Tepary bean

Sesbania exaltata (S. macrocarpa)

Sesbania, coffeeweed

Vigna unguiculata (V. sinensis)

Cowpea

II. Bush or Viny Legumes Best Suited for Long-Term Use (90 days or more)

Scientific Name

Common Name

Calopogonium mucunoides

Calopo

Canavalia enniformis

Jack bean

Canavalia gladiata

Sword bean

Gentrosema pubescena

Centrosema, centro

Clitoria ternatea

Butterfly pea

Crotalaria spectabilus

Showy crotalaria

Desmodium intortum

Greenleaf desmodium

Desmodium uncinatum

Silverleaf desmodium

Dolichos lablab

Hyacinth (lablab) bean

Indigofera hirsuta

Hairy indigo

Phaseolus atropurpureus

Siratro

Phaseolus lathyroides

Phasey bean

Pueraria Phaseoloides

Tropical kudzu, puero

Stizolobium spp. (Mucuna spp.)

Velvet bean

Stylosanthes gracilis (guyanensis)

Stylo

Stylosanthes humilis

Townsville stylo

III. Legume Trees or Shrubs for Cut-and-Carry Green Manuring

Scientific Name

Common Names

Cajanus cajan

Pigeon pea

Calliandra calothyrsus

Calliandra

Gliricida sepium

Madre de cacao

Leucaena leucocephala

Leucaena, ipil-ipil

Mimosa scabrella

Bracatinga

Sesbania bispinosa

Prickly sesban

Sesbania grandiflora

Sesbania

Sesbania sesban

Sesban

(Criteria for green manure crop selection, continued)

It should take relatively little cost, labor, and skill to establish and manage. Remember that green manure and cover crops are closely sown, which requires a large number of seeds per area. Due to their large seed size and demand as a food crop, pulses (edible grain legumes such as cowpeas) may be too expensive or scarce to be used as soil improvers (from 60-120 kg/ha of seed is needed. Small-seeded, non-food legumes (e.g. crotalaria and centrosema) have many more seeds per kilogram and thus require lower seeding rates (about 4-2Q kg/ha); seed cost per kilogram may be lower, too.

• Quick growth, especially in the case of short-term green manuring that's sandwiched in between crops or where rapid erosion protection is needed.

• Ease of eradication, in the case of short-term use. Some perennial tropical grasses (e.g. bermuda grass, pare grass, and kikuyu) propagate by runners and can become invasive and difficult to eradicate. Some perennial tropical legumes like centrosema are vigorous, vining growers best suited to long-term use.

Fertilizing Green Manure Crops: Legume green manure crops should require no nitrogen if the proper strain of rhizobia bacteria is present or added by innoculating the seed (see Chapter 10 under "Pulses"). However, legumes often benefit P and K applications on lowfertility soils. On low-P soils high P tie-up capacity, it may be wise to row-plant the green manure so that the P fertilizer can be band-applied to minimize tie-up.

When to Turn Under Green Manures

If the purpose is to produce the maximum amount of organic matter (or N) in the shortest amount of time, it's best to turn under quick-growing annuals like cowpeas or mungbeans about 40-50 days after sowing; this usually corresponds to the flowering stage in these legumes. Waiting longer will produce more added nitrogen but little extra green material, since the crop's energy is diverted into seed production. However, it may be advantageous to continue growth for cover-cropping purposes. Perennials such as tropical kudzu, stylo, and centrosema are slower growing and usually require 90 days or more to produce sufficient vegetation.

Avoiding toxicity problems: In order to avoid possible seedling injury due to toxic decomposition products, green manure crops should be turned under at least 2-3 weeks before planting the next crop.

What about seed harvesting?: If a legume like cowpeas or beans is grown to maturity for seed harvest (hard, dry seed stage), much less N will be added to the soil when the crop residue is turned under. That's because about 75% of the N (as well as 75% of the P and 60% of the K) in the plants eventually ends up in the mature seeds themselves. Another consideration is that green manure crops are usually sown more densely, which discourages good grain yield.

What about growing legumes among non-legumes to supply nitrogen?: When a legume like beans is interplanted with maize (both for harvest), very little of the legume's N is transferred to the non-legume for 2 reasons. First, most of the N fixed by the legume ends up in the seeds. Second, very little N is excreted into the soil by legume roots or nodules during growth, although there are 2 important exceptions:

• Among the grain legumes, mungbeans (Phaseolus aureus) have the unusual ability to excrete significant amounts of N (up to 40-50 kg/ha) into the soil during growth. Such N can be utilized by a companion crop.

• Most pasture legumes (e.g. clovers, siratro, and stylo) excrete enough N to satisfy the needs of the pasture grasses with which they are often grown in combination.

The Cut-and-Carry Method of Green Manuring

Legume trees and shrubs, such as leucaena and Gliricidia, can be used for green manuring by periodically cutting their branches and carrying them to an adjacent crop (in the case of alley-cropping) or to another field where they can be either worked into the soil or used as a mulch. Leucaena leaves contain 0.5-1.0% nitrogen on a fresh-weight basis. It's estimated that 400-600 kg of N yearly can be supplied by the foliage obtained from one hectare of leucaena cut back to l meter in height every 3 months. (Remember that not all this N is derived from N fixation, however). The fresh weight of the harvested foliage would be roughly 40,000-80,000 kg/ha. The cut and-carry method requires considerable labor, and care must be taken to replenish soil nutrients (other than N) taken up by the green manure crop.

Some Examples of Successful Green Manuring and Cover-Cropping in the Tropics

•The National Maize Program (PNM) in Zaire has been using 2 legumes, crotalaria (toxic to livestock) and soybeans as green manure/cover crops. In this case, the crotalaria has proven superior to soybeans in N production and has supplied all the N required for high maize yields. One problem is that most legumes won't do well on Zaire's low-phosphorus soils (common in the tropics) without additional P as chemical fertilizer.

In parts of S.E. Asia, green manuring between rice crops using legumes is common. In one trial in the Philippines, rice yields were more than doubled (3600 kg/ha vs. 1500 kg/ha) when any of the following green manure crops were used: cowpeas or mungbeans plowed under 45 days after seeding; stylo or tropical kadzu plowed under 90 days after seeding.

MULCHING

Mulching consists of covering the soil surface with a layer of organic matter (or plastic or newspaper) and can have many benefits. Mulching:

• Reduces soil water lose due to evaporation from the surface.

• Suppresses weeds. (However, perennial weeds like nutsedge may be able to grow through mulches, including plastic.)

• Protects the soil from water and wind erosion.

• Modifies soil temperatures. (See below.)

• Keeps vegies like cucumbers and tomatoes fro. ground contact to reduce rotting.

• Encourages earthworms (see next section).

• "Organic" mulches like hay add humus to the soil as they decompose.

• Can be used between planting and seedling emergence to keep soil moist and prevent seed washout. (See Chapter 4 on seedbed preparation.)

Under some conditions, mulching can have disadvantages:

• Organic mulches may attract pests like crickets, ants, slugs, snails, and even termites.

• In cool regions, organic mulches may actually keep the soil too cool for good growth of warm-season vegetables like okra, eggplant, and the squash family.

• In hot regions, plastic mulches may overheat the soil, especially when clear plastic is used.

• In wet areas, organic mulches may encourage stem rota. Keeping it 7-10 cm away from the stems will help.

• Low-nitrogen organic mulches like sawdust and rice hulls may cause a temporary N tie-up if they are worked into the ground before they have decomposed adequately. Although the University of Hawaii recommends mixing the equivalent of 350 cc of urea (45-0-0) into each 100 100 liters of sawdust, this shouldn't be necessary if the sawdust mulch is kept on the soil surface.

The Effect of Mulches on Soil Temperature

• Organic mulches like straw help cool the soil, and generally reduce soil temperature fluctuations; this can be beneficial for cool-season crops like lettuce and cabbage when grown in overly hot weather.

• On the other hand' plastic mulches warm the soil if clear or dark colors are used. Clear plastic can increase soil temperatures in the upper 10-12 cm by as much as 10-12C (1822F), and black plastic by about 3-6 C (5-10F). This may be desirable for warm-season crops like eggplant and squash when grown in overly-cool weather but may produce harmfully high soil temperatures in hot weather unless the mulch is well shaded by the crop's foliage. (It's also possible to cover the plastic with an organic mulch during the hot season.)

• White or reflective Plastic will cool the soil and reduce soil temperature fluctuations by 36C (5-10F).

WHEN AND HOW TO MULCH

Situations Where Mulching may be Beneficial

As shown above, mulching can be beneficial in both dry and wet regions. In fact, the PATS ag trade school in Ponape, Micronesia uses mulch successfully on raised beds in clayey soils under 4500 mm (190") annual rainfall. (They use baits to control slugs attracted to the mulch.) Likewise, dry season vegetable gardens in areas like the Sahel will often benefit from mulching. Mulching can also be used on a larger scale for field crops. In experiments done by IITA (Internal. Institute for Tropical Agric.) in Nigeria, mulching increased maize yields by 23-45 percent and greatly reduced the labor needed for hand weeding which accounts for 50-70 percent of total labor there. (In this case, the mulch consisted of crop residues and weeds killed with a herbicide.) Coconut yields on Pacific atolls have been increased by 100-200% by using cut undergrowth as mulch, rather than burning it, although extra labor is involved.

How to Use and Apply Mulches

Pre-emergence mulches: Applying a pre-emergence mulch to the soil after planting helps maintain good soil moisture for germination and prevents the seeds from being washed out by heavy rain or careless watering; it can also reduce soil crusting problems that adversely affect seedling emergence. Straw or newspaper work well but must be removed as soon as seedlings start emerging or they will quickly become spindly and weak due to lack of sunlight. However, in some cases, pre-emergence mulching attracts harmful insects and even termites. You can also try a "grow-through" mulch of light, fine material like rice hulls or sawdust which doesn't require removal.

Post-planting mulches: Mulch can be applied shortly after the crop comes up. Where crickets and slugs are a problem, it helps to keep the mulch 7-10 cm away from the plants, at least when they're young; this will also help avoid stem-rot problems encouraged by high moisture. Fine materials like rice hulls can be applied about 4-6 cm thick (more is OK too). Coarser mulches like straw need to be applied at least 8-10 cm thick or they may admit enough sun to encourage evaporation and weeds. A newspaper mulch 2-4 pages thick works well but needs to be held in place with soil or rocks. Plastic sheeting for mulching can be as thin as 1 mil (.001"), especially if it is the embossed type that is stronger and more flexible than the smooth kind.

EARTHWORMS

How They Help the Soil

• Earthworms eat soil in order to burrow and to feed on its organic matter. Though their excreted "castings" contain only the nutrients in the consumed soil, this digestion process speeds up the release of available nutrients from organic matter. They're also a good soil conditioner and stimulate beneficial soil microorganisms.

• They mix and redistribute organic matter; under favorable conditions, they'll transport up to 4 kg of soil to the surface per sq. meter per year.

• Earthworm channels improve soil aeration and drainage.

Can Earthworms Turn a Sick Soil into a Healthy One?: A soil with lots of earthworms is usually a very productive one, but it's not a simple matter of cause and effect. Adding them to a poor soil is likely to get you nowhere. They won't survive, let alone thrive, unless the soil is in fairly in fairly good shape to begin with! The best approach is to promote their natural buildup by good soil management. Earthworms need many of the same conditions as plants do in order to prosper.

How to Promote Earthworms in the Soil

• Adjust soil pH if necessary; earthworms don't like very acid soils (below pH 5.5).

• Add lots of organic matter.

• Mulching helps by keeping the soil moist and adding organic matter.

• Good soil drainage is important.

• Overly-high rates of chemical fertilizers will discourage earthworms due to salt buildup; however, there's no evidence that reasonable rates are harmful.

• Some insecticides and herbicides that are applied to the soil are toxic to earthworms.

Using Earthworms for Composting

Under the right conditions, earthworms will consume and digest almost any nontoxic organic waste such as manure, crop residues, kitchen wastes, and even paper and cardboard. The end product is essentially compost. One method of making compost with earthworms is to raise them in wooden-sided beds about 30 cm deep or in shallow pits (with gravel in the bottom for drainage) filled with organic matter. Most fresh manures should first be allowed to partly decompose in order to prevent excessive heat. It will take a month or two for the worms to compost the material. Be sure to keep the beds moist, though not sopping wet. Occasional turning may be needed to prevent the material from compacting.

Earthworm compost can be made from rabbit droppings right under the hutches, using bins or shallow pits. A starter mix of 1/2 droppings and 1/2 fine compost gets them off to a good start. Some lime may be needed to counteract the manure's initial acidity.

Earthworms as Feed: They're very high in protein (about 70% on a dry weight basis) and have been successfully used for poultry feeding.