| Soils, Crops and Fertilizer Use |
|Chapter 4: Seedbed preparation|
-Getting Crops Off to a Good Start
Tillage is the use of implements to prepare land for planting. Many Third World small farmers who lack equipment (or whose land is very steep or rocky) will prepare ground by slashing and burning the vegetation, followed by making seed holes with a planting stick or hoe. In cases where tillage is used, it has 5 main purposes:
• To break up clods and loosen the topsoil to encourage seed germination, seedling emergence, and root growth. Most tractor- and animal-drawn planters require a tilled seedbed for successful operation.
• To chop up and/or bury the previous crop's residues so they won't interfere with the new crop.
• To control weeds. An ideal seedbed is completely free of visible weeds at planting time.
• To incorporate (mix into the soil) fertilizers or liming materials.
• To shape the type of seedbed best suited to the specific soil, crop, and rainfall conditions (e.g. raised beds, ridges, flat beds, sunken beds, etc.).
NOTE: For a more detailed description of tillage equipment, see the PC/ICE Traditional Field Crops Manual.
Hand Implements: Digging hoes, shovels, turning forks, and rakes are very effective for smaller areas. By using the double-digging method (described later in this chapter), the subsoil can be loosened too.
Wooden Plow: Designs date back many centuries; they're animal drawn, and some have a metal tip. They do not invert (turn over) the soil or bury crop residues like a moldboard plow but basically make grooves through the soil. Wooden plows penetrate about 15-20 cm deep.
Moldboard Plow: Depending on its size and the condition of the soil, it penetrates 15-25 cm deep and inverts the furrow slice, making it very effective for burying weeds and crop residues. (Bulky residues like maize stalks must be chopped up first.) Both animal- and tractor-drawn models are used. Moldboard plows aren't as well suited to dry soils, as disk plows and don't handle rocky soils as well. They also don't work well in sticky, clayey soils.
Disk Plow: Better suited than the moldboard to hard, clayey, rocky, or sticky ground but won't bury residues as well; however, this is an advantage in drier areas where leaving residues on the surface cuts down wind and water erosion and reduces moisture evaporation. Nearly all disk plows are tractor drawn.
Rototillers (rotovators) are available in self-powered and tractor-drawn models. They thoroughly pulverize the soil and partially bury crop residues. Heavy duty models can be used for a once-over tillage job. However, their power and fuel requirements are high, and they can easily over-pulverize a soil and destroy its beneficial crumb structure.
Disk Harrow: These are available in animal- or tractor-drawn models and are commonly used after plowing to break up clods, control weeds, and smooth the soil before planting. They are also useful for chopping up coarse crop residues before plowing, but heavier models with scalloped (notched) disks are best for this purpose. Disk harrows are expensive and prone to frequent bearing failure unless regularly greased. Large, heavy duty versions called Rome plows, which are drawn behind big tractors, can sometimes substitute for plowing. Disk harrows cut, throw, and loosen the top 8-15 cm of soil but pack down the soil immediately below that. Repeated harrowing, especially when the soil is moist, can cause compaction in the lower topsoil.
Tillage can either enhance or destroy good soil filth (workability). Sure, plowing and harrowing break up clods and loosen the topsoil, but the stirring and shearing action stimulate the microbial breakdown of beneficial soil organic matter and may also overpulverize the soil. Tractor wheels, animal hooves, (and even foot traffic) can cause soil compaction (especially on wet, clayey soils), which can seriously impair root growth and drainage. In mechanized farming, it's common for a traffic pan (compacted zone) to develop on many soils (even sandy ones) at a point right below tillage depth. Plowing clayey soils when they are overly dry can produce large, brick-hard clods that may first require rainfall or watering before they can be broken up. Tilling clayey soils when they are wet can seriously affect their their filth.
The two implements most likely to harm filth are the disk harrow and the rototiller. Many farmers overuse the disk harrow by making repeated passes to kill weeds or smooth the soil. Unfortunately, harrowing kills one stand of weeds but encourages another by bringing up more weed seeds closer to the surface where there is sufficient oxygen to trigger germination.
The Ideal Seedbed
The only portion of the seedbed that needs to be reasonably clod-free is the narrow row zone where the seeds are planted. In fact, you're actually better off keeping the spaces between the rows in a cloddy condition to discourage weed germination and help maintain filth. Since the 1960's, minimum tillage systems such as plowing and planting in one operation or using specially designed planters to plant directly through crop residues into unplowed ground have become increasingly popular for field crops like maize in the developed countries. These methods have definite potential for the Third World, too. For example, the International Institute for Tropical Agric. (IITA) in Nigeria has been adapting such reduced tillage methods to small farmer conditions (see address in Appendix G). They have developed hand-pushed planters that can successfully plant maize and cowpeas through vegetation that has been slashed down (or killed with a herbicide) and left on the soil surface as a beneficial mulch. No tillage is used with this method.
NOTE: Seedbed preparation is very location-specific and varies with climate, soil type, crop, management level, and available equipment. Local farmers usually have good seedbed skills, so beware of tampering with local methods before thoroughly testing new ones.
What Kind of Seedbed?
There are basically 3 types of seedbeds: flat beds, raised beds, and sunken beds. The best type to use depends much more on the particular climate and soil conditions than on the crop.
Flat beds are used where water availability is adequate and there are no drainage problems. In some areas, crops like maize, sorghum, beans, and potatoes are started out on a flat bed; as the season progresses, soil is thrown into the crop row to mound up the plants; this is called "hilling-up" and is done to control in-row weeds, provide support, and improve drainage. (Potatoes are also hilled up to keep the developing tubers covered with soil.) Hilling-up only works with plants that have enough stem height and leaf clearance to tolerate partial burial.
Crops can also be grown on raised-up beds or ridges. They are especially advantageous for clayey soils under high rainfall or wherever else drainage is likely to be poor. They can also be used in many other situations. Where crops are furrow irrigated, raised beds or ridges are essential so that the water can flow down the furrows between them.
Height of raised beds: Raised beds are usually 10-30 cm high. The best height depends mainly on soil texture and moisture considerations. For example, raised beds are often 2030 cm high on clayey soils under high rainfall where poor drainage is likely to be a problem. On coarser-textured soil under the same conditions, bed height might be 15-20 cm. When raised beds are used in drier conditions, a bed height of 10 cm or leas may be best to avoid excessive moisture loss due to evaporation from the exposed sides.
Width of raised beds: Typically they are 100-130 cm wide.
Raised beds may have several advantages:
• Much better drainage compared with flat or sunken beds.
• They provide a double layer of topsoil, because they're made by dragging in topsoil from the surrounding alleyways. (Because of this, they're also likely to be looser than flat or sunken beds.)
• In temperate regions, raised beds warm up more quickly in the spring, which may benefit cold-sensitive crops and even permit earlier planting.
• Plants on raised beds are easier to reach when doing hand operations such as weeding and thinning.
Raised beds usually aren't a good choice during the dry season, because they dry out more quickly than flat or sunken beds; also, water tends to run off them and be lost into the alley-ways. These disadvantages can be partly overcome by mulching the bed with straw or rice hulls, making a lip around the bed's edge to reduce run-off, and by reducing bed height to 10 cm or less (see Fig. 4-4).
In dry regions, especially on sandy soils with low water-holding capacity, vegetables can be planted in sunken beds (i.e. shallow basins) about 100-130 cm wide and 2-5 cm below the surrounding soil level. Sunken beds conserve water much more effectively than raised beds for 2 reasons:
• Sunken beds don't have the exposed sides of raised beds from where considerable moisture can be lost by evaporation.
• None of the applied water is lost by runoff.
One disadvantage of sunken beds is that some topsoil is lost in the usual method of construction. (They're made by pulling off soil from the bed area and placing it in the surrounding alleyways). This probably won't affect crop growth, as long as the topsoil is of normal depth let least 15 cm) and enough compost or manure is added. Here are 2 ways of building sunken beds without sacrificing topsoil:
• First take off the topsoil, and then replace it after removing enough subsoil to sink the bed enough.
• Make a "pseudo" sunken bed by mounding alleyway soil around the bed's borders. This will work well in clayey soil, but border dikes made of sandy soil may wash out when the bed is watered.
One variation of sunken beds is furrow Planting in which crops like maize, sorghum, and beans are planted in the furrow bottom between two ridges where soil moisture is higher and less easily lost. Soil can then be thrown into the furrow as the season progresses to control weeds and improve drainage if rainfall increases.
Most seedbed preparation methods use a plow or digging hoe to loosen the soil to a depth of 15-20 cm (i.e. the topsoil). There are 2 situations where deeper tillage may be costeffective, though not always, by any means:
• Encouraging roots to enter the subsoil by breaking up a pan (compacted layer) may enable them to tap into a valuable moisture reserve; this can make a crucial difference in a drought, especially for deep-rooted crops (e.g. maize, sorghum) grown under rainfed conditions. Loosening a pan may also improve soil drainage.
• In very hot conditions (e.g. the Sahel during the period from March through June) deep tillage may allow roots to grow deeper into cooler, more hospitable soil.
The value of deep tillage is commonly overrated for a number of reasons:
• Power and labor requirements increase greatly, especially since the subsoil tends to be more compact. With hand tools, deep tillage is seldom practical on anything but small plots.
• About 60-80% of most crops' roots are found in the topsoil, even in high-yielding fields.
• Subsoils that are poorly drained or too acid won't allow much root growth, no matter how well loosened or enriched. Also, it's not unusual for the soil below a hard pan or clay pan to be compacted and poorly drained.
• Loosening a compacted subsoil is likely to be only temporarily effective unless a soil conditioner (sand, rice hulls, etc.) is added, as well as fertilizer. This may be feasible using hand tools but is very laborious.
• On large fields, tractor-drawn subsoilers (long, narrow shanks that penetrate up to 60 cm deep) can be used to break up compacted layers (pans). Results vary from poor to good, depending on the type of pan, its wetness, and the characteristics of the soil below it. Hardpans (natural layers that remain cemented whether wet or dry) and traffic Pans (compacted layers right below normal tillage depth that are caused by machinery traffic) can usually be successfully fractured while dry. However, subsoiling claypans (natural, dense, clayey layers) often gives disappointing results for 2 reasons. First, they're often continually wet (unless the dry season is long) and aren't subject to fracture in this condition. Secondly, soil below such claypans is often compacted and poorly drained, too.
Seedbed "fineness" refers to the degree to which clods are broken down and the soil smoothed over. The need for this depends mainly on seed type, seed size, and whether hand planting or mechanical Planting will be used.
Seed type: Monocot plants like the cereals (maize, sorghum, etc.) have one cotyledon (seed leaf ) and break through the ground in the shape of a spike which helps them handle some cloddiness. Dicot plants (pulses like beans, cowpeas, peanuts, and virtually all vegetables) have 2 cotyledons and emerge from the soil in a much more blunt form, actually dragging the 2 seed leaves (formed from the 2 halves of the seed) with them. They have less clod-handling ability than most monocots, although seed size also makes a difference.
Seed size: As a rough rule, the larger the seed, the less the need for a fine seedbed. Large seeds have more energy and can also emerge from greater depths. A seed like maize is not only large but is a monocot too, so it has especially good clod-handling ability. Peanuts, beans, and most other pulses are large seeded, but this advantage is partly offset because they are dicots. The small seeds of millet and sorghum lack some power, but being monocots is a help. Note that smaller seeds (i.e. lettuce, cabbage, onions, amaranth, require shallower planting than larger seeds (i.e. pulses, okra, maize, squash, etc.) and that a cloddy seedbed makes it difficult to be precise with planting depth.
• Hand vs. mechanical planting: Farmers who hand plant can usually get by with rougher seedbeds for several reasons. It's easier to control planting depth when hand seeding and large clods can be pushed aside. Also, it's common under hand planting to plant several seeds per hole, which provides more power for breaking through the soil.
In this section, we'll cover these useful seedbed skills:
• Seedbed layout for intensive vegetable gardening
• Dealing with heavy clay soils . Dealing with soil crusting problems . Making up a soil mix for nursery seedboxes
SEEDBED LAYOUT FOR INTENSIVE VEGETABLE GARDENING: The Bed-and-Alley System
Western vs. Intensive Seedbed Layout
A typical Western-style vegetable garden, with each of its rows wide enough to walk down, makes poor use of space. In this type of garden, the rows are anywhere from about 60-90 cm apart, even though the plants' foliage and root spread may cover only a portion of this (see Fig. 4-6a). In fact, the usual row spacings recommended by most county extension offices in the U.S. (e.g. 90 cm for peppers and cabbage, 76 cm for turnips, beets, and lettuce, 60 cm for radishes) result in a garden with only about 30 percent of its space occupied by the plants and their roots; the rest of the area is essentially walking space! Such a layout wastes valuable land, water, and fertilizer; it also requires more time and labor for weeding and watering. In addition, providing a walkway between every row greatly promotes harmful soil compaction.
The bed-and-alley system of intensive gardening is an alternative to the wide-row system and may offer a number of advantages under non-mechanized conditions (the system isn't suited to animal- or tractordrawn operations once the beds are planted). Here are the features and possible benefits of of the bed-and-alley system:
• The garden is arranged into beds of closely-spaced plant rows; walkways (alleyways) are placed only between the beds instead of between each row (see Fig. 4-6b).
• The beds' widths can range from about 100-130 cm, which enables all the plants and bed area to be easily reached without walking in the bed. The beds (they can be flat, raised or sunken, depending on soil and rainfall conditions) are best kept to 10 meters or less in length so that alleyways can be run up and down, as well as across the garden to facilitate access. The alleyways can be as narrow as 50-60 cm, although occasional wider ones (up to 100 cm) are useful for wheelbarrow traffic.
Since there's no need to walk in the beds, row spacings can be based solely on what's needed for plant spread, which is usually much less. For example, cabbage rows can be 3045 cm apart (vs. 90 cm), leaf lettuce rows 1520 cm apart (vs. 75 cm), and carrot rows 5-10 cm apart (vs. 75 cm). In the case of root and leaf vegetables, the between-row spacing can be as narrow as the in-therow spacing. Depending on the crop, anywhere from 1.5 to 15 times more plants can be grown per sq. meter of land (bed and alley area included) with the intensive system.
Using 120 cm-wide beds and 60 cm-wide alleyways, the intensive system will give a landuse efficiency of about 65 percent, compared to about 30 percent for a typical wide-row garden. In other words, such an intensive layout would consist of 65 sq. meters of actual bed area and 35 sq. meters of alleyway area per each 100 sq. meters.
• Aside from the savings in space, labor, and water, the intensively-spaced plants eventually form a "living mulch" with their leaves that effectively shades most of the bed's surface. This reduces water losses from surface evaporation, helps suppress weeds, and cools the soil (beneficial for roots of cool-season crops like cabbage grown in hot weather). (Note that under high-rainfall, humid conditions, such close plant spacings may promote the development and spread of some plant diseases, especially fungal leafspots.)
What is Double-Digging?
As we've seen, most land preparation methods loosen the upper 15-20 cm of soil (the topsoil). Double-digging is a method of loosening, as well as improving/enriching, a much deeper layer of soil down to 40-60 cm which includes some of the subsoil, too.
Benefits of Double-Digging
Under appropriate conditions, double-digging may have some important benefits:
• Yields may be increased due to better and deeper root growth and improved aeration and water availability. In very hot areas, it may allow roots to grow deeper into cooler, more hospitable soil.
• In some cases, double dug plots may produce the same yield on less area (1/2 or less in some cases); this means that less land, and, more importantly, less water and labor may be needed.
• Where double-digging results in more production on less land it may make it feasible to use other possibly beneficial practices like crop rotation and green manure crops. (These 2 practices are explained in Chapter 8.)
Limitations of Double-Digging
• Double-digging is not a "cookbook recipe" for increased yields. Like many so-called "improved" practices, it should first be tested locally before promoting it.
• It requires much more time and labor than standard land preparation (probably at least 3 times as much), and therefore is seldom feasible on anything but small plots.
• If the subsoil is very acidic (below pH 5.0-5.5), it may require liming to enable root development there.
• Double-digging will not solve subsoil drainage problems caused by a high water table.
Situations Most Likely to Benefit from Double-Digging
Double-digging is best suited where crops are grown in beds (flat, raised, or sunken) that have alleyways around them. Such a double-dug bed will not be recompacted by foot, animal, or machinery traffic, as would be the case with a wide-row, open-field plot. Beds are most likely to respond well to double-digging under the following conditions:
• Dense, compacted! clayey subsoils
• Deeper-rooted crops like tomatoes, eggplant, okra, squash, and beans may benefit more than shallow-rooted crops like onions, lettuce, cabbage, and radish. However, even some shallow-rooted crops can put down roots 40-50 cm deep with good soil conditions.
• Hot-dry climates where increased root penetration is likely to improve water availability and allow roots to enter cooler soil.
How to Double-Dig
The best way to learn double-digging is to do it. A shovel is sufficient, but a digging fork or pick may come in handy for very hard subsoils. Here are the steps:
1. Stake out a 1 x 4 meter bed.
2. Start at one end and dig out a trench 30-40 cm wide to the depth of your shovel blade. Carry the soil to the other end of the plot and deposit it just outside as in Figure 4-7a.
3. Now add a layer of compost or manure at least 2 cm thick to the bottom of the trench. Work it into the subsoil, ideally to the depth of the shovel blade. It's not necessary to invert the subsoil, as long as the organic matter can be mixed into it. Although some gardening manuals give dire warnings about mixing different soil layers, this is unlikely to cause any problems. Soil conditioners like rice hulls or peanut hulls can be added along with compost or manure; however, if used by themselves, they're likely to cause a temporary nitrogen tie-up. (Refer to the section on soil conditioners in Chapter 8.)
NOTE: Double-digging will be largely ineffective unless you add manure, compost or some sort of soil conditioner to the subsoil; otherwise, it may soon recompact and also will remain infertile, thus discouraging root growth.
4. Now dig the next 30-40 cm-wide trench, but this time throw the soil into the trench you made in Step 2. (See Figure 4-7b.)
5. Repeat Step 3 with the soil in the bottom of the 2nd trench.
6. Continue this sequence until you reach the end of the bed.
7. After loosening and enriching the subsoil in the last trench, fill in the trench with the soil that you carried to that end of the bed in Step 2. (See Fig. 4-7c.)
A shortcut: When double-digging a series of adjacent beds, you can avoid carrying the soil from the first trench to the other end of the bed (see Step 2 above) by using the method shown in Figure 4-8.
How often to double-dig?: Double-digging a plot once a year should be sufficient if the bed-and-alla, system is used, since it will help avoid recompaction. It may take several years of annual double-digging to measurably improve the soil; after that, you may be able to do it less often.
HOW TO DEAL WITH CLAYEY SOILS
Tillage, bed construction, and planting can become very difficult or even impossible when clayey soils are too wet or too dry. This is because they have a very narrow moisture range within which their filth (workability) is satisfactory. Aside from adding sand and organic matter (see Chapter 2), there are several other techniques that will help:
, Whenever possible, allow overly wet, clayey soils to dry out to the point where filth again becomes satisfactory before attempting to work them. Otherwise, you will actually worsen the soil's condition.
• Pre-watering dry, clayey soils a day or two prior to working them can do wonders for filth. Where the entire topsoil is very dry, apply at least 25 liters of water per sq. meter. Allow the soil to dry to a point where it has lost much of its stickiness and can be worked without producing clods that resist being broken down well by hoeing. You may also want to try doing the initial tillage of the soil while it's still dry and then applying water before breaking up the clods.
• Coarse materials like sand and rice hulls are excellent filth improvers for clayey soils. If in short supply, first prepare the bed for planting and then hoe in the material into the top 2-5 cm of soil. This will greatly improve the filth of this shallow planting layer and facilitate seed sowing and seedling emergence. Another way to efficiently utilize limited amounts of soil conditioners is to apply them in narrow strips centered over the rows.
• On clayey soils, seedling emergence from small vegetable seeds can be greatly improved by covering them with sand, compost, or sawdust instead of soil. Covering the seedbed with a pre-emergence mulch of hay or newspaper will help keep the soil moist for sprouting and allow for shallower planting.
HOW TO HANDLE SOIL CRUSTING PROBLEMS
Some soils high in clay or silt and in poor filth will form a cement-like crust at the surface each time it dries out. This can seriously affect seedling emergence, especially in the case of the blunt dicot seedlings (the ones that emerge with 2 seed leaves). Here are some suggestions for handling soil crusting:
• Make regular additions of compost or manure to maintain a healthy level of soil organic matter which helps counteract the tendency to crust.
• Cover small vegetable seed with sand or sawdust instead of the soil.
• Work in sand into the top 2-3 cm of soil.
• Apply a pre-emergence mulch to the soil after planting to help keep the soil surface moist and free from crusting. It also will absorb the impact from rain or overhead watering which encourages crusting by packing the soil. Straw or dried grass works well but must be removed as soon as seedlings begin emerging or they'll quickly become spindly and weak due to lack of sunlight. However, in some cases, such mulches do more harm than good by attracting harmful insects like crickets, ants, and termites. You can also try a "grow-through" mulch of light, fine material like rice hulls or sawdust which doesn't need to be removed at seedling emergence.
HOW TO MAKE UP A SEEDBOX SOIL MIX
If you raise vegetable transplants using seedboxes, you'll quickly find out that straight soil, by itself, seldom works well as a growing medium. When confined to a shallow seedbox or pot, most natural soil becomes very poorly drained in the bottom half of the container, no matter how many drainage holes you make. Even lining the bottom with shells or pebbles won't help much. That's because there's no soil below to provide a suction force to draw the excess water away - gravity by itself just isn't strong enough. The same thing happens when you hang wet clothes on the line - a lot of water collects at their bottom end. So, with seedbox soil you'll want to make up an extra-coarse soil mix that will facilitate drainage yet still hold enough moisture. Another advantage of soil mixes is that they're lighter than regular soil which makes it a lot easier to move the seedboxes around.
Here are a few recipes for soil mixes for seedboxes or other containers - try out your own, too, and ask local farmers for their suggestions.
NOTE: As with in-the-ground nursery seedbeds, seed box soil mixes should be sterilized by heat or other method prior to planting to control diseases and nematodes.
• Mixing rice hulls with soil has proven very successful. One example:
Philippines horticulturists recommend mixing rice hulls with clay loam soil at a 2:1 ratio by volume and then adding 1 gram of single superphosphate (0-20-0) and 1 gram of 14-14-14 per liter at sowing. Another gram of 14-14-14 per liter is applied at 10 days and again at 20 days after sowing. Rice hulls have a strong capacity to tie up nitrogen and are also low in other nutrients, so that's why the chemical fertilizer is added. (N tie-up is explained in Chapter 6.)
• A 1:3 sand-compost mix. Rotted coconut husk fibers passed through 1/4-3/8 inch (6-10 mm) hardware cloth can be used to make a great instant compost.
• A 1:1:1 sand-soil-compost mix.