| Soils, Crops and Fertilizer Use |
|Chapter 2: Trouble-shooting soil physical problems|
This chapter focuses on diagnosing and managing soil physical problems that affect productivity. Soil fertility problems are covered in PART II of this manual.
As explained in Chapter 1, it's difficult to make any useful generalizations about the soils of the tropics and subtropics. Soil-forming factors like climate, parent rock, time, topography, vegetation, and management interact in countless patterns. It's not unusual to find two or more soils on one small farm that vary markedly in texture, depth, slope, and other important features.
Here are the best ways of getting to know your area's soil's:
• Visit with farmers and walk through their fields with them. They're the ones most intimately involved with the land and can provide a wealth of useful information on local soils, their behavior, and productivity.
• Arrange for a soils field tour of your area with an agronomist or extension agent.
• Consult soil survey reports or other soil studies on your area. Soil specialists have devised several taxonomy systems to classify soils, first into orders and groups made up of hundreds of soils, progressing down to a very specific series consisting of several closely related soils that share many similar Profile features. (A soil profile is a vertical slice of soil that includes the topsoil, subsoil, and some of the parent material below.) Of the several taxonomy systems, the one developed in the 1970's by the the USDA in cooperation with other countries has become the most widely used. The terms oxisol and ultisol used in Chapter 1 refer to two soil orders in in this system that comprise hundreds of soils formed under tropical and subtropical conditions. (However, not all warm climate soils belong to these two groups, as explained in Chapter 1.)
NOTE: When reading a soil survey report, don't be intimidated by the technicalities and fancy terms. What's most important to farmers and extension workers is how a soil behaves when farmed - not what order or series it belongs to.
HOW TO EVALUATE A SOIL AND DIAGNOSE PHYSICAL PROBLEMS
Using a shovel and a homemade device to measure slope, it's fairly easy to evaluate the 6 major physical characteristics that determine a soil's behavior and management needs:
• WATER-HOLDING CAPACITY
Let's cover them one at a time. But wait a minute, we haven't said anything about SOIL COLOR - where does it fit in?
A soil's color doesn't necessarily provide useful information on its characteristics and yield potential. For example, it's commonly believed that dark colors (especially black) indicate high organic matter content and, therefore, high natural fertility. This is often true in temperate regions like the prairie grasslands of the Great Plains (USA) where there is a direct relationship between soil color and humus content - the blacker the soil, the more humus it contains and the more fertile the soil. However, this correlation isn't universally valid, because soil humus in warmer regions has a more brownish coloration. Also, parent rock itself can make a soil black. In fact, many black soils in the tropics and elsewhere owe their color not to high humus content but to a reaction of the calcium in their limestone parent material with only a small amount of humus.
Distinct red and yellow colors usually indicate very old and weathered soils likely to be acidic and low in natural fertility; their clay portion usually contains a high amount of ''tropical"-type clays (hydrous oxides of iron and aluminum, and 1:1 clays like kaolinite) that are lower in negative charge but less sticky when wet than soils high in "temperate" type clays (see Chapter 1).
Subsoil color is also a valuable indicator of how well drained a soil is as will be explained in the section on drainage in this chapter. Now, on to the 6 mayor "vital signs" of soil physical health.
Texture refers to the relative proportions of sand, silt, and clay in a soil (see Figure 2-1). Note that humus content technically has nothing to do with texture. A soil's texture has a big influence on its productivity and management needs, because it affects filth, waterholding capacity, drainage, erosion potential, and soil fertility.
Texture usually varies with depth: As explained in Chapter 1, the subsoil is usually more clayey than the topsoil.
There are 3 broad soil textural classes: Sandy, Loamy, and Clayey; they are further subdivided as shown in Table 2-1.
TABLE 2-1 TEXTURAL CLASSES OF SOIL
Sandy Loams (CT)
Sandy clays (FT)
Loamy sands (CT)
Fine sandy loam (CT)
Silty clays (FT)
Very fine sandy loam (MT)
Silt loam (MT)
Clay loam (FT)
Sandy clay loam (FT)
Silty clay loam (FT)
* "Coarse-textured" (CT), "medium-textured" (MT), and "fine-textured" (FT) are other adjectives used to describe soil texture. Coarse-textured and fine-textured soils are also referred to as "light" and "heavy" soils respectively.
Checking Out Soil Texture in the Field
For farming and extension work, you don't need to determine the exact percentages of sand, silt, and clay. In fact, just being able to place a soil in one of the 3 broad textural classes (i.e. sands, loams, clays) may be sufficient. However, it's usually helpful to be more specific, and, with the help of Table 2-2, you shouldn't find this difficult. A good way to begin is to first determine whether the soil is sandy, clayey, or loamy, and then fine tune your diagnosis.
THE PROS AND CONS OF SANDY, LOAMY, AND CLAYEY SOILS
• Easily tilled
• Resistant to compaction caused by animal, foot, or machinery traffic.
• Absorb water readily.
• Usually well drained unless the water table is close to the surface as can happen in low areas and depressions.
Disadvantages of Sandy Soils
• Low water-holding capacity (about half that of clay loams and clays); tend to dry out quickly.
• More leaching of plant nutrients due to low negative charge and more downward movement of water (because of lower water holding capacity).
• Tend to be lower in natural fertility (but not always) due to greater leaching and low content of nutrient-bearing clay.
The term "loam" is a bit confusing, because it conveys nothing about sand, silt, and clay content. A loam isn't simply an equal mixture of the three, either. As shown in Figure 2-1, loam soil contains about 45% sand, 40% silt, and 15% clay. That's because it takes much more sand than clay to influence soil behavior. Ideally, a loam combines all the advantages of both sandy and clayey soils without having any of their bad points. A clay loam has enough additional clay clay to exhibit some of the negative features of clay soils, but not enough to be classified as a clay. Likewise, a sandy clay has enough extra sand to have some moderate problems with water-holding capacity and excessive leaching, but not to the extent of a true sandy soil.
• Good water-holding capacity (about twice that of sands).
• Less leaching of plant nutrients, due to higher negative charge and less downward water movement because of higher water-holding capacity). (Remember, however, that "tropical" clays can have a very low negative charge.)
• They tend to be higher in natural fertility than sandy soils, but not always, especially those whose clay minerals are mainly "tropical'' types.
Disadvantages of Clayey Soils
• Harder to till, not only in terms of power required, but also regarding ideal moisture range for tillage. If plowed when too wet, they compact and stick. If worked when too dry they're overly hard and cloddy.
• More prone to Poor drainage, due to slower downward movement of water. (Not a problem on a slope).
• More prone to soil compaction by animal, foot, or machinery traffic.
• Their slow water intake rate encourages excessive runoff on slopes.
• Soils high in clay (or silt) tend to crust over upon drying which can inhibit seedling emergence.
Beware of Overgeneralizing!
Don't interpret the above comparisons of sandy, loamy, and clayey soils too rigidly. Many clayey soils aren't poorly drained or high in natural fertility. Likewise, all sandy soils aren't low in natural fertility. Of the 3 groups, clayey soils are probably the most variable in their traits.
Is Natural Fertility Important?: Probably not as much as it once was, since most farmers have access to chemical or organic fertilizers. Fertility is usually much easier to improve than physical problems like poor drainage, insufficient depth, or excessive clay; however, farmers without enough organic fertilizer may not have funds or credit for purchasing chemical fertilizer.
HOW TO IMPROVE SANDY AND CLAYEY SOILS
NOTE: Refer also to the section on clayey soils in Chapter 4.
• Add organic matter to either: Compost, manure, and green manure crops will greatly benefit these soils (as well as loams). Aside from adding nutrients, they loosen up clayey soils and bind together sandy soils. They also improve the water holding capacity of sands and increase their negative charge. Organics like rice hulls' millet hulls, cottonseed hulls, and peanut hulls (shells) don't add many nutrients but are valuable for loosening up clayey soils. Organic soil conditioners are covered in detail in Chapter 8.
• Try mulching: Covering the soil with a layer of straw, dried grass, or leaves, etc. will help reduce water evaporation losses from sandy soils. Mulching clayey soils will eventually add organic matter and encourage earthworms, both of which will have a loosening effect.
• Reduce animal, foot, and machinery traffic over clayey soil, especially when wet, to help minimize compaction.
• In poorly drained clay soils, plant crops on raised beds or ridges to prevent the plants from getting "wet feet". (Raised beds are covered in Chapter 4).
• Planting on a flat bed or sunken bed is often recommended for sandy soils (and sometimes others) in dry areas or where dry spells are common. (See Chapter 4).
• Add sand to clay or clay to sand: From what you've read, you should be able to interpret this poem:
"Clay to sand is like a bird in hand Sand to clay is like throwing money away"
As the poem implies, you'd have to add a lot more sand to a clayey soil than clay to a sandy soil to modify its behavior. The third line of the jingle might be, "A little bit of clay goes a long way". Check the soil texture bar graphs (Fig. 2-1) again and see how much sand content is needed for a soil to rank as a sandy soil compared to clay for a clayey soil. Whether you try to add sand to clay or vice-versa, this remedy is only likely to be practical on very small plots, especially when adding sand.
Tilth refers to a soil's physical condition. A soil in good filth is easily worked, crumbly, and readily takes in water when dry. A soil in poor filth is hard to work, overly cloddy or loose, and absorbs water slowly when dry.
What influences filth?: Texture, organic matter, and moisture content all play a role.
A soil's filth isn't static: It can vary markedly with changes in soil moisture content, especially on some clayey soils which can be worked only within a very narrow moisture range without being too hard or too sticky.
How to Maintain or Improve Soil Tilth
Improving filth by adding sand to clay is only practical on smaller plots and will still require considerable labor.
Routine additions of organic matter to the soil are very helpful.
Land drainage or the use of raised beds or ridges may help alleviate excessive moisture that's causing poor filth.
Time tillage operations: Under favorable moisture conditions, plowing and hoeing may improve filth by breaking up clods and loosening hard ground. But when done when too wet or too dry, tillage can leave the soil worse off than before.
Don't overdo tillage: Stirring and shearing the soil aerates it which stimulates fungi and bacteria to accelerate the breakdown of valuable humus. Tillage may loosen the topsoil but it often compacts the subsoil, especially when done with tractor or animal-drawn equipment on wet, clayey soils.
Choose crops carefully: Some crops like cotton, peanuts, tobacco, and vegetables require frequent traffic down the rows for spraying and cultivating. Soil filth will suffer and compaction increase unless these crops are rotated with others like grains and forage crops that require less field traffic and return more organic matter to the soil.
How Soils Hold Water
About half a soil's volume is pore apace which is occupied by varying amounts of air and water, depending how wet the soil is.
Water is held in the pore spaces in the form of films adhering to the soil particles.
The smaller pores in the soil are called micropores the larger ones are macropores. Macropores don't hold water well, because the water films become too thick to adhere well to the surrounding soil particles.
All Soil Water isn't Available to Plants
Soils hold water very much like a sponge, so we'll use this analogy to explain some basic soil water principles. Get a sponge and a pail of water, and follow along:
• Dip the sponge in the water and then hold it over the pail. Some of the water is draining fro. the sponge, even though you're not squeezing it. This water is being lost from the macropore spaces in the sponge where the files become too thick to be held against the pull of gravity. It's the same way with soil after a rain or irrigation. Water begins to move out of the macropores downward through the soil. This is drainage water. It moves down until it reaches the water table (where water ponds) or runs into drier soil where it becomes held in the micropores.
• Available vs. unavailable water: The water that remains in the sponge after natural drainage is all held in the micropores. It'- the same with the soil. A soil at this stage is said to be at field capacity (micropores full). Now squeeze the sponge. At first, it's easy to extract water, but then it becomes harder and harder. Again, it's the same with the soil. Only about half the water held at field capacity is actually available to plant roots. As the soil dries out, the water films become thinner and are held increasingly tightly to the soil particles, making it harder for roots to get water. When the permanent wilting point is reached, plants will remain wilted (and may die) unless water is added, even though the soil is far from completely dry. This remaining, unusable water is called unavailable water.
The Difference Between Water-Holding Capacity and Drainage
At first, it might seem contradictory that some soils can have both good water-holding capacity and good drainage, but these characteristics are compatible. That's because drainage takes place only from the macropores, and water-holding capacity resides in the micropores. The films of micropore water are resistant to being drained away by gravity. It's the macropores that allow a soil to retain enough air for the roots as long as drainage isn't impeded.
How Soil Texture and Organic Matter Influence Water-Holding Capacity
Soil texture has the biggest influence on water-holding capacity. As shown by Table 2-3, clays and clay loams have about twice the water-holding ability of sandy soils. That's because, in terms of pore space, clayey soils have a much larger proportion of waterholding micropores compared to sands. Surprisingly, they also have more total pore space, too, which is why dry clay usually weighs less than dry sand, given equal volume.
Humus can help in some cases: Humus will help increase the water-holding capacity of sandy soils but won't help much on clayier soils that already have good water-holding ability.
Water-Holding Capacity and Water Penetration
Figure 2-2 illustrates the important concept that sandy soils are more prone to leaching losses than clayey soils because of their lower water-holding capacity. If lettuce plants with roots 30 cm deep were being grown on the sandy loam and the clay soil in Fig. 2-2, the clay soil could receive almost twice as much water per application as the sandy loam without having leaching losses. It would also require watering only about half as often as the sand. Total amount of water needed per week would be the same for both soils, however.
Drainage refers to the soil's ability to get rid of excess water (water in the macropores) through downward movement by gravity. It is affected by topography, texture, filth, depth, and the presence of pans (compacted or cemented zones). Nearly all major crops need fairly good drainage so that their roots can obtain enough oxygen; some exceptions are rice and most varieties of taro (Colocasia esculenta).
Poorly drained soils adversely affect crop yields in several ways:
• Roots lack adequate oxygen, since the macropores are largely filled with water.
• Soil-borne fungal and bacterial diseases are encouraged.
• Nitrate nitrogen (a nutrient) is subject to loss by a process called denitrification (see Chapter 6).
• Manganese and iron may become soluble enough to injure plant roots.
Although clayey soils are more likely to have drainage problems, they also may occur on sandy soils in cases where the water table is close to the soil surface. (The water table is the upper surface of the ground water, below which the soil is completely saturated with water.)
How to Spot Drainage Problems
You and farmers can easily spot areas of poor drainage in a field. Here's what to look for:
Topography: Poor drainage is most likely to occur on level fields or in low spots where water tends to collect after a rain or irrigation. Soils with even a gentle slope seldom have drainage problems but are likely to have the opposite problem of excessive water runoff.
Presence of Hardpans or Claypans: A hardpan is a hardened, cemented layer a few centimeters thick, usually located in the lower topsoil or upper subsoil. It remains hard even when wet, and restricts drainage and root growth. A claypan is a thicker, dense clayey layer in the subsoil which will soften somewhat when wet. It still impedes drainage and root growth. Dig a pit to check for such pans.
Crop Appearance: Crops growing in poorly drained areas will be stunted and yellow compared to surrounding portions. Beware, though, that other conditions such as nitrogen deficiency and disease can produce these symptoms. Suspect poor drainage only when stunting and yellowing are associated with low spots or areas of standing water.
Standing Water: Any area where water ponds for a day or two after rainfall or irrigation is likely to be poorly drained.
Subsoil Color: Red, reddish brown, or Yellow subsoil colors indicate very good drainage. That's because the presence of sufficient air allows the soil's iron and manganese to remain in the oxidized form, indicated by bright colors. Dull greys and blues indicate a reduced state (little oxygen) which means poor drainage. Some soils in wet-dry climates have subsoils with alternate streaks of bright and dull colors. This color pattern is called mottling and indicates fluctuations in soil drainage (i.e. good in the dry season, poor in the wet season) caused by the seasonal variation in the height of the water table.
How to Test Soil for Poor Drainage
The hole test: Dig a hole 60-90 cm deep and fill it with water; allow it to drain, and refill it again. In a well drained soil, the water level should fall by 2-3 cm an hour and disappear in 24 hours. However, if poor drainage is being caused by a hardpan or claypan, this test won't be valid, as you will have overcome the problem by digging through them.
Checking for a high water table: Poor drainage in low spots is often caused by a high water table. Ideally, the water table should be at least 100 cm below the soil surface, at least during the cropping season. When digging, you can easily tell when the water table has been reached, since water will begin to pond in the hole.
NOTE: In some cases, a high water table can actually benefit crop growth by supplying water to the roots during long dry spells by upward capillary movement (as long as it's not high enough to affect drainage in the major root zone area). However, there's always a risk of poor drainage in wet years on such land.
Dealing with Drainage Problems
First, determine what is causing the drainage problem before deciding which of the methods below will be effective.
• Seedbed styles for poorly drained areas: Growing crops on raised beds or ridges can alleviate drainage problems that aren't serious. See Chapter 4.
• Breaking up pans: This can be done with digging hoes and picks or with tractor-drawn sub-soilers (narrow shanks that penetrate 40-50 cm deep). Hardpans can sometimes be permanently fractured and loosened. Claypans, however, tend to reconsolidate after being loosened, especially since they're usually moist and don't tend to fracture. On small plots, the best way to permanently loosen a claypan is by double-digging the soil and adding an organic soil conditioner to the pan area. (Double-digging is covered in Chapter 4.)
• Drainage ditches for surface water: These are shallow, wide ditches that follow the natural depressions in the field to conduct water away. Make sure the outlet is satisfactory, so one farmer's drainage problems won't be passed on to another.
• Drainage ditches for subsurface water: Shallow ditches remove only surface water. To remove excess subsurface water, deeper and more numerous ditches can be used. They will "pull" (attract) this excess water from the soil between them. These ditches are usually spaced about 15-45 meters apart, depending on the soil (the closer spacings for clayey soils) and are dug 30-60 cm deep. Top widths range from about 2-5 meters and bottom widths about 1.5-2 meters. Ditches with V-shaped sides permit the passage of farm machinery. Of course, the ditches must be designed to convey the water off the field and eventually into a natural drainage way such as a stream.
• Drainage tile or Plastic pipe: Drain pipe can be rayed 80-100 cm underground to drain saturated subsoils and conduct the water off the field. Short sections (30-40 cm long) of 10-12 cm diameter clay pipe can be laid end to end in a trench and covered with straw, building paper, or earth to facilitate water entry and retard plugging. Flexible, perforated plastic pipe may also be available for this purpose in your country. The pipe or tile are laid at a slight slope (about 25-50 cm per 100 meters of length) and lead to an outlet such as a ditch or canal. The distance between the tile or pipe lines varies from about 10-20 meters on clayey soils to 30-90 meters on sandy soils. If the land has a natural drainage way, running such an underground drainage line along this path can speed up the removal of water from these areas of accumulation.
• Land leveling will fill in depressions and lower high spots, although the high spots may end up losing lots of topsoil. Animal-drawn scrapers can be fabricated locally.
Seedboxes have special drainage problems: See Chapter 4.
Soil depth refers to the depth of the topsoil plus subsoil and can be easily determined with a shovel. Soils can be classified as being deep or shallow as follows:
Depth (Topsoil + Subsoil)
90 cm +
Less than 25 cm
Actual vs. Usable Depth: There's often a big difference between actual depth and usable depth, because the factors listed below can also limit root penetration:
• Excessive subsoil compaction.
• Hardpans and claypans ( explained in the drainage section).
• Poor drainage.
• Excessive subsoil acidity (very low pH).
• Potential rooting depth of the crop itself; some are naturally much deeper-rooted than others. (See Table 5-1 in Chapter 5 on water management.)
• Overly shallow watering can restrict depth of roots, since they will not grow into dry soil.
The Value of Deep Rooting
Deep rooting isn't necessarily essential for good crop yields. Some shallow soils can produce excellent yields if well managed. However, there are benefits to encouraging deep rooting:
• Better drought tolerance.
• Better nutrient uptake since the roots explore more soil.
• In irrigated crops, deeper rooting allows more water to be applied per application and more time between waterings. This can be very helpful in areas where farmers use furrow irrigation and receive water from the main ditch on an erratic schedule.
How to Encourage Deeper Rooting
• Use raised beds or ridges since they actually increase soil depth and provide a double layer of topsoil. However, they're not suited to dry conditions, because they dry out too fast.
• Double-digging will help encourage root growth into previously uninviting subsoil.
• Avoid overly shallow watering; this is most likely to occur on clayey soils because of their high water-holding capacity.
• Fertilizer use will stimulate deeper rooting.
A field's slope has a marked influence on the amount of water runoff and soil erosion caused by flowing water. Slope is usually measured in terms of percent. A 10 percent slope has 10 meters vertical drop per 100 meters horizontal distance (or 10 ft. per 100 ft.). A 100 percent slope equals 45 degrees. Soil conservation measures become necessary on land with as little slope as 1-2 percent.
You can measure slope with a homemade device. This is covered in Chapter 3 on soil conservation.