Chapter 1: Down to earth - Some Important Soil Basics

What is soil, anyway?

Most soils evolve slowly over centuries through the weathering of underlying rock and the decomposition of plants. Others are formed from deposits laid down by rivers and seas (alluvial soils) or by wind (loess soils).

Soils have 4 basic components:

• Mineral particles: sand, silt, and clay

• Organic matter

• Water

• Air

A sample of typical topsoil contains about 50 percent pore space filled with varying proportions of air and water, depending on the soil's current moisture content. The other 50 percent of the volume is made up of mineral particles (sand, silt, clay) and organic matter; most mineral soils range from 2-6 percent organic matter by weight in the topsoil. Organic soils like peats are formed in marshes, bogs, and swamps, and contain 30-100 percent organic matter.

Why do soils vary so much?

Your host country is likely to have dozens of different kinds of soils. In fact, even a small farm often has 2 or more types that vary markedly in their management problems and yield potential. The reason is that there are 6 soil-forming factors that determine the type of soil that develops in a particular spot:

• Climate: The higher the rainfall and temperature, the more rapid and complete the weathering process is. For example, the speed of chemical weathering reactions doubles for every 10°C (18°F) rise in temperature.

• Type of Parent Material: Soils are formed from a wide variety of parent material including many types of rock, vegetation, and animal life (e.g. the soil of Pacific atolls is formed largely from coral). Rock varies a lot in its mineral makeup and other qualities. Some rocks like granite and sandstone are acid and tend to form more acidic soils than basic rocks like limestone and basalt.

• Vegetation: Soils formed under grassland differ from those formed under forest, although there are also variations within these 2 groups. For example, soils formed under pine trees tend to be more acidic than those formed under other tree species.

• Topography exerts a big influence on erosion and drainage (the relative amounts of water and air in the soil pore space). In the tropics, red and yellow soils usually form on land with some slope since they need good drainage for their genesis. Black and grey soils are most common in depressions where drainage isn't as good.

• Time: Soils change over time as they weather, a process that takes place over thousands of years. Soils vary in age a lot.

• Farmer Management: Farming practices like land clearing, tillage, and cropping influence soil development by affecting erosion, pH, and organic matter, etc.

Topsoil vs. subsoil

Dig down about 50 cm in most soils and you will have exposed 2 distinct layers: the topsoil and part of the subsoil. The topsoil is the uppermost layer and has these features:

• It's usually darker in color than the subsoil since it contains more organic matter from decaying plants and their roots.

• It's more fertile than subsoil, due to having more organic matter and because fertilizers are usually added to the topsoil only.

• It's usually looser and less compacted than the subsoil, mainly due to its higher organic matter content and to plowing (or hoeing).

• The topsoil is usually about 15-25 cm thick. On cultivated soils, topsoil depth is about equal to tillage depth since this determines how deep organic matter and fertilizers are worked into the soil.

• About 60-80% of the roots of most crops are found in the topsoil since it's a better environment for root growth than the subsoil (i.e. more fertile, less compact).

The subsoil is located between the topsoil and the parent rock (or material) below. Aside from being lighter in color, less fertile, and more compact, it's usually more clayey; that's because downward water movement has transported some of the tiny clay particles from the topsoil into the subsoil.

The role of subsoil: It would seem that we could dismiss subsoil as not having much influence on crop growth. However, this isn't so for 2 good reasons:

• Subsoil is an important storehouse of moisture, especially since it's usually much thicker than the topsoil, and the moisture isn't lost as easily by evaporation. The higher clay content of subsoils makes for higher water holding capacity, too. This moisture reserve is very useful during dry spells, even though there are fewer roots in the subsoil. For example, it's estimated that half the moisture needed to grow a maize crop in the U.S. Corn Belt is already stored in the subsoil at planting time; rainfall during the crop's growth provides the rest but would fall far short by itself to produce good yields.

• Subsoil characteristics like clay content and compaction have a big influence on drainage (the ability to get rid of excess water).

Making Topsoil out of Subsoil: If little topsoil remains due to erosion, you can convert subsoil into productive topsoil. All it takes is hefty additions of organic matter like compost, manure,or green manure (see Chapter 8 on organic fertilizers) for a few years, but this isn't often feasible on large plots.

The mineral side of soil: sand, silt, and clay

The mineral part of soil is composed of varying amounts of sand, silt, and clay. Their characteristics have a big influence on soil behavior and management needs.

Sand

• Of the 3 kinds of mineral particles, sand is the largest in size; about 50 sand particles laid side by side would equal 1 centimeter (125 per inch).

• Sand is mainly quartz (silicon dioxide) and contains few plant nutrients.

• Moderate amounts improve soil drainage, aeration, and filth (workability).

Silt

• It consists mainly of ground up sand particles (quartz), which are often coated with clay.

• It contains few nutrients in itself except those that might be in the clay coating.

• Silt particles are too small to help improve drainage and aeration.

Clay

Clay particles are the smallest of the 3 (about 4000 of them laid side by side would equal 1 cm). Farmers know that clay has a big influence on soil behavior. High clay content usually makes for harder plowing, more compaction, and poorer drainage, but it does assure good water-holding capacity. Aside from this, clays have 3 other important features:

• Source of plant nutrients: Unlike sand and silt, clays are aluminum-silicate minerals that also have varying amounts of plant nutrients such as potassium, calcium, magnesium, and iron, etc. A good part of a soil's native fertility can come from its clay portion.

• Clays have a negative charge: This makes them act like tiny magnets to attract and hold those plant nutrients that have a positive (+) charge like potassium (K+), calcium (Ca++), magnesium (Mg++), and the ammonium form of nitrogen (NH.+). This helps greatly to keep these nutrients from being carried downward beyond the root zone by rainfall or irrigation. (The term leaching is used to describe this type of loss.)

• Tremendous surface area: Each clay particle is really a laminated structure consisting of tiny plates. This lattice arrangement plus small particle size gives clays an amazing amount of surface area for attracting and holding positively-charged nutrients. In fact, one cubic centimeter of clay particles contains about 1-3 square meters of surface area.

All Clay isn't the Same

There are several different types of clay, and most soils contain at least two. Understanding some basics about clay types will help you interpret the soils in your work area. It's important to understand the difference between temperate clays and tropical clays and why both types are found in the tropics.

• Temperate clays: These are 2:1 silicate clays such as montmorillinite and illite that dominate the clay portion of most temperate zone soils but may also be found in the tropics. The 2:1 figure refers to the ratio of silicate to aluminum plates in a clay particle's laminated structure. Soils with a good amount of these temperate clays are very sticky and plastic when wet; some kinds such as montmorillinite shrink and swell readily, forming large cracks upon drying out. They also have a relatively high negative charge (good for holding positively-charged nutrients>.

• Tropical clays: These are 1:1 silicate clays, such as kaolinite, and hydrous oxide clays of iron and aluminum that often make up most of the clay portion of old, well drained soils in the tropics and subtropics, mainly in areas with at least 6 months of rainfall. These clays have lost lots of silicate due to centuries of weathering and leaching. Unlike the 2:1 clays, these "tropical" clays are much less sticky and plastic and are easier to work with, even when clay content is high. However, they usually have much less negative charge and lower natural fertility than temperate clays. Soils whose clay portion is largely "tropical" can usually be identified by their red or yellow colors.

Distinguishing "tropical" soils from "temperate" soils

Note that "tropical" clays don't necessarily make up the major portion of the clay in all soils of the tropics. In fact, temperate clays are surprisingly common, especially in younger soils or those formed under drier conditions or where drainage isn't good. A true tropical soil (one whose clays are mainly I:1 or hydrous oxides) requires good drainage, centuries of weathering, and lots of rainfall and leaching to form. Iikewise, not all clays in the temperate zone are 2:1 clays, especially in areas that may have once been tropical thousands of years ago. Some soils are mixes of both types.

Spotting "tropical" soils: A distinct red or yellow color, especially in the subsoil may be one indication. Such soils are unlikely to form in depressions but are found on gentle to steep slopes where drainage is good.

The extent of tropical soils in the tropics: Overall, true tropical soils account for about half the soils in the tropics and often exist side by side with "temperate" ones. They're fairly diverse themselves and are grouped into 2 broad categories based on the current USDA (U.S. Dept. of Agric.) soil classification system:

• Ultisols: Their clays are mainly 1:1 types, along with varying amounts of hydrous oxides of iron and aluminum, and their workability is usually goad. They are moderately to very acidic and may have a high capacity to "tie up" added phosphorus, preventing its full use by plants.

• Oxisols: The most strongly weathered and leached of all soils. They're acidic and have high clay contents (mainly of hydrous oxides), but don't tend to be very sticky when wet. Like ultisols, they may tie up added phosphorus readily. One well known member of this order (group) are laterite soils whose subsoils are rich in a clayey material called plinthite that contains red mottles (blotches) and highly weathered oxides of iron and aluminum. Plinthite can harden irreversibly into ironstone (formerly called laterite) when exposed by erosion, as has occured following deforestation. Note that true laterite soils at risk of ironstone fomation are estimated to amount to less than 10% of all tropical soils.

Organic matter - a soil's best friend

Most cultivated soils contain about 2-4 percent organic matter by weight in the topsoil. Despite its small proportion, organic matter has a remarkably beneficial effect on soil behavior and crop yields, especially in the form of humus (partially decomposed organic matter that has become dark and crumbly; humus continues decomposing, but at a slower rate). Humus benefits the soil in many ways:

• It can greatly improve overall soil physical condition (filth), especially on clayey soils.

• Humus helps reduce soil erosion by wind and water, because it acts as a helpful "glue" to bind soil particles together into "crumbs" (called aggregates) that improve water intake rates and lessen runoff. Such "crumbs" are also more resistant to being moved by wind or flowing water.

• It's an important storehouse and supplier of nutrients (especially nitrogen, phosphorus, and sulfur) which are slowly released for use by plant roots as organic matter decomposes. Estimates are that for each 1 percent organic matter in the topsoil, 600 kg/ha of maize can be produced without additional fertilizer.

• It increases the water-holding capacity of sandy soils (but not clay loams and clays whose water-holding capacity is already high).

• Humus has a high negative charge that helps prevent plus-charged nutrients from leaching. Per equal weight, humus has up to 30-40 times the negative charge of the lesser charged clays (i.e. tropical clays) and can account for the major part of a soil's nutrientholding ability. In addition, negative charge improves a soil's buffering capacity (the ability to resist changes in pH; see Chapter 6).

• It helps prevent Phosphorus and other nutrients from being "tied up" by the soil (i.e. being made unavailable to plants; see Chapter 6).

• Recent research has confirmed the observations of many organic gardeners and farmers that a high soil organic matter level can reduce the incidence of some soil-borne diseases and root-attacking nematodes. It also stimulates the growth of beneficial soil bacteria, fungi, and earthworms.

Organic Matter Does Wonders for Soil. BUT It's Hard to Maintain

Although forest or grassland soils have very healthy levels of organic matter (6-9 percent) in their untouched state, such levels can quickly decline once the land is cleared and put into crop production for several reasons:

• If the land is cleared by burning, much organic matter is destroyed.

• Plowing and hoeing aerate the soil, which stimulates soil microorganisms to speed up the breakdown of organic matter. Although this speeds up the release of nutrients from the organic matter, it can also result in a drastic decline in soil humus unless large, routine additions of organic matter are made.

• Forests and grasslands recycle huge amounts of organic matter back to the soil by leaf fall and root decay, but most crops (especially annual row crops like maize and peanuts) can't even come close to matching this. Row crops also expose the soil to higher temperatures which speed up the loss of organic matter. That's one reason why soil fertility and yields rapidly decline in 2-3 years under shifting cultivation (slash-and-burn agriculture).

Maintaining or Increasing Soil Organic Matter

Except on small plots, maintaining or increasing soil organic matter isn't likely to be easy for 2 reasons:

• It takes a huge amount organic matter to raise a soil's humus level by even one percentage point (i.e. from 3 percent to 4 percent). Each 1 percent of organic matter equals about 22,000 kg/ha (2.2 kg/m2).

• Soil organic matter is lost more quickly in the tropics, due to higher temperatures; breakdown occurs about 3 times as fast at 32°C (90°F) as at 16°C (61°F).

In an experiment in New York, adding 56,000 kg/ha of stable manure per year for 25 years raised the topsoil's organic matter level by only 2 percentage points!

On the bright side: The good news is that you don't have to increase the percentage of organic matter in a soil in order to improve it. Why? Because when new additions of organic matter are made, the decomposition process releases compounds that provide many of the benefits listed above. You can probably raise organic matter levels on small plots, but on large areas it's more realistic and almost as beneficial to make routine additions of organic matter to keep the breakdown process active and help stabilize organic matter levels.

Some Suggestions for Encouraging a Healthy Turnover of Soil Organic Matter

• Return all crop residues to the soil except in the case of special insect and disease problems. It's OK if livestock feed on crop residues, as long as the manure is returned to the land (see Chapter 8).

• Don't prepare land by burning if there's a feasible alternative.

• Use compost, manure, and green manure crops wherever practical (these are covered in Chapter 8).

• Limit tillage operations like plowing, disking, and hoeing to the minimum needed for adequate seedbed preparation and weed control.

• Rotate low-residue crops like vegetables and cotton with higher-residue crops like maize and especially forage crops such as grasses and legumes.

• If liming is needed to correct excessive soil acidity, avoid excessive applications, because they accelerate the breakdown of organic matter by soil microbes. Avoid liming a soil to a pH above 6.5. (see Chapter 11.)

The role of soil microorganisms

The soil is a thriving biological laboratory, and a teaspoonful easily contains a billion microorganisms such as fungi and bacteria. Some cause plant diseases, but most are beneficial to agriculture. Some examples:

• Humus production: Many kinds of soil bacteria and fungi decompose organic matter into crumbly humus that does all those great things for the soil. The compounds produced by decomposition are also beneficial.

• Release of plant nutrients from organic matter: Most of the nitrogen, phosphorus, and sulfur in fresh plant residues is tied up in the unavailable organic form which plants can't use. Soil microbes change these tied-up nutrients into available inorganic (mineral) forms which plants can use.

• Mycorrhizae are a kind of mushroom fungi commonly found in most soils and infest the roots of many plants and trees. They cause no harm but actually enhance the host's uptake of plant nutrients, especially phosphorus (P); they also improve water uptake, lessen the toxicity of salinity or excess aluminum, and stimulate the growth of other beneficial microbes like rhizobia. They may even secrete growth-promoting hormones. In return, the plant provides the fungi with simple sugars for food. It's believed that mycorrhizae play a particularly important role in aiding the P uptake in some crops like sweetpotatoes and cassava (manioc) which seem to tolerate soils with low levels of available P. In the case of sterilized field or greenhouse soils that lack the fungi, considerable savings in phosphorus fertilizer have sometimes been obtained by innoculating them with a mycorrhizae culture, notably in the case of citrus nurseries. Topsoil from a disease-free, actively growing organic garden is likely to contain an especially good population of the fungi, and a few shovelfuls can be transferred to a new plot to encourage development. (However, mycorrhizae do not colonize the roots of beets, spinach, chard, and brassica [crucifer] family plants such as cabbage, broccoli, radish, turnip, and pak choy.)

• Nitrogen fixation by rhizobia: Several kinds of bacteria "fix" (capture) nitrogen from the air and convert it to a form that plants can use. The most important type are rhizobia bacteria (of the genus Rhizobium) that live in small nodules on the roots of legumes. (Legumes are plants that produce their seed in pods such as beans, peas, and peanuts.) The rhizobia have a symbiotic (mutually beneficial) relationship with legumes. The bacteria live off sugars provided by the plant and supply their host with nitrogen. Some legumes such as cowpeas, peanuts, mungbeans, soybeans, and pasture legumes like clovers receive all the N they need from the rhizobia if the right strain is present.

• Other kinds of N fixation:

•• Blue-green algae (cyanobacteria) inhabit flooded rice soils and fix N. Free-living types (i.e. those requiring no host) fix modest amounts of N, and farmers in Egypt, India, and Burma purposely inoculate their rice paddies with these algae.

•• The Azolla plant is a low-growin8, aquatic fern which harbors a type of N-fixing, bluegreen algae (Anahaena azollae) in its leaves. Azolla has been used as a green manure and also intercropped (grown in combination) with flooded rice for centuries in China and Viet Nam and can supply considerable N to the rice plants. (For more information on Azolla, refer to the section on rice in Chapter 10.)

•• Azotobacter are free-living, N-fixing bacteria commonly found in unflooded soils of warm areas.

•• Casuarinas are pine-like trees used for firewood, soil stabilization, and windbreaks in warm climates. Although not a legume, they do fix N, thanks to an association with an Actinomycete bacteria of the genus Frankia.