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.)