BUILDING SOIL FOR BETTER CROPS

Part 2

When soil organisms and roots go about their normal functions of getting energy for growth from organic molecules they “respire” — using oxygen and releasing carbon dioxide to the atmosphere. (Of course, as we take our essential breaths of air, we do the same.) An entire field can be viewed as breathing as if it is one large organism. The soil is like an organism in another way too — a field also may get “sick” in the sense that it becomes incapable of supporting healthy plants.

The organisms living in the soil, both large and small, play a significant role in maintaining a healthy soil system and healthy plants. One of the main reasons we are interested in these organisms

is because of their role in breaking down organic residues and incorporating them into the soil. Soil organisms influence every aspect of decomposition and nutrient availability. As organic materials are decomposed, nutrients become available to plants, humus is produced, soil aggregates are formed, channels are created for water infiltration and better aeration, and those residues originally on the surface are brought deeper into the soil.

We classify soil organisms in several different ways. Each organism can be discussed separately or all organisms that do the same types of things can be discussed as a group. We also

can look at soil organisms according to their role in the decomposition of organic materials. For example, organisms that use fresh residues as their source of food are called primary (1°), or first-level, consumers of organic materials. Many of these primary consumers break down large pieces of residues into smaller fragments. Secondary (2°) consumers are organisms that feed on the primary consumers them14 selves or their waste products. Tertiary (3°) consumer then feed on the secondary consumers.

Another way to treat organisms is by general size, such as very small, small, medium, large,

and very large. This is how we will discuss soil organisms in this chapter. There is constant interaction among the organisms living in the soil. Some organisms help other organisms, as when bacteria that live inside the earthworm’s digestive system help decompose organic matter. Although there are

many examples of such mutually beneficial symbiotic relationships, an intense competition occurs among most of the diverse organisms in healthy soils. Organisms may directly compete

with each other for the same food. Some organisms naturally feed on others — nematodes may

feed on fungi, bacteria, or other nematodes, and some fungi trap and kill nematodes.

Some soil organisms can harm plants either by causing disease or by being parasites. In other

words, there are “good” as well as “bad” bacteria, fungi, nematodes, and insects. One of the goals of agricultural production systems should be to create conditions that enhance the growth of beneficial organisms, which are the vast majority, while decreasing populations of those few

that are potentially harmful.

SOIL MICROORGANISMS

Microorganisms are very small forms of life that can sometimes live as single cells, although many also form colonies of cells. A microscope is usually needed to see individual cells of these organisms. Many more microorganisms exist in topsoil, where food sources are plentiful, than in subsoil. They are especially abundant immediately next to plant roots, where sloughed off cells and chemicals released by roots provide ready food sources. These organisms are important primary decomposers of organic matter, but they do other things, such as providing nitrogen through fixation to help growing plants. Soil microorganisms have had another direct importance for humans — they are the origin of most of the antibiotic medicines we use to fight various diseases.

Bacteria

Bacteria live in almost any habitat. They are found inside the digestive system of animals, in the ocean and fresh water, in compost piles (even at temperatures over 130°F), and in soils. They are very plentiful in soils; a single teaspoon of topsoil may contain more than 50 million bacteria.

Although some kinds of bacteria live in flooded soils without oxygen, most require wellaerated soils. In general, bacteria tend to do better in neutral soils than in acid soils. In addition to being among the first organisms to begin decomposing residues in the soil,

bacteria benefit plants by increasing nutrient availability. For example, many bacteria dissolve phosphorus, making it more available for plants to use. Bacteria are also very helpful in providing nitrogen to plants. Although nitrogen is needed in large amounts by plants, it is often deficient in agricultural soils. You may wonder how soils can be deficient in nitrogen when we are surrounded

by it — 78 percent of the air we breathe is composed of nitrogen gas. Yet plants as well as animals face the dilemma of the Ancient Mariner, who was adrift at sea without fresh water: “Water, water, everywhere nor any drop to drink.” Unfortunately, neither animals nor plants can use nitrogen gas (N2) for their nutrition. However, some types of bacteria are able to take nitrogen gas from the atmosphere and convert it into a form that plants can use to make amino acids and proteins. This conversion process is known as nitrogen fixation.

BUILDING SOIL FOR BETTER CROPS

Part 1

Farmers sometimes use the term soil health to describe the condition of the soil. Scientists usually use the term soil quality, but both refer to the same idea — how good is the soil in its role of supporting the growth of high yielding, healthy crops?

How would you know a high quality soil from a lower quality soil? Most farmers or gardeners would say that they know one when they see one. Farmers can certainly tell you which of the soils on their farms are of low, medium, or high quality. They know high quality soil because it generates higher yields with less effort. Less rainwater runs off and fewer visible signs of erosion are seen on the better quality soils. Less power is needed to operate machinery on a healthy soil than on poorer, compacted soils. Soil scientists are working together with farmers and agricultural extension personnel to try to come up with a widely accepted definition of soil health and to determine what factors (pH, bulk density, aggregate stability, etc.) need to be measured to estimate a soil’s quality. The first thing many might think of is that the soil should have a sufficient supply of nutrients throughout the growing season. But don’t forget, at the end of the season there shouldn’t be too much nitrogen and phosphorus left in highly soluble forms or enriching the soil’s surface.

Leaching and runoff of nutrients are most likely to occur after crops are harvested and before the following year’s crops are well established. We also want the soil to have good tilth so that plant roots can fully develop with the least amount of effort. A soil with good tilth is more spongy and less compact than a soil with poor tilth. A soil that has a favorable and stable soil

For soil thou art…

—BOOK OF GENESIS

Structure also promotes rainfall infiltration and water storage for plants to use later. For good root growth and drainage, we also want a soil with sufficient depth before there’s a restricting layer. We want a soil to be well drained, so it dries enough to permit timely field operations. Also, it’s essential that oxygen is able to reach the root zone to promote optimal root health — and that happens best in a soil without a drainage problem. (Keep in mind that these general characteristics do not hold for all crops. For example, flooded soils are important for paddy rice production.)

We want the soil to have low populations of plant disease and parasitic organisms so plants grow better. Certainly, there should also be a low weed pressure, especially of aggressive and hard-to-control weeds. Most soil organisms are beneficial and we certainly want high amounts of organisms that help plant growth, such as earthworms and many bacteria and fungi.

A high quality soil is free of chemicals that might harm the plant. These can occur naturally,

such as soluble aluminum in very acid soils or excess salts in arid region soils. Potentially harmful chemicals also are introduced by human activity, such as fuel oil spills or application of sewage sludge with high concentrations of toxic elements. A high quality soil should resist being degraded. It also should be resilient, recovering quickly after unfavorable changes like compaction.

THE NATURE AND NURTURE

OF SOILS

Some soils are exceptionally good for growing crops and others are inherently unsuitable; most are in between. Many soils also have limitations, such as low organic matter content, texture extremes (coarse sand or heavy clay), poor drainage, and layers that restrict root growth. Iowa’s loess-derived prairie soils are naturally blessed with a combination of silt loam texture and high organic matter contents. By every standard for assessing soil health, these soils — in their virgin state — would rate very high. We can compare them with a person who is naturally very healthy and has great athletic abilities. Many of us are not quite so lucky and Nature has given us qualities that may never make us great baseball players, swimmers, or marathon runners, even if we tried very hard. The way we care for, or nurture, a soil modifies its inherent nature. A good soil can be abused through years of poor management and turn into one with poor health, although it generally takes a lot of mistreatment to reach that point. On the other hand, an innately challenging soil may be very “unforgiving” of poor management and quickly become even worse. For example, a heavy clay loam soil can be easily compacted and turn into a dense mass. Both the naturally good and poor soils can be pro ductive if they are managed well. However, they will probably never reach parity, because some limitations simply cannot be completely overcome. The key idea, however, is the same that we wish for our children — we want our soils to reach their fullest potential.

Copy from : Building Soils for Better Crops 2ND EDITION --Fred Magdoff and Haroldvan Es---