WW-07399     2000  

THE SOIL SCIENTIST

This section gives background information about how soil works. Understanding the basics will help you apply management recommendations found in this and other publications in this series.

What Is Soil Made Of?

It takes thousands of years for rock to develop into soil, and hundreds of years for rich organic layers to build up. Soil is made of air, water, mineral particles, organic matter, and organisms. Half of soil is pore space. Generally, pores are about half filled with water and half air, though the proportion varies greatly depending on weather, plant water use, and soil texture. Most of the solid portion of soil is mineral particles. Organic matter may make up only 5% to 10% of the volume of soil (less than 5% of the weight), but it is critical in holding soil particles together, storing nutrients, and feeding soil organisms.

Mineral particles are divided into three groups based on their size: clay, silt, and sand. Soil texture depends on the proportion of particles from each of these groups. (See the texture triangle below.) For example, a loam has similar proportions of all three classes of particles. A sandy loam is higher in sand; a clay loam is higher in clay.

Soil structure, or how soil is put together, can be as important as what it is made of. Most soil particles are held together in aggregates of many particles. The size and stability of these aggregates determine the size of pores. Soil texture is difficult to change, but farming does impact soil structure.

Soil Texture Triangle Particle diameters: clay: <0.002mm silt: 0.002mm to .05mm sand: 0.05mm to 2mm

How Does Soil Work?

Regardless of the kind of farming you do, the rules of soil chemistry, physics, and ecology are the same. Four processes make your soil what it is: formation of soil structure, nutrient cycles, water cycles, and life cycles of soil organisms.

1. Soil structure

Soil particles range in size from gritty sand particles as large as 2 mm (1/16 inch) to microscopic clay particles 1000 times smaller. These particles rarely exist separately in the soil. They normally combine into clumps called aggregates or peds. A few particles bind into tiny microaggregates. Microaggregates, in turn, combine to form larger aggregates. Ideally, soil will have a wide range of aggregate sizes and pore sizes. Aggregates in healthy soil will be stable and resist breakage when tilled, hit by rain, or otherwise disturbed.

How soil structure develops

How do aggregates form? Several biological, physical, and chemical processes interact to form aggregates and then stabilize them. Microbes decomposing organic matter create compounds that cement soil particles together. Synthetic polymers ("soil conditioners") and sugars excreted from roots can have a similar cementing effect. Humus - the stable organic compounds - also binds soil particles together.

Fungal hyphae and fine roots surround and stablize aggregates. This is why surface residue and other organic matter improve soil structure - the residue is food for fungi and bacteria that in turn help form and stabilize soil aggregates.

Larger organisms (such as insects and earthworms), enhance soil structure when they burrow through the soil and deposit fecal pellets that become stable soil aggregates.

Physical and chemical processes are also important to the formation of soil aggregates - especially smaller aggregates. Particles are physically pushed closer together by freezing and thawing, wetting and drying, and roots pushing through the soil.

At the molecular level, electrochemical charges bond clay particles together. In arid areas, this process may be disrupted by large amounts of sodium (Na+) ions. In tropical and semitropical soils, iron oxides can cement together aggregates.

How do surface crusts form? Rain drops break apart soil aggregates and then fine clay particles clog the spaces between aggregates and form a crust on the soil.

Improving soil structure

Several of our six soil-friendly practices impact soil structure:

Organic matter management. Regular additions feed the organisms that build soil structure.

Tillage practices. Over time, tillage increases the decomposition of soil organic matter and breaks up aggregates - especially when tillage is done in wet soil. Residue left at the surface protects surface aggregates from rain and encourages the growth of fungi that help stabilize aggregates.

Compaction prevention. Compaction pushes aggregates together and eventually breaks them down.

Crop choices. The dense roots of grasses, small grains, and pastures stabilize soil aggregates and improve structure.

2. Nutrient cycles

Soil is the storehouse for the nutrients that plants need. It is a dynamic environment in which soil organisms, chemistry, and physics are continually acting to change the form of plant nutrients.

Plant roots draw most of the nutrients they need from the soil solution - the water and dissolved minerals in soil pores. The amount of nutrients in the soil solution at any one time is just a fraction of that needed by plants over the course of the year. The soil solution must be continually replenished from the store of nutrients in minerals and organic matter. Many farmers also contribute nutrients to the soil solution by adding readily available fertilizers once or more each year.

Soil particles and plant residue are made of large quantities of nutrients that are unavailable to plants. Eventually, the soil (mineral) particles weather into sand, silt, clay molecules, or mineral ions. Plant residue eventually decomposes into mineral ions or reforms into humus.

Plant nutrients exist in several forms. Adapted from Brady and Weil, 1996, p. 22.

Clay and humus are not absorbed by plants, but they hold nutrients (mineral ions) on their surfaces. The amount of places available on clay and humus to hold nutrients is called the "exchange capacity" of the soil.

Cation exchange capacity is the number of places available - the storage capacity - for positively charged ions, including calcium, magnesium, sodium, and potassium ions (Ca2+, Mg2+, Na+, and K+). Nutrient ions attached to the exchange sites on clay and humus are released into the soil solution for use by plants and soil organisms such as bacteria and fungi.

As plants draw nutrients out of the soil solution, more may be released into solution from exchange sites on clay and humus. Fertilizers added to the soil solution do not remain unchanged, waiting to be used by plants. Like minerals from other sources, they become attached to exchange sites, are used by microorganisms, and are transformed.

Roots get nutrients in three ways: 1) the root grows into an area where the soil solution has not been depleted of nutrients; 2) after a root depletes the nutrients near it, nutrients will diffuse into the deficient area; and 3) water flows towards the root and carries nutrients. This means that prolific root growth and adequate water are essential to plant nutrition.

What is soil fertility?

Where is the most fertile land on your farm? What makes it more productive than other fields? The soil may test high in nutrients, but it probably has additional characteristics that make it your best land. Soil fertility is not just the amount of nutrients, but whether plants can get the nutrients when they need them. In other words, a fertile soil will have:

• Good rooting environment. To grow and find nutrients, roots (and mycorrhizal fungi) need well-drained soil with a crumbly, uncompacted structure.
• Adequate water. Soil with good "tilth" will have good water infiltration and water-holding capacity.
• High organic matter. Organic matter is a source of many nutrients, improves the rooting environment, and helps hold water in the soil.
• Active soil community. Soil organisms release and retain nutrients, protect plants from pests, and even enhance plant growth. Their activity depends on food availability, pH, and moisture and temperature levels.
• Appropriate pH. When pH changes, many nutrients can become either more or less available to plants, depending on the nutrient. pH also affects microbial activity. For example, Rhizobia form nitrogen-fixing nodules poorly in acid soils.

Improving nutrient cycling

Several of our six soil-friendly practices are important to nutrient cycling:

• Organic matter management. Organic matter supplies nutrients for plants and feeds the soil organisms responsible for cycling nutrients. It is a "slow release" fertilizer.
• Tillage practices. Tillage triggers the decomposition of organic matter and the release of nutrients, and mixes nutrients throughout topsoil. Excessive tillage reduces organic matter and the nutrient-holding capacity of your soil.
• Compaction prevention. Preventing compaction improves the ability of roots to grow through soil to reach available nutrients.
• Fertilizer management. You have choices about which form and how much of a nutrient to use, and when and how to apply it.
• Crop choices. Each crop affects nutrient cycling differently and encourages a different mix of soil organisms. Deep-rooted crops draw up nutrients that other crops cannot reach. Legumes can add nitrogen to the soil.

4. Water Cycle

If you have watched crops suffer through a drought or have seen yields jump after installing drainage tiles, then you understand that water is a fundamental factor for good crop yields. Like animals, plants need large amounts of water every day. Yet, too much water deprives roots of air and makes soil susceptible to compaction. Water carries soil, nutrients, and other substances, so water management is essential to controlling erosion and pollution.

Three major water processes occur in soil: water gets into the soil (infiltration), is held by the soil, and drains out of the soil. How these processes occur depends on soil type and management.

Infiltration is the rate at which water gets through the surface and into the soil. With higher infiltration, more water will be available to plants and less will run off the surface, erode soil, and wash away nutrients. Crop residue, living plants, or a rough soil surface will slow down the flow of water so more can infiltrate. A soil crust reduces infiltration and can be minimized by leaving surface residue, improving organic matter levels, and enhancing biological activity.

Available water holding capacity is the amount of water soil can hold for plant use. After water gets into the soil, the surface tension of water holds it in soil pores against the pull of gravity. Because of surface tension, small pores in fine-textured soils such as silt and clay loams hold more water than the large pores in sandy soils. Organic matter also holds large quantities of water. The maximum amount of water that a soil can hold against the pull of gravity is called the field capacity. Generally when a soil is at field capacity, half the pores are filled with water.

Plants cannot use all of the water in soil. As water evaporates and is drawn out of soil by plants, the water content gradually declines until plants can no longer extract the small amount of remaining water held tightly to soil particles. This remaining amount of soil water is called the wilting point. Clays have a high wilting point. They hold water more tightly than coarser-textured soils and so less of the water is available to plants. Thus, although silt loams hold a bit less water (have a lower field capacity) than clay loams, more of it is available to plants. Salts prevent roots from absorbing water and thus increase the wilting point.

Drainage or percolation is excess water that soil cannot hold that moves out of the rooting zone so roots and organisms can get air. After a heavy rain, the soil will be saturated (all the soil pores are filled with water). Many roots and soil organisms will die if the excess water does not quickly percolate out and allow air into the pores again.

Changes in Soil Water. Field capacity minus wilting point is the amount of water available to plants.

Improving the availability of water

Several of our six soil-friendly practices increase field capacity and improve infiltration:

• Organic matter management. Organic matter significantly increases the water-holding capacity of soil in two ways. It absorbs and holds large amounts of water, and it improves the structure of the soil - increasing the total volume and size of pores that can hold water and preventing soil crusting.
• Tillage practices. Leaving residue on the surface slows runoff and prevents crusting. Residue encourages populations of earthworms and other burrowing organisms and water infiltrates rapidly into their burrows.
• Compaction prevention. Compaction reduces water holding capacity by reducing the number and size of soil pores.
• Erosion control. Erosion reduces the depth of your soil and its water-holding capacity.

4. Life cycles of soil organisms

Soil organisms were mentioned in connection with all three of the previous soil processes. Their life cycles are an integral part of the formation of soil structure and nutrient cycles, and their activity is highly dependent on the water status of soil. Temperature, food supply, and pH are other factors that determine what lives in your soil and when they are active. The table on the previous pages, "A Year in the Life of Your Soil," shows how the annual cycles of organisms link to other soil processes.

The food web of organisms living underground is at least as diverse and complex as the ecosystem of plants and animals living above ground. Practically all the energy for the food web comes from the sun. Plants and other photosynthesizers convert the sun’s energy and carbon dioxide into the carbon compounds used by other organisms. Plant-eaters create compounds that other organisms need. As bacteria, fungi, earthworms, microscopic insects, and other organisms consume and transform carbon compounds, they release carbon dioxide back into the atmosphere and make nutrients available to plants. Farmers depend on these life cycles for their livelihood.

Improving the soil ecosystem

Several of our six soil-friendly practices can improve your soil’s ecosystem:

• Organic matter management. Soil organisms need a regular supply of organic matter for food.
• Increase diversity. It is generally beneficial to increase the diversity of organisms in the soil. Diversity is increased by using different management practices, including long crop rotations, changing tillage practices from year to year, supplying several types of organic matter, or creating buffer strips or other vegetation that adds variety to the landscape.

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