Soil consists of four important parts: mineral solids, water, air, and organic matter. Mineral solids are sand, silt, and clay and mainly consist of silicon, oxygen, aluminum, potassium, calcium, and magnesium. The soil water, also called the soil solution, contains dissolved nutrients and is the main source of water for plants. Essential nutrients are made available to the roots of plants through the soil solution. The air in the soil, which is in contact with the air above ground, provides roots with oxygen and helps remove excess carbon dioxide from respiring root cells. When mineral and organic particles clump together, aggregates are formed. They create a soil that contains more spaces, or pores, for storing water and allowing gas exchange as oxygen enters for use by plant roots and soil organisms and the carbon dioxide (CO2) produced by organisms’ leaves the soil.
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, high-quality, and healthy crops? How would you know a high-quality soil from a lower-quality soil? Most farmers and 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 signs of erosion are seen on the better quality soils. Less power is needed to operate machinery on a healthy soil than on poor, compacted soils.
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 one with poor tilth. A soil that has a favorable and stable soil structure also promotes rainfall infiltration and water storage for plants to use later. For good root growth and drainage, we want a soil with sufficient depth before a compact soil layer or bedrock is reached.
We want a soil to be well drained, so it dries enough in the spring and during the following rains 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 desirable for cranberry and 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 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 and sodium in arid 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.
THINK LIKE A ROOT!
If you were a root, what would you like from an ideal soil? Surely you’d want the soil to provide adequate nutrients and to be porous with good tilth, so that you could easily grow and explore the soil and so that soil could store large quantities of water for you to use when needed. But you’d also like a very biologically active soil, with many beneficial organisms nearby to provide you with nutrients and growth-promoting chemicals, as well as to keep potential disease organism populations as low as possible. You would not want the soil to have any chemicals, such as soluble aluminum or heavy metals that might harm you; therefore, you’d like the pH to be in a proper range for you to grow. You would also not want any subsurface layers that would restrict your growth deep into the soil.