1.3: What Kind of Soil Do You Want?

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 well the soil is functioning for whatever use is being considered. The concept of soil health focuses on the human factor—the anthropogenic influence—that is increasingly significant due to many years of intensive management. This is different from the inherent differences in soils that are the result of the natural factors that formed the soil, such as the parent material, climate, etc. Thereby, an analogy with humans is apt: We may have some natural differences from our genetic backgrounds (taller or shorter, fairer or darker, etc.), but our health still strongly affects the way we can function and is greatly influenced by how we treat our bodies.

In agriculture, soil health becomes a question of how good the soil is at supporting the growth of high-yielding, high-quality and healthy crops. Given this, how then would you know a high-quality soil from a lower-quality soil? Most farmers and gardeners would say 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, and oftentimes they refer to how dark and crumbly it is. 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. But there are other characteristics that we’d like a soil to have. These can be condensed into seven desirable attributes of healthy soils:

1. Fertility. A soil should have a sufficient supply of nutrients throughout the growing season.
2. Structure. We want a soil with 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.
3. Depth. For good root growth and drainage, we want a soil with sufficient depth before a compact soil layer or bedrock is reached.
4. Drainage and aeration. We want a soil to be well drained so that 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 enter the root zone and just as important that carbon dioxide leaves it (it also enriches the air near the leaves as it diffuses out of the soil, allowing plants to have higher rates of photosynthesis). Keep in mind that these general characteristics do not necessarily hold for all crops. For example, flooded soils are desirable for cranberry and paddy rice production.
5. Minimal pests. A soil should have low populations of plant disease and parasitic organisms. 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.
6. Free of toxins. We want a soil that 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 when sewage sludge with high concentrations of toxic elements is applied.
7. Resilience. Finally, a high-quality soil should resist being degraded. It should also 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 the 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, and you wouldn’t want to be in a soil that somehow became contaminated with toxic chemicals. You would also not want any subsurface layers that would restrict your growth deep into the soil