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More Dirt on Dirt


A YEAR AGO, I RECOMMENDED having soil tested every few years in the interests of maintaining a productive garden (see “The Dirt on Dirt“. A traditional soil test measures pH (acidity/alkalinity) and the amounts of various nutrients that are present and available to plants (some standard tests also measure organic matter content)—information that can enable you to make informed decisions about what plants to grow (some, for example, can tolerate more acidity than others) and what natural fertilizer or other amendments might result in better crops.

Should you want to dig deeper and learn not only about pH and available nutrients, but also about your soil’s physical and biological condition—factors that translate into soil health—you might consider a relatively new but more expansive test called the Cornell Soil Health Assessment (CSHA). It costs more the standard Dairy One test ($85 versus $12), but it gives a more holistic picture of the soil and will also suggest steps to take, beyond the standard applications of fertilizer and lime, that will lead to an enduring and bountiful garden.

In addition to the data that a traditional soil test provides, CSHA measures the following 10 indicators of overall soil health (some of this information is a little technical).

1. Aggregate stability

Most people view soil as a solid medium. But it’s not as solid as you might think: 50 percent of a typical garden soil is pore space containing air and water that provides the highways and byways for plant roots and soil organisms to move about; 45 percent of an average soil is minerals and very small rock fragments (this is the solid portion); the remaining 5 percent of a typical soil is organic matter.

Soil aggregates are small clumps made up of mineral particles, moisture and organic matter. A soil with good aggregate structure will have sufficient pore space and will be better able to withstand erosion by wind and water. The CSHA determines how resistant a soil’s aggregates are to breaking apart when water, simulating rain, is poured over them. In other words, it’s a measure of aggregate stability.

2. Soil texture

The solid or mineral portion of a soil is made up of particles of clay, silt and sand, with clay being the smallest, sand the largest and silt somewhere in between. The relative proportions of these three describe a soil’s texture. Soils with a high percentage of clay hold the most water—sometimes more than you may want them to. These soils are prone to flooding and often are described as poorly drained. Soils with an excess of sand tend to drain quickly. This can be a problem during droughty periods. Soils with a nice mixture of clay, silt and sand are usually the best. They are called loams.

In addition to its relevance to water retention, a soil’s texture will influence the interpretation of several of the other indicators.

3. Available water capacity

Even during dry periods a soil should be able to retain some moisture for plant and microbial use. How well it can do this depends on aggregate stability, pore space and organic matter content. It also depends on the soil’s texture or the size of its mineral particles. The CSHA determines how much water a disturbed sample of soil can retain for plant use.

4. Surface hardness

Soil compaction occurs when a soil is subjected to heavy equipment or excessive human or large animal foot traffic, especially under wet conditions. Compacted soils have small and degraded aggregates, resulting in less pore space for air and water, which makes it harder for plant roots, microbes and larger soil organisms to get around. Soils that have a lot of clay are more prone to compaction than those with lots of sand.

A device called a field penetrometer is used to measure resistance to penetration. It looks like a long-stemmed, sturdier version of a soil thermometer. Applying slow, even pressure, the penetrometer is pushed straight down into the top 6 inches of soil. It indicates on a circular dial the soil’s resistance in pounds per square inch. As long as the penetrometer doesn’t encounter a hard object like a rock, it will give a fair indication of how compacted your soil is.

5. Subsurface hardness

Soil compaction can occur at much greater depths than 6 inches. Readings taken when a penetrometer is pushed into the soil from 6 to 18 inches will indicate how hard the soil is at these depths. This, too, is useful information, since plant roots can easily extend 18 inches down in search of water and nutrients, provided the soil is not too compacted to receive them.

6. Organic matter

Organic matter can be divided into three categories: the living, the dead, and the very dead. The roots of live plants are living organic matter, as are live soil organisms, from the myriad microscopic fungi and bacteria all the way up to the various insects, beetles, earthworms and even small burrowing animals that make their home in the soil.

Dead organic matter includes soil organisms whose lives have ended, the roots of dead plants, manure, crop residues and any other vegetative debris that has found its way into the soil or is still on the surface. It is this form of organic matter that provides food for the vast array of herbivorous (i.e. non-predatory) soil organisms, from the very small to the quite large.

Very dead organic matter is material that has undergone complete decomposition at the hands of soil organisms and reached a stable state. It is what we call humus. Because it has a gummy, sponge-like quality, humus retains moisture and nutrients very well. It also helps preserve soil aggregates. But it is not a source of food for soil fauna.

The CSHA will determine what percentage of your soil is composed of organic matter and in what forms. Generally speaking, the more the better.

7. Active carbon

This is a measure of how much carbon in the soil’s organic matter is available as food for the various soil organisms. It is essentially the dead or un-decomposed form of organic matter described above. Active carbon is the raw energy that drives the microbial community. It is a leading indicator of soil health.

8. Root health

Healthy plant roots are whitish, coarse textured and prolific, and they should be without lesions (which suggest the presence of pathogens and/or parasites). The people conducting the CSHA take some of each soil sample they receive and grow beans in them. After six weeks the bean plants are removed from their containers and the roots are washed and rated for overall health.

9. Soil respiration

This new addition to the CSHA lineup measures the abundance of microbes in your soil and how biologically active they are. It determines how rapidly nutrients are being converted into forms that plants can use and the rate at which decomposition of organic matter is occurring. As microbial activity increases so too does the formation of humus and aggregate stability.

10. Soil protein

Soil proteins are found in soil organic matter. They are primarily a source of stored nitrogen, which soil microbes convert (mineralize) into forms plants can use. The addition of organic matter to the soil in the form of crop residues, cover crops and plant debris, will positively affect the formation of soil proteins. So will a robust microbial community.

If your curiosity has been piqued and you’d like to learn more about what’s going on in your soil, check out the Cornell Soil Health Assessment website (www.soilhealth.cals.cornell.edu). There’s plenty more information available than is covered here, including an online training manual that includes descriptions of testing procedures and their relevance. You can pull up a submission form and follow instructions for taking the soil sample (8 cups) that is required. This modest step could reap rewards in any garden and help you gain a deeper understanding and appreciation of the vast, dynamic and fascinating world that lies under your feet.

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