From protozoa, pathogens, parasites and predators to birds and mammals, all are important to the development and maintenance of healthy soil.
Dorn A.W. Cox, Ph.D., a student in natural resources and earth systems sciences at the University of New Hampshire and director of GreenStart, a nonprofit providing technical education and practical agricultural examples, helps other farmers understand, observe and measure these biological contributors as well as chemical and physical elements that contribute to soil health. Cox, part of four generations working his family’s Tuckaway Farm in Lee, N.H., explains how the Cornell Soil Health Test, designed for the northeast and New York state, can help guide management decisions leading to improved soil health and increased crop yields.
“Soil health,” Cox cautions, “changes slowly over time. The Cornell Soil Health Test (CSHT) should serve as a management guide, not a prescription. Different management approaches can mitigate the same problem, and one management practice can affect multiple factors. Every field and every farm is different.”
Healthy and unhealthy soils
Healthy soils are characterized by good tilth, sufficient (but not excess) nutrients, depth sufficient to support crop growth, and the capacity to both store and drain water. Healthy soils are also free of harmful chemicals. Plant disease and parasitic organism populations are low in healthy soils, while beneficial organism populations are high. Weeds are minimal.
In healthy soils, chemical, physical and biological processes are working well together. Among chemical processes are nutrient storage and release. Healthy soils are physically able to support plants, to move and store water, to resist erosion, to foster root growth, and to allow air circulation and movement of organisms. Healthy soil supports a microbial community and the biological processes necessary to suppress pests, promote nitrogen mineralization and decompose organic matter.
Cornell Soil Health Lab, http://soilhealth.cals.cornell.edu/index.htm
Organic matter – living, dead and very dead – is an important component of healthy soils. Organic materials provide habitat. Living organic matter (protozoa, pathogens, parasites, shredders, predators, grazers, birds and mammals) all work together to create humus. Soil organisms make nutrients available, suppress disease and produce hormones, which encourage plant growth, while plant roots create pores and feed other life within the soil. Recently dead organisms and crop residues supply soil with energy and nutrients, while very dead, very decomposed organic materials (aka humus) hold nutrients and water and store carbon.
Unlike healthy soils, unhealthy soils may be cloddy, crusty and hard. Plants grow poorly, and crop yields are low and declining. Runoff and erosion may be evident, and plants rapidly become stressed during both wet and dry periods. Disease pressure is high. Intensive tillage, soil erosion and decreased organic matter contribute to the decline of otherwise healthy soil. Once begun, soil may continue to degrade as subsoil compacts and aggregates break down. “Aggregates are like unfilled rooms in a house – interesting stuff goes on in those empty spaces,” says Cox. “Small and intermediate pores become the habitat for small organisms and for water storage.” Loss of aggregate stability makes the surface susceptible to surface compaction or crusting. The crusty surface resists water penetration. Wind and water easily erode soil, resulting in lost organic matter and topsoil. Ironically, increased precipitation results in less water storage in the soil. Then, with access to significantly less water, fewer kinds of soil organisms survive. Plants are unable to access nutrients, decrease in number or die.
Measuring what is wrong
Traditional chemical soil tests measure soil nutrients and are used to determine the types and amounts of fertilizer needed to grow particular crops. “Measuring indicators of soil processes, as is done in the Cornell Soil Health Test, is more helpful in identifying problems and pointing to solutions,” says Cox. Developed for the northeast, the CSHT measures indicators of 15 agronomically essential soil processes within three categories: physical, biological and chemical (Table 1).
Accurate assessment of physical, chemical and biological aspects of soil health depends on careful sampling. Approximately 6 cups of soil is required for each field being tested. To collect samples, first scrape away surface debris. In each of five locations within the field (located at the five points of the letter W), use a trowel to collect the top 6 inches of soil. Put the soil in a 5-gallon bucket. Thoroughly mix together the five soil samples. Scoop out 6 cups of the composite soil and place it in a zippered plastic bag labeled with identifying information and date. Keep the sample refrigerated. For detailed instructions, see Soil Sampling Protocol at http://soilhealth.cals.cornell.edu/exten sion/test/sampling_instructions.pdf.
Use express shipping to send the sample together with a completed grower and field information sheet (including penetrometer readings) to the Cornell Soil Health Lab. In many areas, free pickup is available through the Dairy One Cooperative network. A comprehensive test of soil for vegetable production, organic production or for diagnosis of soil problems is $75 and takes seven weeks. A basic test of soil for grain, forage and nonagricultural crops is $45 and takes approximately four weeks. For a sample of results of a comprehensive test on a no-till field see http://soilhealth.cals.cornell.edu/extension/test/g149%20no%20till%20for%20manual.pdf.
Growing a pathogen-susceptible bean is part of the comprehensive test and is one reason for the increased time required for completing the test.
Fine root structures continued downward 20 inches in this low-till field.
Building better soil
Once problem areas have been identified, soil health can be improved by modifying with organic or chemical amendments, rotating crops, growing cover crops, and reducing or modifying tillage.
“All organic matter is not created equal,” says Cox. In amending soil, the goal is to balance organic matter needed to decompose and provide nutrients with organic matter needed to accumulate water, carbon and nutrients. Each type of organic matter (for example, manure, compost, cover crops or leaves) has a different balance. Biochar (charcoaled plant matter) is a useful way to add carbon. Biochar decomposes much more slowly than uncharred biomass.
Invaluable in enhancing soil, cover crops can break up compaction, add nitrogen and store water. Cover crops can also help attract beneficial insects and slow erosion. On clay soil, cover crops help springtime drying. A helpful tool in selecting a cover crop for fields used to grow vegetables is www.hort.cornell.edu/bjorkman/lab/covercrops/decision-tool.php.
Till or until …
Whether or not or how much to till has been the subject of much discussion. “Decreasing tillage,” says Cox, “helps prevent or alleviate soil compaction and has been shown to improve many soil processes.” In addition to reducing erosion and runoff by retaining residues atop the soil, reducing tillage also saves time and fuel. “When tillage is reduced, up to 80 percent less soil is moved,” says Cox. “In addition, lighter tillage can be done later in the spring, right before planting, giving soil more time to dry.”
While Cox has found reducing tillage to be an important part of soil improvement plans, he cautions against abruptly stopping. “In soil ‘addicted’ to tillage, tillage should be decreased gradually,” says Cox. “Don’t try to stop tilling all at once. Begin working toward no-till by first rebuilding soil health.” He also notes that reducing tillage may be less successful in lighter soils and in dry springs.
Before investing in new equipment to reduce tillage, Cox recommends borrowing or renting and testing the equipment in strip trials. After planting and turning in a cover crop, sample the soil. CSHT results will help you in designing a long-term plan for improving soil health.
Assisted by his dog Nula, Dorn Cox digs downward to observe and compare soil structure and compaction in low-till and plow-tilled soil.
“Use the CSHT results as a guide,” says Cox. “The test results will not only point to nutrient deficiencies, but will also identify biological and physical factors which could be modified in order to improve the health of your soil. Begin your plan by first addressing the items noted in red on the CSHT. Target management practices for this year and the years to follow. Then look forward to improved crops grown in healthy soil.”
Kathleen Hatt is a freelance writer and editor and has been a frequent contributor to Growing since its inception in 2002. She lives in Henniker, N.H.