Soil Health Concepts Leading to On-Farm Prescription, Soil Health is About Function
This is the second in my series of Soil Health Concepts Leading to On-Farm Prescription. Soil Health is defined as the capacity of soil to function. Recently, Tennessee Association of Conservation Districts (TACD), at their 75th annual convention, passed a resolution. The resolution summarized said that cover crops or any other conservation practice is not soil health, but soil health is the capacity of soil to function.
I want all potential readers when downloading this article see the importance of soil function. There are five recognized general functions, 1. Productivity and bio-diversity, 2. Handling of water (partitioning water), 3. Nutrient storage and cycling, 4. Filtering and buffering, and 5. Plant support. I am sure you can expand on this list. The bottom line is that soil health is about improving the function of soil. No-till, cover crops, nutrient management, pest management, crop rotations, etc. are tools to achieve better soil health.
Conservationists, educators, consultants, and others need to assist farmers in understanding of soil function. So many years we all treated symptoms, such as soil erosion. Many conservation practices have been installed to treat erosion or symptoms. Soil health is about going beyond a current level and improving that level of soil function. We do not need to continue to just conserve or add band aids to soil. Furthermore, if farmers treat soil based on function, soil aggregation will improve, water infiltration is improved resulting in less run-off and erosion. The root of the problem (in this case is the lack of roots) is the soil's ability not being able to handle intense rainfall due to poor soil structure.
Many have seen the statistics that agricultural soils have lost about half their original soil organic matter. "The 1938 Yearbook of Agriculture, Soils and Men" states that Soil Organic matter is the heart of the soil. Using the human body as an analogy, we would not be able to survive without our heart. Soils are severely limited with 50% losses of soil organic matter. Soil's most limiting factor to meet its potential to function is carbon. That is active carbon as well as total carbon. Carbon is a significant percent of soil organic matter and both terms will be used interchangeable.
The five principles of soil health focus on carbon cycling and improving soil biology of the soil. Let's discuss each in a practical setting to help farmers understand these concepts to potentially prescribe specific practices on their farms. Let's begin with keeping the soil covered. In nature, we see soil is always covered. In most settings in Tennessee, soils were formed under deciduous forests. When soils are not disturbed, the soil is always covered. The cover protects rainfall impact on the soil surface. Residue on soil surface is food for soil biology; it is also habitat for soil biology. Not only is it habitat for soil biology, it is also regulator for moisture and temperature. Soils retain more moisture and soil is cooler in summer and warmer in winter. When farmers distribute their crop residues evenly, soil is protected and soil biology can begin to multiply and more importantly to function. Soil biology consumes the carbon (crop residue) and aggregates the soil and cycles nutrients.
Understanding in an agro-ecosystem, a farmer will be growing crops which is not natural. We promote mimicking nature not emulating it. We expect farmers to grow agronomic crops but mimic nature by leaving residues of grain and fiber crops. Crops like vegetables may need to be removed due to disease. Follow integrated pest management for best decision making on specialty crop residues. By leaving residues on the soil surface, we need to assure even distribution of residues from the combine. Uneven distribution leads to areas of too little resulting in bare soil or more apt to have weed infestation. Too much residue leads to colder wetter soils and potential problems in planting. Even distribution provides temperature regulation, food and habitat for soil biology, erosion control armor, and mulch to protect from moisture losses. Adding cover crops exacerbate the above benefits by adding additional cover with diverse carbon-nitrogen ratios resulting in different periods of decomposition. Cover crops should be managed to reach boot stage or dough stage for grass and early bloom for legumes.in order to give sustained cover throughout the growing season. There must be balance for nutrient cycling, so the conservation planner should select species for carbon to nitrogen ratio of 28 to 32 to 1 ratio. As soils mature in soil biology, carbon to nitrogen ratios can increase to up to 37 to 1. Cover crop residue can also enhance weed suppression. Again, farmers need to allow covers to grow in order to reach height in order to smother weed germinating seedlings. Planting green can help farmers maximize cover when planting earlier spring crops such as corn. When killing covers earlier, say 14 days before planting, covers will not reach potential heights for weed suppression. Another factor for leaving cover is rolling and crimping to lay cover evenly on soil surface for better soil coverage. This provides better potential for soil coverage resulting in soils functioning better when covered. Another factor is when cover is laid flat by a roller, voles have less area to hide and more apt to be seen by hawks and other predators.
In addition to keeping the soil covered, the second principle of improving soil health is minimizing disturbances. For decades and even centuries, we tilled the soil. With that soil organic matter has decreased by 50%. We have experienced high soil erosion, runoff, and water quality issues. Going back to deciduous forest, and how we should mimic nature in an agro-ecosystem. Disturbing soil in a natural system would be like an annual fire to burn excess fuel in the forest. Minimum disturbance in a cropping system is a no-till planter or applications of nutrients if any tool is used to inject. It is a disturbance but the disturbance is needed to seed crops and cover crops. We need to teach farmers the need to no-till. Tillage breaks down soil aggregates resulting in poor soil structure. When soil structure is changed to platy structure or soil has no structure, water will not infiltrate easily. Tillage is detrimental to soil biology due to loss of food, habitat, and environment regulators discussed in the section keeping soil covered. In most cases organic farming usually practice tillage. In those cases, farmers need to rotate more with grass, add more amendments to assist in alleviating from tillage. The best way to improve soil is not to till. Minimizing disturbances protect the residues and soil aggregates. It is essential we produce residues in order for minimizing disturbances to make a difference. For example, you cannot leave cotton residue only and expect soil protection when no-tilling. The section of keeping the soil cover is essential to produce residues through good crop rotations and seeding cover crops. Keeping the soil cover and minimizing would fit under the area of protection principles. Whereas, the next three principles would be categorized as adding or enhancing. In a football analogy, the first two principles would be under the defense coordinator, protecting what we have. The following three would be under the offence coordinator, that is add or enhance to existing carbon, soil structure or existing amounts of residue.
Principle three is a continuous growing root. Basically, if you do not have something growing, the field and soil is degrading. In natural systems, plants are perennial with some annuals. They are always growing or either dormant waiting to produce carbon. In order to have active carbon into the soil, plants must be present to intercept sunlight energy. Green plants must be present. In an agroecosystem, we must have green plants growing to increase active carbon. The green plant intercepts sunlight and converts sunlight energy to energy into the plant through photosynthesis and the product is sugars which is a carbohydrate or simply carbon. The carbon moves through the plant. Scientists report that 5-30% of carbon gained in photosynthesis are leaked and or shared with symbiotic relationships with rhizobia bacteria in legumes and arbuscular mycorrhizal fungi. Through these actions soil biology is provided energy and the food web thrives while providing approximately 95% of soil's functions through contributions. Aggregating soils is a major contribution from soil biology that requires active carbon. Active carbon is broken down by soil biology and soil carbon or soil organic matter is the end product. Of course active carbon is used for nutrient cycling as well as build soil structure as a result of soil particles being aggregated into soil structure.
One of my favorite activity on the job is show farmers the results of cover crops, crop rotations, and no-till with a shovel. They get excited seeing soil aggregation that is similar looking to chocolate cake mix. Nice and crumbly with spherical shapes providing porosity for water and air movement. I also like to find a bare spot in or near the field where vegetation is not present. We dig there and find soils with no structure (no aggregation) and no major signs of life. This phenomenon can happen in one season showing that we must have continuous green plants growing to provide a continuous supply of carbon. Absence of green plants equal soil degradation.
The fourth principle is also addition and it is adding diversity. I live near the Smokey Mountains National Park. It is world renowned for its biological diversity. Plants are found from Georgia to Canada within the park. As I watched these soils for decades, they function in spite of having very low pH, steep soils and shallow to rock. Water coming off those 70% + slopes are pristine clean. The soils infiltrate large quantities of water quickly. Diversity of vegetation results in diversity of soil biology. We want biodiversity in our agricultural fields. We must have crop rotations but many times that is limited by markets. I hope one day farmers can work together to increase diverse markets. It would be better agronomically and economically. However, in the present grain and fiber crops, many crop rotations are two to four different crops, more in vegetable production. Adding multi-species cover crops is the farmers' way to mimic nature and add diversity. Many Haney (ARS Soil Health Test) and phospholipid and fatty acids (PLFA) soil tests show this finding. Several Soil Conservation Districts have funded some of these tests. Respiration, wet extractable carbon and nitrogen, carbon and nitrogen ratios, and soil health calculations all improve in time with additions of cover crops and diversity from cocktail mixes. The PLFA test shows consistent improvements in total microbial biomass, usually in year three but that is dependent on length of time cover crops remain growing in the spring. Terminating cover crops at ankle height restricts many potential benefits. Farmers are finding that there is a balance of leaving covers growing as long as possible and still plant spring crops in a timely fashion. Whether one tests with Haney tests or PLFA tests, there are many anecdotal benefits from diversity. There are less weed pressures due to choking out effect of covers and allelophathy from some of the species. There is generally more beneficial insects. Of course, IPM methods will enhance even more beneficial insects. There are less disease pressures with good diversity. Yield is seen consistent or slightly increasing. Net profit improvement is common in most of these systems. Reducing inputs and maintaining or increasing yields brings more profits.
The final principle is not as present on the majority of farms as the other four. This principle is integrating livestock into the agronomic field by adding temporary water and fencing. This of course is another variable. It requires more management. Most soil testing such as Haney and PLFA show no more total gains in carbon but increases soil biology (higher and more diverse microbial biomass) more quickly resulting in soil function improvement. The saliva, manure, and urine that are important in soil health in grazing systems can benefit an agronomic field. We see higher microbial biomass in PLFA tests and much higher respiration in Haney tests. The low carbon to nitrogen ratio from adding livestock and jump kick soil biology. Grazing management is essential to keep this positive toward soil health. Improper grazing such as grazing continuously or in saturated conditions will cause soil compaction. Normally, remove livestock during consistent wet conditions. Graze mostly in late winter or early spring taking half of vegetation and leaving half of vegetation. Always allow the cover crops to recover before terminating to plant the next crop. Farms that cannot integrate livestock would still improve the food web by additions of manure and or compost. Following good nutrient management principles are important and applying by soil test analysis will help the farm stay in check with nutrients.
In summary, cover crops, crop rotations, grazing management are not soil health. Soil health is about function. Soil functions will only improve when we add carbon to our soils. We do that by following the five principles of soil health. Your local NRCS office and Soil Conservation District can provide you information and guidance to assist you technically and financially to improve your soil health.