Farmers who improve soil health increase the amount of soil carbon being stored, and recycled. The increased carbon flow increases microbial numbers and efficiency leading to improved plant photosynthesis. The entire soil ecosystem functions at a higher level. The overall effects are healthy plants that have less disease and insect issues and higher overall yields.
Soils high in soil organic matter (SOM) allow carbon to cycle in many different forms.
The soil microbial community adjusts to these changes in the quantity of recycled carbon and nitrogen and this has an effect on the amount of phosphorus, sulfur, and micro-minerals released in the soil. Many of these trace minerals like manganese, iron, copper, and zinc are essential micro-nutrients needed to suppress disease. Plants activate plant proteins into enzymes to fight diseases using these micro-nutrients as co-factors. All soils have pathogens but healthy soils can tolerate these pathogens because healthy plants have the ability to fight them off. Since most insects attack diseased plants, both diseases and insect pests decline in healthy soils.
The type of nitrogen used to feed the plant can play a significant role in plant disease suppression. Soils high in SOM have plenty of carbon and nitrogen recycling. Since most soils are now lacking adequate SOM, the form of nitrogen primarily as nitrate or ammonium can affect the disease cycle and possible pathogens. Some pathogens are enhanced by nitrate and suppressed by ammonium. Others are the exact opposite. Most (but not all) soil-borne pathogens are enhanced by the presence of nitrate (Kempf & Huber, 2018). Ideally, farmers should use about 20-30% of their corn nitrogen as nitrate (especially in corn starter fertilizer) to promote vegetative growth and about 70-80% as ammonium forms to promote reproductive growth. The added bonus is both disease and insect pest suppression.
Soil compaction can have an impact on disease organisms. Saturated or water-logged soils in compacted soils become nutrient depleted, especially in micro-nutrients leading to more disease. This corresponds to the impact of reduced vs oxidized environments, since ammonium is the reduced form of nitrogen, and nitrate is the oxidized form of nitrogen. In general, reduced environments (ammonium) are very disease suppressive, and oxidized environments (nitrate) are disease enhancing (Kempf and Huber, 2018).
The amount of oxygen and water impacts oxidation, reduction, and nutrient form. Soils that are very dry but highly aerobic (lots of oxygen) can be as detrimental as fields that are saturated (too much water) and anaerobic (no oxygen). The ideal situation is a healthy soil with plenty of SOM and pore space so that when it rains 0.5-1.5 inches and/or with adequate moisture allows reduced micro-nutrients (plant available forms) and ammonium to recycle in the soil. The soil is a micro-environment meaning that both reducing and oxidizing conditions can occur at the same time only short distances apart. In most cases, for healthy plants, we want to avoid the extreme conditions at either end. Pest (disease and insect) suppression is all about balance.
The best way to reduce disease in crop fields is crop rotation with biological microbial buffering. Changing the plant community allows a balance of microbial predators which consume many of the disease pathogens. Crop rotations also promote beneficial microbes which recycle essential micro-nutrients. Crop rotation disrupts disease cycles and allows micro-nutrients to be made plant available as co-factors to activate enzymes to fight off disease organisms.
Cover crops are similar to crop rotation and are another way to suppress disease pathogens and improve micro-nutrient availability. Oats is one of the best cover crops for disease suppression because it makes manganese available (decreases Fusarium), decreases soil compaction, and improves soil structure. Cereal rye helps soybeans by reducing soybean cyst nematodes, Rhizoctonia, and Phytophthora. Brassicas like kale, rape seed, and radish makes sulfur and other micro-nutrients available while breaking the disease cycle similar to crop rotation. Reducing the use of glyphosate can be beneficial because glyphosate is a chelator which ties up many micro-nutrients (especially manganese) and glyphosate promotes Fusarium by tying up manganese.
To promote healthy soils that resist pests, crop rotation and cover crops are extremely beneficial. The form of nitrogen fertilizer (ammonium vs nitrate) affects disease cycles. Soil conditions like alleviating soil compaction and improving SOM can play a large role in keeping pests away. Doctors say you are what you eat, so produce and eat healthy food. Healthy soils and healthy microbes lead to healthy plants and animals eaten by healthy people.