Manure benefits soil health

Date:12-08-2022

By James J. Hoorman
Hoorman Soil Health Services

Manure is a great fertilizer for improving soil health, commonly used before commercial fertilizer. If manure is applied correctly, using the 4R’s (right source, right rate, right time, right place) and proper best management practices, manure greatly improves crop growth and also increases biological activity, leading to improved soil health. Some of the environmental benefits include: increasing soil carbon and reduced atmospheric carbon, reduced soil erosion and runoff, reduced nitrate leaching, and reduced demand for commercial nitrogen fertilizer derived from natural gas.

Manure increases soil organic matter because it has nutrients plant require for adequate growth (N-P-K, micronutrients), so plants grow better and faster, producing more roots and crop residue to build soil carbon. Manure consists of carbon residues which the plants can use in the form of carbon dioxide for increased photosynthesis. Adequate soil carbon is limiting plant growth, so manure and carbon may boost plant growth significantly. Building soil organic matter is the best way to gauge soil productivity. The organic carbon in manure provides the energy and nutrients for active, healthy soil microbes; not only stabilizing those nutrients so they are not lost to the environment (leaching, runoff, volatilization) but also providing those nutrients to the plant in a form they can use.

Manure often is not as concentrated as commercial fertilizer, but it tends to have less salts and have more plant available nutrients. Salts are positive and negative charges on crop nutrients. Because manure nutrients tend are tied up with organic carbon, these charges are neutralized which allows the plants to use them efficiently. Too many salts may burn or desiccate a plant, requiring more water for the plant. Some ″hot manures″ do occur, where animals are fed a highly concentrated diet that produces manure high in salt content. Generally, small doses of manure are beneficial while large doses may cause plant stress.

Dr. Mark Risse, University of Georgia did an extensive literature review on the environmental benefits of manure. Risse found ″The ability of manure to maintain or build soil organic matter levels has a direct impact on enhancing the amount of carbon sequestration in cropped soils. Manure organic matter contributes to improved soil structure, resulting in improved water infiltration and greater water-holding capacity leading to decreased crop water stress, soil erosion, and increased nutrient retention. An extensive literature review of historical soil conservation experiment station data from 70 plot years at 7 locations around the United States suggested that manure produced substantial reductions in soil erosion (13%-77%) and runoff (1%-68%). Additional studies during years following manure application suggest a residual benefit of past manure application.″

Other studies have found that surface application of manure is similar to crop residue on the soil surface. Both surface applied manure and surface crop residue significantly reduce soil erosion by reducing raindrop impact from detached soil particles. Surface applied manure tends to coat the soil surface and reduce rainfall impact similar to crop residue. In the short-term, surface manure applications reduce soil erosion. Again, use some common sense here. Small applications of manure can be beneficial with very light rains, but either large manure applications or heavy rains can negate these benefits. When dealing in soil health and environmental benefits, everything needs to be applied in moderation.

Organic N (N tied to carbon) in manure is more stable than the inorganic N (bare N) in commercial fertilizer. Organic N is released slowly as the soil warms and follows crop growth and crop needs for additional N. Commercial fertilizer applied as nitrate or ammonia (inorganic N sources) which easily converts to soil nitrates, are easily leached from the soil. Nitrate-N is quite soluble and easily moves with the water, so it is quite mobile, subject to leaching and runoff. Heavy rains, melting snow, or excess irrigation often lose soil nitrates. In heavy clay soils, denitrification or loss of N to the atmosphere can result in 40-60% loss of soil N in saturated soils. Manure is a slow-release N source which is more closely timed to plant growth and plant needs.

Commercial N fertilizer is expensive to produce from natural gas and also contributes to greenhouse gases. A 200# N application from anhydrous ammonia requires 3300 cubic feet of natural gas to supply the N requirements for corn per acre. Substituting manure saves not only natural gas, but also mined phosphorus and potassium, significantly reducing energy costs. Manure is a great source of crop nutrients and offers some environmental benefits only if producers use the 4 R’s and best management practices to minimize losses. Apply manure to a cover crop with 90% crop residue mimics Mother Nature.