Fall Nutrient Management

 

Crops flourish and grow quickly in the spring. The first cutting of hay may be 50% higher than any other cutting. It’s not just due to more water. Increased spring growth comes from plant available nutrients (PAvN) from dormant microbes. Usually, this spring flush lasts 30-45 days, but with good management, this growth (and yield) flush may last all summer. However, it starts with fall nutrient management.

All soil nutrients are part of a biological system. Each element is like a component or part in an engine. If one component is lacking or missing, the engine may not run as well or even stop running. Soil nutrients, especially micronutrients, are the activators to many biological processes. Over the winter; microbes release nutrients when they die, are consumed by others, but also when they are active.

Plant available nutrients (PAvN) include many elements. The major positive charged elements called cations include ammonium (nitrogen), potassium, calcium, magnesium, and even sodium. The minor positively charged cations include iron, zinc, manganese, copper, and cobalt. Major negatively charged anions include nitrate (nitrogen), phosphorous or phosphate, sulfur (sulfates), and chlorine. Minor (needed in small amounts) anions include boron, molybdenum, iodine, and silica. Most minor elements activate enzymes and speed up biological growth and enhance yield.

To increase PAvN, farmers need to evaluate their farms limitations. First, what factors are limiting PavN? Bare soils, excess tillage, poor soil structure, soil compaction, low microbial and biological activity, herbicides, insecticides, excessively wet or dry soils all may limit nutrient release. Second, farmers need to evaluate or measure PAvN. Soil test are the cheapest and most common tool, but are not always the best for evaluating PAvN. Tissue tests can be used during the growing season but can be rather late for applying fertilizer. The Haney tests (fall or spring) can be used to evaluate PAvN. Some farmers use SAP, however; SAP analysis results can quite variable and need to be taken often during the growing season. Scouting and visual observations of plant growth can also be used to verify test results. No one procedure or test is perfect and generally several tests give a more complete picture of PAvN. 

The big focus needs to be on increasing soil microbial growth, since they are the source of most PAvN. Farmers should plan for good crop residue management. Ideally, at least 80% of crop residue should be decomposed by spring. Soil temperature (above 500F), soil moisture, good mix of soil microbes (balance of fungus for hard to decompose residues and bacteria for sugars) plus other soil fauna (springtails, mites) all help decompose crop residue. Harvesting crops late and excessive use of insecticides and other limiting factors may reduce crop residue breakdown.

Planting cover crops with various root systems (fibrous, tap roots, deep vs shallow) all enhance soil biology including increasing soil microbes and larger soil fauna populations to enhance PAvN. Deep plant roots allow air and oxygen to penetrate deeper into the soil so that more soil nutrients can be released. Plants roots are a great source of carbon, which is the most limiting soil element in our soils. Plants need carbon in the form of carbon dioxide for photosynthesis. Good fall and winter residue breakdown enhances soil aggregation for improved soil structure, promotes less disease organisms (better aeration), and improves conditions for spring planting. Also, adding livestock or manure increases soil microbe levels for increased PAvN.

Farmers have several ways to stimulate soil biology. On-the-farm; compost, manure, even compost teas are known to increase soil biology. Results can be quite variable and not always consistent so be careful. Off-the-farm; foliar sprays, bio-fertilizers, and inoculants are often used to stimulate the biology. Usually, the quantity and quality of nutrients or microbes is known and they are easier to use, but again results depend upon many different soil conditions. Farmers should also evaluate their crop varieties. Most modern crop varieties are now bred for fertilizer that is spoon fed like in a hydroponic system. Farmers using no-till and cover crops may need to use older crop varieties with a more robust root system with roots that are leakier (exude more sugars) to stimulate soil biology that improves PAvN.

In general, plants prefer microbially digested nutrients. One advantage for water quality is that PAvNs are tied up in the biology and are less likely to leach or runoff the soil. Some nutrients can leach from crop residue (10-20%) but ideally plants prefer to get their nutrients directly from the microbes after 80% of the crop residue has been decomposed. Adapted from Fall Nutrient Management webinar, David Miller/John Kemp.