The Basics: How Nutrients Are Released From Soil Organic Matter
In temperate mineral soils, levels of soil organic matter vary from approximately 0.4 percent to 10 percent. Despite making up a very minor portion of the soil, organic matter has a significant impact on the fertility and structure of the soil. Ninety-five percent of soil nitrogen (N) and forty percent of soil phosphorus (P) are estimated to be found in soil organic matter; under the correct circumstances, this can supply all of a crop’s N and P requirements. A soil containing 3 percent organic matter is estimated to have between 2,000 and 4,000 pounds of total nitrogen per acre, and between 100 and 300 pounds of phosphorus per acre. When soil microbes eat organic stuff and eventually perish, they release these nutrients.
The “active” proportion, or 10–20% of the total organic matter in the soil, is easiest for soil organisms to break down. The main source of replenishment for this active fraction is the addition of organic matter (compost, manures, cover crops, and crop residues). This active organic matter is broken down by soil organisms, which account for an additional 10–20% of soil organic matter. These organisms deliver their nutrients to plants when they die. Humus is the organic stuff that remains in the soil. Since soil organisms break down humus more slowly, it provides less readily available nutrition. On the other hand, humus is crucial because it offers cation exchange sites, which retain nutrients in the soil and keep them accessible to plants.
The rate at which organic matter amendments to soil degrade influences the rate at which nutrients are made available to crops. The carbon-to-nitrogen ratio of the amendment, the kind of soil, the temperature and moisture content of the soil, and the crop being produced are some of the variables that influence how quickly organic amendments break down. Being a component of the more active organic matter fraction, green manures break down easily and release nutrients rather quickly. Composts break down more gradually and include more stable, humic organic materials. Because of this, compared to green manures, most composts give nutrients to crops more gradually.
Decomposition of organic materials is accelerated in the rhizosphere, the region directly surrounding roots. The release of organic molecules by roots into the soil, including vitamins, amino acids, and carbohydrates, promotes the proliferation of microorganisms in this area. Numerous of these organisms break down organic materials, which releases nutrients for the crop. The plant species or kinds that most effectively support these nutrient-releasing microbes have not been the subject of much research. Such data could be used in the future to pinpoint crop varieties that work well in organic systems. The residues of cover crops add to soil organic matter when they are routinely included in a rotation. As bacteria proliferate and die, more nitrogen is released into the atmosphere. This is because the organic matter fuels their growth.
Legumes can be included in a cycle as green manure (vetch or clover) or as a harvested crop (alfalfa, for example). With organic crop rotations, legumes are especially valuable since they supply nitrogen to the system. Legumes’ roots are home to specialized bacteria called Rhizobium spp., which transform atmospheric nitrogen (N2 gas) into nitrogen that plants can use. This bacterial-legume relationship fixes different amounts of N, depending on the type of plant, environment, soil type, crop management practices, and length of crop growth. Legumes give nitrogen (N) to the crop that comes after them when they are cultivated carefully.
Nitrogen Scavenging And Conservation By Nonlegume Winter Cover Crops
Due to their larger root systems, winter-hardy cereals and grasses are able to scavenge soil nitrates in the fall more effectively than legumes, which reduces nitrogen loss in the late fall and winter & is considered to be best crop rotation for soil fertility. Since cash crop harvests in the northeastern US sometimes continue into late summer and fall, vegetable growers most frequently utilize minor grains (wheat and rye) as winter-hardy cover crops. Because grain residues from these cover crops decompose more slowly than those from legume cover crops, they will transfer trapped nitrogen and other nutrients to succeeding crops more slowly when they are introduced in the spring.
In certain situations, such as when high carbon-to-nitrogen ratio heavy crop or cover crop residues (30:1 or more) are tilled into the soil, soil N may be absorbed by soil microbes as they consume the carbon-rich residues, rendering it unavailable to plants in the short term (immobilized). Small-grain legume cover crops can decrease N immobilization by giving microorganisms more nitrogen when they break down leftovers. As an alternative, waiting around two weeks following the inclusion of residues before planting a cash crop usually provides enough time for nitrogen to cycle through microorganisms and back into the soil. By incorporating nonlegume cover crops when they are still young and green, Nimmobilization issues are also lessened.
The possibility of overwintering cover crops depleting soil water is a crucial factor to take into account. While cover crops can enhance soil water-holding capacity and infiltration, the temporary loss of soil water in the early spring might lower the yields of succeeding cash crops in arid springs. In this case, irrigation or the early incorporation of cover crops may be necessary to preserve soil water. Conversely, cover crops can aid in the drying out of soggy fields in the spring.
So by this we can understand that organic crop rotation is the Best crop rotation for soil fertility.