Question

The most suitable crop rotation to increase the level of nutrient in the soil is

Answer 1

Soil organic matter and clay particles hold large stores of plant nutrients. These reservoirs, however, are not all available to the crop. In an organic crop rotation, the grower manages soil organic matter and nutrient availability by incorporating different crop residues, cycling among crops with different nutrient needs, using cover crops, and adding organic soil amendments. Most crops deplete soil nutrients during their growth cycle. Some of these nutrients leave the farm as harvested products, and the rest return to the soil as crop residues. The nutrients in residues may or may not be available to the next crop. Crop roots and residues improve soil fertility by stimulating soil microbial communities and improving soil aggregation. This improved soil physical environment facilitates water infiltration, water holding, aeration, and, ultimately, root growth and plant nutrient foraging. This section will review different ways that crop rotations affect soil fertility.

Understanding the basics of how nutrients are added to and released from soil organic matter will help the farmer in choosing crop sequences and amendments to optimize organic crop fertility. Certain fractions of soil organic matter contribute to plant nutrition more than other fractions. To effectively plan organic crop rotations to meet crop nutrient needs, several factors should be considered. Legume crops, which capture atmospheric nitrogen and “fix” it into forms available to plants, can be used strategically in rotations to meet the needs of nitrogen-demanding crops. Cover crops used after a cash crop capture surplus plant-available nutrients and conserve these for following crops. Cash crops themselves vary in their nutrient demands (see Appendix 1); considering their needs helps make the most efficient use of the available soil nutrients in a rotation. Finally, other types of organic amendments, such as compost and manures or approved mineral fertilizers, can supplement nutrients at targeted times during a rotation. Each of these topics is discussed in the sections below.

The Basics: How Nutrients Are Released from Soil Organic Matter

Levels of soil organic matter range from about 0.4 percent to 10 percent in mineral soils in temperate regions. While organic matter is a relatively small fraction of the soil, it has large effects on soil structure and soil fertility. Soil organic matter contains an estimated 95 percent of soil nitrogen (N) and 40 percent of soil phosphorus (P), and with the right levels and conditions it may provide all of the N and P needs of a crop. Estimates of total nitrogen in a soil with 3 percent organic matter range from 2,000 to 4,000 pounds per acre; estimates of phosphorus range from 100 to 300 pounds per acre. Soil microorganisms release these nutrients when they consume organic matter and subsequently die. The rate of this nutrient release is affected by the availability of carbon sources (energy for the soil microbes), soil temperature, soil moisture, tillage, types and numbers of soil organisms, and quality of the soil organic matter.

A portion (10–20 percent) of the total soil organic matter has been termed the “active” fraction and is most easily decomposed by soil organisms. This active fraction is replenished primarily by additions of organic matter (cover crops, crop residues, manures, compost). Soil organisms, which make up another 10–20 percent of soil organic matter, decompose this active organic matter. Upon death, these organisms release their nutrients to plants. The remaining soil organic matter is humus. The humus is more slowly digested by soil organisms and therefore is not a large source of available nutrients. Humus is very important, however, because it provides cation exchange sites, which hold nutrients in the soil and thus maintain their availability to plants.