Crop rotation: a method of increasing soil quality and crop yield
Until recently, all agricultural production in the world was organic. After 1945, agriculture became increasingly dependent on chemical inputs such as fertilizers and pesticides. The use of chemicals obviously resulted to an increase in food production to meet the nutritional needs of an ever growing world population. However, this came with some costs to the environment; chemicals in food and drinking water reducing their quality, loss of soil organic matter, and soil degradation due to erosion. Producing sufficient amount of food while protecting the environment is therefore one of the major challenges of this century. Attention is now directed towards resource efficient crop production systems which utilizes less chemicals and with little or no damage to the environment.
One of the ways in achieving a sustainable, healthy food production system would be to rotate a variety of crops including cereals and legumes in the rotation. Crop rotation has been used over the ages as a method of maintaining productivity and soil fertility and increase yields. As most pest and disease thrive in a small number of crops, planting non-host crops in a rotation drives down their numbers. The benefits are reduced costs on pesticides and reduced pesticide use. Reduced pesticides encourages the growth of beneficial organisms such as bacteria, fungi and predatory insects and spiders which further reduce the need for pesticides with some benefits to the environment.
Crop rotation is also known to increase fertility. When nitrogen-producing legumes such as alfalfa, peas, lentils or beans are included in the rotation, subsequent crops are provided with substantial amounts of nitrogen which is crucial for growth. This is particular important for farmersâ€™ fields as current research has shown that soil nitrogen synthesized by legumes remain longer in the fields compared to nitrogen added to the fields from chemical fertilizers. Each crop grown in a rotation support the growth of different soil microbes. Colonization of the rhizosphere, the region of the soil close to the roots, by microbes is important. Rhizobacteria which may grow around the roots, aid plants by providing nutrients modulating growth and defending against diseases. An association between a mycorrhizal fungi and most plants is crucial for the absorption of some minerals such as phosphates. Thus increasing soil microbial diversity by increasing the number and diversity of crops in rotations will improve nutrient uptake.
Cover crops are known to provide protection and improvement between the main crops. Cover crops potentially can re-cycle nutrients which could be lost through leaching during off-season periods. Leguminous cover crops could increase nitrogen content by fixing atmospheric nitrogen, converting then into soil nitrates. The addition of compost and manure will increase the organic matter and microbial content of the soil and therefore its health and fertility. Crop residue left in the field and allowed to decompose also increase soil organic matter. When crop residue does not interfere with seeding and growth of new crops, they should not be removed because they improve soil organic matter.