How Do Plants Get Nutrients in the Soil in a Biological Farming System?

Plants take up nutrient elements from the soil through their roots. Plants need nitrogen, phosphorus and potassium in large amounts; very often, these elements are not available in adequate quantities in the soil. Other essential nutrients such as boron, calcium, copper, iron, magnesium, manganese, sulphur, zinc and others are needed in smaller or trace amounts, and these are often adequately available. If nutrient elements, along with water, are not available in adequate quantities at the time the plant needs them, growth and development will be affected adversely.

Biological farming systems make nutrient elements available in the soil through judicious management of nutrient cycles. One important goal of a biological farming system is to provide essential nutrient elements to crops by maintaining or increasing soil fertility with the use of plant residues, animal manure, legumes, composts, green manure cropping, crushed rock minerals and other natural inputs. Supply of essential trace elements and minor nutrients comes from wood ash, crushed minerals and the release of inorganic nutrients already in the soil through biological additives.

Organic matter benefits the soil in many ways: improving soil water-holding capacity; enhancing soil structure; binding and releasing mineral nutrients; serving as food for microorganisms that recycle soil nutrients; and being mineralised to nitrogen, phosphorus and sulphur. Reserves of essential plant nutrients are created with the flow of mineral nutrients between living and non-living components of the soil.

Nitrogen is made available by raising legumes in rotation with the regular crop. Unlike inorganic fertiliser nitrogen, leguminous nitrogen is steadily and gradually released throughout the cropping cycle if temperatures are sufficiently high to allow microbial action. Different legume species and cultivars fix different quantities of atmospheric nitrogen. Management practices and physical factors are also significant determinants of nitrogen-fixing, and these include soil pH, temperature, drainage, the timing of harvest, and the turning under of foliage for green manure.

Phosphorus does not leach as readily as nitrogen, but in acid or alkaline soils it easily converts into forms not immediately available to plants. The amount dissolved in water determines phosphorus availability for plants. Organic farmers may apply rock phosphate instead of acid-treated phosphate to their fields. However, rock phosphate is significantly less effective than acidulated phosphate. It is possible in some areas to defer application of acidulated phosphates for several years. Manures and organic wastes can be applied to partially replenish phosphorus, but replacement applications of rock/acidulated phosphates will eventually be needed. It is not possible for a farm to attain self-sufficiency in phosphorus.

Potassium in immediately available form is usually present in adequate amounts in subhumid and arid regions from weathering of minerals. Humid regions and highly organic soils may need regular replenishments of potassium. Some forage crops (e.g. alfalfa and clover) take up large amounts of potassium, thus the hay or silage should not be harvested but turned under for green manure. Leguminous forages have potentially high levels of this nutrient; this implies that manure from animals consuming such forages should be conserved and returned to the land, for use by the next crop.