The Importance of Early Season Phosphorus Nutrition

Phosphorus (P) fertilization is a major input in crop production on the Great Plains, as many soils lack sufficient P for effective crop production. To optimize crop nutrition, P must be available to the crop in adequate amounts early in the growing season. Phosphorus is needed from the earliest stages of crop growth, because it is important in nearly all energy-requiring processes in the plant. Phosphorus stress early in the growing season will reduce crop productivity more than P restrictions later in the year. Plants have adapted to increase their ability to access P and avoid deficiencies. When under P stress, plants will use their resources to extend their root system to improve the chance of contacting P. Some plants can modify rhizosphere pH or excrete organic acids to increase the uptake of P. Plants such as canola or wheat can increase root proliferation in high-P regions to enhance P uptake while others will form associations with vesicular arbuscular mycorrhizae. Plants differ in strategies adopted and in efficiency of P absorption. Understanding how different plants respond to P deficiencies and fertilization can help in designing P management practices most suited to each crop type. Phosphorus placement in or near the seed-row helps supplies P to the crop early in the growing season. Other ways of managing P could include increasing seed concentrations of P, using tillage system and crop sequencing to improve mycorrhizal activity, or maintaining adequate P levels in the soil. Since plants differ in their ability to access P from the soil and fertilizer applications, different practices may be needed for different crops. We need to more accurately predict early P supply from the soil to avoid overfertilization or crop deficiencies. By fertilizing only where P limits crop production and by improving fertilizer efficiency, we can optimize economic of crop production and avoid environmental problems. Introduction Phosphorus is a critical nutrient for plant growth, since it is involved in cellular energy transfer, respiration, and photosynthesis. Phosphorus is also a structural component of the nucleic acids of genes and chromosomes and of many coenzymes, phosphoproteins and phospholipids (Ozanne 1980). An adequate supply of P is essential from the earliest stages of plant growth. Early season deficiencies of P can lead to restrictions in crop growth from which the plant will not recover, even when P supply is increased to adequate levels. In most crops, only a small amount of the phosphate is actively involved in metabolism (Bieleski 1973; Lefebre and Glass 1982). If P supply is adequate, the majority of the P is non-metabolic and stored within the vacuole as inorganic P. Concentrations of stored inorganic P tend to vary to a great extent with external P availability, while concentrations of metabolically active organic P are more stable. The length of time required for a P deficiency to affect plant growth depends on the extent of P reserves in the plant. High concentrations of stored P from the seed or from luxury uptake early in the season will produce reserves of available P that can protect the plant from short-term P deficiencies later in the plant’s life cycle. Under P stress, the inorganic reserves are depleted while the metabolic phosphate levels remain essentially unaffected.