Soil Nutrient management
Phosphorus
Forms of Phosphorus Taken up by Plants
Plant roots absorb phosphorus mainly as orthophosphates (H2PO4¯, HPO42¯) which is controlled by soil pH. As soils become more acidic, dihydrogen phosphate (H2PO4¯) is the primary species taken up, but excessive soil acidity can result in phosphorus deficiency as aluminum and iron concentrations increase, “fixing” phosphorus. Conversely, as soils become more basic or alkaline (pH 7.2 to 8.5), hydrogen phosphate (HPO42¯) is the primary species taken up, but a high pH can result in phosphorus deficiency as calcium phosphate precipitates.
Phosphorus Deficiency in Plants
The classic phosphorus deficiency symptom is a reddish-purple coloring of leaves and stems, usually associated with stunting or small non-thrifty looking plants (Figure 10.5). Such deficiency coloring usually occurs early in the growing season. While these classic deficiency symptoms are often associated with phosphorus deficiency, reddish coloring of leaves and stems also can be induced by any disruption of plant sugars. Such coloring often appears naturally in sorghum and sometimes during cold wet springs on corn when no real deficiency exists. Phosphorus deficiency reduces leaf expansion, leaf surface area, and fewer, smaller leaves.
Phosphorus Toxicity in Plants
The buildup of phosphorus in croplands can cause plants to grow poorly and even die. Excessive soil phosphorus reduces the plant's ability to take up potassium and required micronutrients, particularly iron and zinc, even when soil tests show there are adequate amounts of those nutrients in the soil. Because organic farmers cannot use chemical fertilizers, they often apply large amounts of manure to boost their nitrogen levels.
Mobility of Soil Phosphorus
Unlike nitrogen, phosphorus is relatively immobile in soils because it tends to precipitate into solid forms. Clay particles tend to retain or fix phosphorus in soils. Consequently, fine-textured soils such as clay loam soils have a greater phosphorus-fixing capacity than sandy, coarse-textured soils.
Management of Soil Phosphorus
Fixation of soil phosphorus increases with time of contact between soluble phosphorus and soil particles. Consequently, more efficient utilization of fertilizer phosphorus is generally obtained by applying the fertilizer shortly before planting the crop. This practice is especially effective on soils with high phosphorus-fixing capacities. Banding of fertilizer for row crops is also much more likely to increase the efficiency of fertilizer phosphorus on soils of high phosphorus-fixing capacity than on soils of low phosphorus-fixing capacity.
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