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Importance of Soil Organic and Inorganic Phosphorus to Plant Growth at Low and High Soil Temperatures

ABSTRACT

The plant-available P in a group of soils was estimated at soil temperatures of 20° and 35° C on the assumption that the response to P obtained in a green-house experiment followed the Mitscherlich law. By multiple linear regression the dependence of the plant-available P on the following soil P fractions was calculated: (1) inorganic P soluble in a solution 0.03 N to NH₄F and 0.025 N to HCl; (2) organic P soluble in hot 1% K₂CO₃ and hydrolyzed by KOBr; (3) organic P soluble in hot 1% K₂CO₃ and not hydrolyzed by KOBr; and (4) organic P not soluble in hot 1% K₂CO₃. At 20° C the plant-available soil P was determined by inorganic fraction 1. Organic fraction 2 had no appreciable effect. At 35° C the plant-available soil P was determined by both inorganic fraction 1 and organic fraction 2. Organic fractions 3 and 4 were of no importance at either temperature. A probable cause of the observed results is that at 20° C the rate of organic P mineralization was low and the plant was dependent on inorganic fraction 1 originally present. At 30° C the plant was supplied with inorganic fraction 1 plus additional inorganic P from the mineralization of organic fraction 2.