Relationships between Forms of Soil Phosphorus, Some Indexes of Phosphorus Availability and Growth of Sudangrass in Greenhouse Trials

Sudangrass (Sorghum sudanense) grown in two greenhouse trials was used to measure P status and response to added P of 29 soil horizons. Forage was harvested six tinles over a period of 295 days. Chemical methods of assessing soil P status were total P, Bray No. 1 soluble P, and Olsen sodium bicarbonate soluble P. Other methods for assessing P status were Fried and Dean “A” and Dean “a” values. The chemical forms of P determined were aluminum, iron, calcium, and organic phosphates. Yield of dry matter, dry matter response to added P, percentage yield, P content of forage, yield of P, response of yield of P to added P, and recovery of added P were used as plant growth parameters for linear correlatiou and regression with chemical methods of assessing soil P status, forms of soil P, and with “A” and “a” values. Olsen sodium bicarbonate soluble P, Bray No. 1 soluble P, “A’, and “a” values correlated significantly with aluminum, iron, organic and total phosphates. Aluminum phosphate explained 87, 86, 78, 76, and 81% of the variations in Olsen sodium bicarbonate soluble P, Bray No. I soluble P, “A” value, “a” value first harvest, and “a” value six harvests, respectively. Yield of dry matter, percentage yield, and yield of P correlated significantly with aluminum, iron, organic and total phosphates, Olsen sodium bicarbonate soluble P, Bray No. 1 soluble P, “A” value, and “a” values. Dry matter response to added P and P content of forage correlated significantly with the same methods of assessing soil P status, except these did not correlate with total P content. Response in yield of P to added P and percent recovery of added P did not correlate significantly with any of the methods of assessing soil P status nor with forms of soil P. The level of calcium phosphate was high in all soils but did not correlate with plant growth parameters and was indicative of little soil weathering.