Relationship of phosphorus isotherms to sodium bicarbonate extractable phosphorus as affected by soil properties
Klages, M.G.; Olsen, R.A.; Haby, V.A.
Soil Science 146(2): 85-91
We compared sodium-bicarbonate-extractable P and the P requirement based on a sorption isotherm. The latter more accurately predicted yield response in a set of dryland winter wheat fertilizer trials. When compared in soil samples from all parts of Montana [USA], there was no apparent relation between levels of the two tests. Graphical presentation suggested a negative relationship when soils with allophane clays were deleted from the group. Two other factors that seemed to affect the relationship were clay content and the presence of carbonates. Regression of values by the two tests was significant only in samples with carbonates. Phosphorus requirement by isotherm was also significantly related to NH4OAc-extractable Ca in samples with carbonates and to DTPA-extractable Al in carbonate-free samples. Phosphorus was added to selected soil samples, and change in P status was measured by both tests after 15 weeks of wetting and drying. The experiment gave differing results when evaluated by each of the two tests. High clay reduced recovery of added P in a group of soils dominated by smectite and illite, as measured by NaHCO3 extraction, although initial levels were similar. High clay increased initial isotherm P requirement of these soils, but did not affect recovery of added P. Dominant clay minerals also affected recovery of added P by increase in NaHCO3-extractable P, but not by decrease in isotherm P requirement. Initial NaHCO3-extractable P and isotherm P requirement in these soils were both highest in soils with high DTPA-extractable Al and low NH4OAc-extractable Ca.