- Chaves, Dialma Miler
University of Maine Graduate School
The present, investigation was conducted in order to test, the hypothesis that phosphorus availability is dependent on the adsorption maximum, as determined by the Langmuir sorption isotherm, and quantity-intensity parameters. A greenhouse experiment involving four Maine acid soils, four degrees of phosphorus saturation (0, L2.5, 25, and 50 percent) as determined by the adsorption maxima, and two tree species: jack pine (Pinus banksiana Lamb.) and hybrid Poplar (Populus x euramericana cv. Heidemi j) was designed to test this hypothesis. The adsorption maxima as determined by the Langmuir binary equation were 3.94, 4.43, 5.65, and 3.29 mg P/g soil for soiLs 1, 2, 3, and 4, respectively. The solubility products attributed to variscite were much less than the values reported in the Literature for the mineral variscite. The amount of available phosphorus as determined by the Bray No. 1 method was highly correlated with the adsorption maxima (r= 0.996). The Bray No. 1 phosphorus also increased (r= 0.966). The increase in pH and CEC values resulting from increasing the percent phosphorus saturation suggests the exchange of aquo and hydroxo groups for phosphate ions on the surface of clays and aluminum and iron compounds' (Bray No. linearly with percent phosphorus saturation The optimum phosphorus concentration in soils I value) for jack pine yield was about 250 mg P/kg of soil. For hybrid poplar the optimum phosphorus concentration was in excess of 250 mg P/Rg of soil. The optimum phosphorus concentration in jack pine tissues appeared to be approximately A.27 percent. It was not possible to establish the optimum phosphorus concentration in hybrid poplar tissues, because the phosphorus concentration did not reach a maximum even at 50 percent phosphorus saturation' phosphorus concentration in the soil solution increased sharply with phosphorus saturation. The monocalcium phos-phate potentials increased from -10.16 to -7.57 kcaL/kg when the percent phosphorus saturation increased from 0 to 50 percent. This supports the hypothesis that the energy of adsorption decreases as phosphorus saturation increases. The optimum phosphorus concentration in jack pine was attained for soils Lr2, and 4 when the monocalcium phos-phate potential values were about -9.0 kcal/eq. For hybrid poplar the optimum phosphate varied from -9.4 to -8.6 kcal/ew.
The quantity-intensity relationships were consistently curvilinear when the quantity factor was phosphorus by the Bray No.1 method and the intensity factor was either phosphate ion activity in the soil solution or monocalcium phosphate potential. However, the q-I relationship became linear when the intensity factor used was log ah2po4-. This made it Possible to estimate the buffer capacities of the soils directly from the slope of the regression line. The buffer capacities were 525, 564, and 412 mg kg-1 mole-1 1, for soils 1,2,3, and 4, respectively.