| Chemical composition, and solubility of five rock phosphates (RP) were evaluated by infrared, X-ray diffraction, and chemical methods. The main mineral found in all sources was fluorapatite. However, the chemical composition of the apatite varied among the various sources. The highest F (4.1%) and CaO (52.2%) and the lowest R(,2)O(,3) content were attained with Morocco RP and the lowest by Brazil RP. The best correlation with plant response was obtained using the absolute citrate solubility index. The unit cell a-dimension decreased as the isomorphous substitution of CO(,3)('2-) and F('-) for PO(,4)('3-) in the apatitic lattice increased. The higher the isomorphic substitution, the more reactive was the rock.; Sources of RP were evaluated using a Redding soil (pH 5.9) cropped to alfalfa in the greenhouse. Various application rates were used. Yield, P concentration, total P uptake and HCO(,3)-P were higher by Morocco RP and lower by Tennessee and Brazil RPs. Both results of the greenhouse and the laboratory rank the reactivity of the rocks as follows: Morocco > Central Florida > Peru > Tennessee and Brazil.; Grain sorghum was grown in potted soils in the greenhouse. Different rates of Morocco and Brazil RPs per pot were either mixed with 33%, 50% or 100% of the soil volume. Mixing RP with the whole soil resulted in the highest yield, P concentration and total P uptake by the Redding soil (pH 5.9). San Timeteo soil (pH 8.4) was not responsive to RP application or method of placement.; Alfalfa seeds were grown in potted calcareous San Timeteo soil. Different S:RP ratios (0:1, 0.5:1, 1:1) were used. Sulfur addition did not affect DM production. However, a moderate change in pH (0.6-0.8 units) was observed. The greatest P concentration was achieved when sulfur was included. No statistically significant differences were found among the different S treatments. |
