Agronomic Effectiveness of Dolomite Phosphate Rock Fertilizers: A Greenhouse Experiment.

Soil culture experiments were conducted in greenhouse to evaluate the agronomic effectiveness of activated dolomite phosphate rock (DPR) from central Florida. Lettuce (Lactuca sativa) was used as an indicator crop. 2.5 kg soil per pot (dry weight basis) was spiked with 150 mg/kg N (as urea), 120 mg/kg K (as KCl) and 100 mg/kg P. The experiment consisted of 14 different P treatments, including control (no P), original DPR with high- (DPR5, 24% P₂O₅), medium- (DPR1, 21% P₂O₅) and low-grade DPR (DPR7, 18% P₂O₅), activated DPR by HA (HA-DPR1, HA-DPR5, and HA-DPR7) and LSS (LSS-DPR1, LSS-DPR5, and LSS-DPR7), superphosphates made from the DPRs (SP1, SP5, SP7), and commercial di-ammonium phosphate (DAP). Each treatment had three replications. Pre-germinated lettuce seeds were sown and the plants were thinned to 4 seedlings in one week after germination. Soil moisture was maintained at 70% by adding deionized water. After 3-week growth, leaf chlorophyll concentration and plant height were measured, and shoot and root were harvested separately. Morphological properties of fresh roots were measured. The shoots and roots were oven dried and biomass yields were recorded. Total P and other macro-and micro-nutrients (N, K, Ca, Mg, Fe, Mn, Cu, Zn, B, and Mo), as well as toxic metals (Cd, Pb, Cr, Ni, and Co) in shoot and roots were analyzed. Simultaneously, pH, EC, and available Pin soil samples were determined. Results showed that application of DPR powder improved lettuce growth, but failed to support normal growth of lettuce. Addition of DAP or SPs significantly enhanced plant growth, as reflected in the significant increase in plant height and leaf/root biomass yield. Application of activated DPRs resulted in similar or better results, as compared to DAP/SP fertilizers.