Evaluating the efficacy of various phosphate fertiliser sources for oil palm seedlings

Isotope dilution techniques were used in a glasshouse experiment to compare seven P sources for oil palm seedlings grown on Rengam series soil (Typic Paleudult). The P sources were triple superphosphate (TSP) and six phosphate rocks from North Carolina, USA (NCPR), Tunisia (Gafsa PR), Jordan (JPR), Morocco (MPR), Christmas Island (CIPR) and China (CPR). The percent P derived from fertilisers (%PdfF) in the 3, 6, 9 and 12 months of growth ranged from 81% to 99%, indicating the poor P supplying power of the soil used. TSP was far superior than PR in supplying the required P at all times of measurement. Total amount of P taken up during the 12 months growing period was equivalent to 15.0% of the added P as TSP, it was 5.2% from NCPR, 4.2% from JPR, 4.1% from MPR, 3.2% from GPR, 4% from CIPR and 2.2% from CPR. The PR effectiveness based on the amounts of fertilizer P taken up by the oil palm seedlings at 12 months of growth was in the sequence of triple superphosphate > North Carolina PR > Gafsa PR Jordan PR Morocco PR Christmas Island PR > China PR. This was due to the reactivity of these P sources when applied into the soil, triple superphosphate being water soluble is immediately available. PR sources reacted with the soil solution with time, making P slowly available. PR solubilised by neutral ammonium citrate (NAC) expressed as percentage of rock was shown to correlate better than 2% citric acid and 2% formic acid with plant P uptake. Thus this method of extracting P from PR can be used as a basis for comparing PR effectiveness to oil palm seedlings.