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Fluid Journal : Winter 2011
Winter 2011 The Fluid Journal 12 Two California studies demonstrate the complexation of phosphorus (P) with organic acids results in prolonged availability, plus more movement away from point of application. Greater Phosphorus Efﬁciency Results From Improved Mobility and Prolonged Availability The Fluid Journal • Ofﬁcial Journal of the Fluid Fertilizer Foundation • Winter 2011 • Vol. 19, No. 1, Issue #71 Tom Gerecke, Dr. Husein Ajwa, Dr. Charles Krauter, and Jerome Pier Phosphorus fertilization of crops is notoriously inefficient. Phosphorus fertilizer reactions determine the dominant, potentially-labile P forms in soils and hence the proportion of soil P that can be released to the soil solution. In broad terms, P binding to soil particles results from surface adsorption or precipitation. The relative contribution of these two processes is related to soil properties, fertilizer form, and cultural practices. Production agriculture is experiencing increasing scrutiny regarding applied P rates versus crop P demand. Improved P availability allows growers to more closely match application rates to crop demand. In this era of increasing environmental awareness and cost consciousness, applied P fertility research is still under way. Two similar field studies completed in California demonstrate that the complexation of P with organic acids derived from leonardite in a patented, proprietary technology results in prolonged P availability with more movement away from the point of application.Mendota, California This was the site of one study conducted by Dr. Husein Ajwa of the University of California in an off-station field. Its soil was a Cerini clay loam with a pH of 7.8, NaHCO3-P of 12 ppm, and about 1.5% CaCO3. The field was bedded up for processing tomatoes on 60-inch centers on 200-foot-long plots. Methodology. Standard herbicides were used to eliminate weeds and no crop was planted so no roots took up soil P. The soil was pre-irrigated to ensure moisture uniformity. Fertilizer treatments to moistened soil were made once through drip irrigation lines for one hour. Fertigation was followed by 4-3/4 hours of irrigation to ensure all fertilizer was out of the lines and applied uniformly. Further irrigations were applied twice weekly in amounts calculated to generally replace moisture lost to evaporation. Over the six-week duration of the trial, this amounted to about 6 acre-inches of water. At specified time intervals, soils were sampled in a 3" x 3" grid pattern down and away from an individual drip emitter at right angles to the drip line. Soil samples were taken to the lab, extracted by each of four different methods to test the many soil P fractions present. Fractionation methods include extraction with: • water or dilute salt solution • buffered alkaline solutions (sodium bicarbonate) • dilute concentration of strong alkaline solutions (sodium hydroxide • dilute concentration of strong acids coupled with heat (HCl). The water-extractable fraction represents the P present in the soil solution. This available P fraction is very small, but readily accessible to plant roots. The sodium bicarbonate- extractable fraction represents the adsorbed P and the highly soluble calcium phosphate pool in the soil, accounting for one to five percent of the total soil P. The sodium bicarbonate- extractable P method was developed for estimation of plant-available soil P, and is commonly used in alkaline soils as an index of soil P status to determine fertilizer P requirements. The sodium hydroxide-extractable fraction represents the P bound in iron oxides. The organic P and total P fractions can be extracted by boiling with NaOH and with HCL in a digestion block, respectively. The lab methodology used resulted in excellent P recovery, accurate to 0.25 ppm. Samples were taken at 2, 14, 28, and 42 days after the application. The trial was conducted with four replicates of each of the following treatments in a randomized block design: • 50 lbs/A P2O5 from ammonium polyphosphate (10-34-0) • 50 lbs/A P2O5 from ortho phosphate- merchant grade phosphoric acid (0-52-0)
Early Spring 2011