Phosphate rock fertilisers to enhance soil P status and P nutrition on organic cropping farms : a thesis presented in partial fulfilment of the requirements for the degree of Master of Plant Science at Massey University

Abstract

The soils used by the East Coast Organic Producers Trust (ECOPT; the grower group that this study is targeted towards) have exceptionally low soil Olsen P concentrations (ca. 6 mg/L). These and other limitations (e.g. poor weed and pest and disease control) result in many ECOPT growers being unable to produce economic yields on anything other than small scale gardens. Fertilisers and manures are seldom used by these growers, which exacerbates the problem. Thus, the object of this research was to provide information to ECOPT on which fertilisers and application strategies would provide the best returns on their phosphorus (P) fertiliser investment. The experimental work was carried out in two parts. A laboratory study tested a range of phosphate rock (PR) based fertilisers and application rates; Ben Guerir reactive phosphate rock (RPR; 67, 133, 267, 533 and 1,333 mg P/kg soil), BioPhos and BioSuper (267 and 1,333 mg P/kg soil) and a no fertiliser Control. Soil fertiliser mixtures were incubated for 155 days and periodic measurements of PR dissolution, soil pH and Bic-P (analogous to Olsen P but expressed in µg/g) were undertaken. The field study used fewer application rates and two application methods; banded and broadcast. Broadcast plots were applied at 678 mg P/kg soil (488 kg P/ha); banded RPR was applied at 236, 678 and 1475 mg P/kg soil (40, 115 and 250 kg P/ha respectively) and banded BioPhos and BioSuper at 678 mg P/kg soil (115 kg P/ha). A Control was also included. Fertilisers were applied in October 2004 and changes in soil pH and Bic-P were measured in the broadcast plots only over a 344 day period. Potato (Solanum tuberosum L. cv. Desiree) was the test crop. Regression analysis was used to generate exponential equations to describe the changes in Bic-P over time (∆Bic-P). Differences between fertilisers in the amount of P dissolved and pH fluxes were used to explain the differences in ∆Bic-P. BioSuper dissolved quicker and generated greater ∆Bic-P than RPR and BioPhos, which were similar. Higher application rates produced greater increases in Bic-P than lower rates but decreased the % of P applied that dissolved. The increase in Bic-P over time from fertiliser application was much slower in the field compared with the laboratory. This was put down to differences in experimental conditions; mainly soil pH and soil aggregate surface area. Potato tuber yield (mean = 35 t/ha) did not respond to any of the fertiliser treatments despite a significant increase in P concentration of the shoots mid-way through the season in all broadcast treatments (shoot P concentration was not analysed in the banded plots). Water and N availability were the main limiting factors in this season as the crop was not irrigated and soil N supply was insufficient to produce a full canopy. Phosphorus response curves generated using the fertiliser response model PARJIB (Reid, 2002), and an economic analysis, indicated that for RPR and BioPhos the optimum economic application rate was 200 kg P/ha and for BioSuper it was 100 kg P/ha (applied every third and second year respectively).