THE AGRONOMIC EFFECTIVENESS OF REACTIVE PHOSPHATE ROCKS 1 - EFFECT OFTHE PASTURE ENVIRONMENT

The agronomic effectiveness of directly applied North Carolina reactiv e phosphate rock was determined for 4 years from annual dry matter res ponses at 26 permanent pasture sites across Australia as part of the N ational Reactive Phosphate Rock Project. Fertiliser comparisons were b ased on the substitution value of North Carolina reactive phosphate ro ck for triple superphosphate (the SV50). The SV50 was calculated from fitted response curves for both fertilisers at the 50% of maximum yiel d response level of triple superphosphate. The reactive phosphate rock was judged to be as effective as triple superphosphate in the Ist yea r (and every year thereafter) at 4 sites (SV50 >0.9), and was as effec tive by the 4th year at 5 sites. At another 9 sites the reactive phosp hate rock was only moderately effective with SV50 values between 0.5 a nd 0.8 in the 4th year, and at the final 8 sites it performed poorly w ith the 4th year SV50 being less than 0.5. Pasture environments where the reactive phosphate rock was effective in the 1st year were: (i) th ose on sandy, humic or peaty podsols with an annual rainfall in excess of 850 mm; (ii) those on soils that experienced prolonged winter inun dation and lateral surface flow; and (iii) tropical grass pastures in very high rainfall areas (>2300 mm) on the wet tropical coast on North Queensland. The highly reactive North Carolina phosphate rock became effective by the 4th year at sites in southern Australia where annual rainfall exceeded 700 mm, and where the surface soil was acidic [pH (C aCl2) <5.0] and not excessively sandy (sand fraction in the Al horizon <67%) but had some phosphorus (P) sorption capacity. Sites that were unsuitable for reactive phosphate rock use in the medium term (up to 4 years at least) were on very high P-sorbing krasnozem soils or high P -sorbing lateritic or red earth soils supporting subterranean-clover-d ominant pasture, or on lower rainfall (<600 mm) pastures growing on so ils with a sandy Al horizon (sand component >84%). No single environme ntal feature adequately predicted reactive phosphate rock performance although the surface pH of the soil was most closely correlated with t he year-4 SV50 (r = 0.67). Multiple linear regression analysis found t hat available soil P (0-10 cm) and the P sorption class of the surface soil (0-2 cm), together with annual rainfall and a measure of the sur face soil's ability to retain moisture, could explain about two-thirds of the variance in the year-4 SV50. The results from this Project ind icate that there are a number of specific pasture environments in the higher rainfall regions of Australia where North Carolina reactive pho sphate rock can be considered as an effective substitute P fertiliser for improved pasture.