EFFECTS AND INTERACTIONS OF P-FERTILIZER FORMS AND RATES OF LIME ON ACLOVER RYEGRASS SWARD/

Triple superphosphate (TSP), Arad phosphate rock (APR), and ''Longlife '' super-phosphate (LL) were compared for their effectiveness as P fer tilisers on a ryegrass/clover sward in New Zealand to which lime had b een applied at 0, 1.25, 2.5, and 5 t ha(-1). The P fertilisers were ap plied annually for five (LL) or six (TSP and APR) years at 24 kg P ha( -1) yr(-1); there was also a nil P control. The trial design was a fac torial of four P treatments x four lime treatments. DM yield responses were initially negligible but increased to a maximum of 27% by Year 5 . Mean response to P fertiliser was 5% in spring and 16% in summer and autumn. There were significant DM responses to TSP and LL from Year 3 onwards and to APR from Year 4 onwards. Over the final three years, T SP and LL yielded significantly more DM than APR, but there was no sig nificant difference in DM yield between TSP and LL. Herbage %P, herbag e P uptake, and soil Olsen P were much more sensitive discriminators b etween P fertilisers than was DM production and showed significant dif ferences between all fertilisers throughout most of the experiment, wi th the ranking of effectiveness being TSP>LL>APR>nil P. All measuremen ts showed APR to be a relatively ineffective P fertiliser, giving only 41% of the response to TSP in Year 6 when averaged over all lime trea tments. Herbage chemical analysis showed effectiveness of APR to be ma rkedly reduced by 2.5 and 5 t ha(-1) lime. Calculations based on resid ual PR in soil at the end of the experiment indicated that dissolution rates of APR with nil, 1.25, 2.5, and 5 t ha(-1) lime were 23%, 20%, 7%, and 9% per year, respectively. The slow dissolution of APR was att ributed to its relatively low reactivity as measured by solubility in formic acid. The PR content of LL (North Carolina PR) also proved very ineffective, with an average dissolution rate of 13% per year which w as unaffected by lime application rate. It is suggested that granulati on of LL depressed dissolution of its PR component. Lime at 5 t ha(-1) raised soil pH from 5.6 to 6.5 one year after application, with inter mediate rates having a pro rata effect. Soil pH subsequently declined, but with considerable year-to-year fluctuations, at rates proportiona l to the lime application rate. There were no positive DM responses to lime but there were significant depressions in Years 5 and 6. Lime re duced herbage %P in APR treatments only. Lime significantly reduced Ol sen P in control and all fertiliser treatments throughout the experime nt, but only with APR was there an associated reduction in herbage %P.