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The Kaipara clay soils and associated soil types occupy some 36 000 ha in Northland. The soils are naturally high in P and P fertilisers have been used sparingly in the past but with increasing farming intensity a fall in soil Olsen P values (to < 20) is occurring and responses to P fertilisers are becoming more common. Lime is widely used. In December 1996, a field trial, conducted over 6 years, was established on a Kaipara clay soil near Ruawai. The first phase (1996-1999) consisted of six rates of P (0, 20, 40, 60, 80, 100 kg P/ha) with or without lime at 5 t/ha applied once. The initial pH was 5.8 and Olsen P was 16. Results indicated a marked rates response to P without lime but with lime a marked Psparing effect occurred equivalent to 50-60 kg P/ha. The effect was consistent over 3 years. An explanation for this finding is due to lime improving plant growth probably by N mineralisation and N uptake, thus giving improved root growth and allowing those roots to explore a greater volume of soil and absorb more P. Plant chemical analyses and soil P fractionation studies indicated that more P was being taken up by plants on the lime/no P plots. The second phase (2000-2003) involved splitting plots and applying additional lime to some to give a pH range from 5.6-7.0. Results indicated no benefit to increasing pH above 6.0 but there was a suggestion that the reapplication of lime itself was beneficial. The P sparing effect of lime will best be utilised where soil Olsen P levels remain above 20.

Lime-phosphate interactions on a Kaipara clay soil in Northland