Phosphate sorption was measured by the method of Barrow (1980) using a laboratory incubation procedure for up to 60 d on four soils which had different mineralogies but medium to high phosphate retention. All the soils had slow reactions where phosphate sorption continued, but at a decreasing rate, with time. The rate of decrease in the slow reactions was similar on all the soils. Phosphate became less available to plants during the slow reactions, and results of a pot trial with white clover showed that, on all the soils, phosphate incubated with the soils for 218 d was about 65% as effective as phosphate incubated for 10 d.When 700 mg P kg-' was added to allophanic soils (Andisols), about 100 mg kg-l was strongly adsorbed, about 200 mg kg-I became unavailable in about 200 days and the remainder was weakly adsorbed. A similar result was obtained on Waiarikiki soil (Inceptisol), which contained ferrihydrite and Al-humus as the predominant reactive species. On the Kerikeri soil (Oxisol) about 150 mg P kg-' became unavailable with time as a result of reactions with geothite, hematite and Al-humus.The phosphate uptake by the microbial biomass was similar to the uptake by the clover, and immobilization of phosphate in the biomass can contribute to the loss of availability of phosphate in soils.

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The studies performed in 1982 and 1983 indicated that the response to 2,4-D varieA significantly among the species (Hume 1987

Development and nodulation of the forage legume sainfoin (Onobrychis viciifolia Scop.) were observed under glasshouse conditions for 50 days after sowing. Growth changes caused by different nitrogen (N) nutrition were investigated from Day 50. The quantity of nodule tissue produced compared favourably with values reported for a range oflegumes. In spite of this, the symbiotic N 2fixing system of sainfoin appeared unable to provide sufficient N to meet the plant's requirements. The relative growth rate of the whole plant, and the relative N accumulation rate were substantially lower in plants dependent on N 2 fixation compared to plants provided with 210 mg/litre nitrate N. Development of the below-ground portion of the plant was rapid relative to that of the top. Relative growth rates, top: root + nodule ratio, and leaf area ratio of sainfoin were lower than values reported for other legumes. The mode of N nutrition affected dry matter distribution, with proportionately much greater below-ground development in plants dependent on symbiotic N 2 fixation. This resulted in lower leaf area ratios and hence probably reduced the availability of energy for symbiotic N 2 fixation. The application of 35 mg/Iitre nitrate N to plants previously solely dependent on symbiotic N 2 fixation appeared to substitute for, rather than supplement, N 2 fixation. Applying 35 mg/Iitre nitrate N to plants previously given 210 mg/Iitre nitrate N resulted in nodulation, the initiation of symbiotic N 2 fixation, and a substantial change in dry matter distribution in favour of the root system.