Three forms ofTrifolium ambiguum(a diploid, a tetraploid, and a hexaploid) and a single cultivar of Trifolium repens (cv. Grasslands Huia) were grown in a factorial field experiment for five years near lindabyne, N.S.W. Monocalcic phosphate at five applied rates had been either mixed through the surface 10 cm of soil or spread on the surface of the experimental area before planting. The plants were sampled in the autumn and spring of each year to measure growth of tops; roots were excavated 1 year after planting. Extractable P levels in the soil were measured on five occasions. Both species responded markedly to applied P.T. repensproduced much more foliar dry matter thanT. ambiguumin the first two years at all levels of P; dry conditions then decimated the stand and the subsequent weak regeneration was almost entirely from seed.T. ambiguumgrew so little in the first year that responses to P were not measurable but the persistence of this species was such that in later years, growth in the absence of applied P and response to P were greater than for T. repens. The hexaploid became more responsive to P than the diploid and tetraploid forms. Root production byT. ambiguumwas up to three times that ofT. repens1 year after planting, the greater development taking place in each of the 0-5, 5-10, and 10-15 cm layers of soil sampled. In both species, shoots responded almost linearly to applied P over the range 0-160 kg.ha-1of P. Response byT. repensroots was also linear over that range but roots ofT. ambiguum(especially the diploid) were less responsive. Mixing the fertiliser through the surface 10 cm was slightly more effective than surface application for shoot production by both species, especially in the early years. Only the hexaploid benefitted in root production from mixing. It is concluded that an insensitivity to P supply previously observed in young stands ofT. ambiguumresulted from the inherent slow early growth of the above-ground parts and not to positional unavailability of the surface-applied P.
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