SOIL-P RESOURCES, PLANT-GROWTH AND ROOTING CHARACTERISTICS IN NUTRIENT POOR UPLAND GRASSLANDS

A field study was undertaken to establish the demand for P by mixed he rbage, manipulated by cutting regimes, and the extent to which orthoph osphate alone in soil solution could meet this demand from three cambi sols derived from different parent materials. Differences in soil type s were sufficient to produce significantly different rooting patterns at each site. Yields for 7- and 10-cm treatments generally exceeded th ose for swards cut to 2- and 4-cm. The highest yields were from plots cut once at the end of the season, or when herbage was cut in June and October only. Yields fell in the second season by an average of 30%. Two cuts in the season resulted in almost twice the P uptake compared with other treatments, leading to the view that a ''silage'' cut stimu lated root growth. Rooting was deepest in Tarves Association soil (Dys tric cambisol), densest in Insch Association soil (Eutric cambisol) an d intermediate in Foudland Association soil (Dystric cambisol) but her bage yield at each site was similar Whole season mean P and N content in roots ranged from 1.0 to 3.4 and from 8.1 to 27.9 mg g(-1) dry weig ht, respectively. The lowest values were in once cut herbage and were half those in herbage cut in June and October only. Data for the total P resources of the soils, extractable P, and shoot and root P at each site are presented together with data for P in soil solution (princip ally organic) from an associated soil solution study. There was a disp arity between daily uptake and orthophosphate in soil solution. These findings suggested that it was probable that soluble organic forms of P are important for P nutrition in these nutrient poor soils, and coul d account for the excess of observed P uptake (from soils low in P) ov er that predicted by mechanistic mathematical models.