Effects of different levels of zinc and phosphorus on the growth of subterranean clover (Trifolium subterraneum L.)

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Abstract
Four water culture experiments are described in which the effects of the addition of all combinations of graduated amounts of phosphorus and zinc on the growth of subterranean clover were studied.In the first experiment, the variety Dwalganup was used, and in the other experiments the varieties Clare and Edenhope. Varietal differences in the symptoms of zinc deficiency are described. Phosphorus toxicity symptoms occurred in the variety Clare but not in Edenhope. The effects of increases in zinc or phosphorus level, or both, on growth of top + root (T + R) and on percentage top are described. A highly significant variety x phosphorus interaction effect on T + R was recorded at adequate zinc levels, which indicated that the phosphorus requirement of Edenhope was considerably greater than that of Clare. By contrast, tolerance to high levels of zinc in the substrate was greater in Clare than in Edenhope. In the presence of added phosphate, the initial T + R response to small, but still insufficient, additions of zinc to the substrate occurred in root growth more than in top growth, whereas when a level of zinc adequate for normal growth was supplied, top growth of the plants benefited more than root growth. Highly significant zinc x phosphorus interaction effects on T + R occurred. No relationship existed between zinc concentration and the occurrence of zinc deficiency symptoms. The critical zinc level was not a point of narrow range, but included a wide range of values, depending on the age of the plants and the phosphate level. The depression in T + R induced by high phosphate when the zinc level was insufficient was not due to a decrease in zinc concentration resulting from the high phosphate treatment, but such plants gave a high value for the phosphorus/zinc ratio. The results suggest that zinc is essential to phosphate utilization by the plant. Evidence of redistribution of phosphorus from tops to roots, and of decreases in the total amount of phosphorus in phosphorus-deficient plants, was obtained in one experiment between days 27 and 60.