This study examined the hypothesis that accumulation of P to toxic levels in plant tissues may explain reported effects of high P in increasing the Zn requirement of plant tissues. It also examined the contribution of other mechanisms to the complex interaction of Zn and P fertilizers on plant growth.
Three levels of Zn and P fertilizers were applied to a siliceous and a ferruginous sand with low and high capacities to react with P, respectively. Application of P fertilizer induced or enhanced symptoms resembling Zn deficiency in subterranean clover (Trifolium subterraneum L.) in three distinct ways:
1) On the ferruginous sand but not on the siliceous sand, it induced Zn-deficiency by depressing Zn absorption.
2) Where P limited growth, additional P depressed Zn concentrations and induced Zn deficiency in plants by promoting growth thus diluting available Zn supplies.
3) Where P did not limit growth, additional P increased P concentrations to toxic levels producing necrotic symptoms in old leaves and depressing growth.
Where plant tops had high Zn concentrations (> 40µg/g), P toxicity produced nectrotic symptoms in the absence of any of the characteristic symptoms of Zn deficiency. Where plant tops had low Zn concentrations (< 16µg/g), the necrotic symptoms developed in addition to the characteristic Zn-deficiency symptoms of bronzing and chlorosis of old leaves and “little leaf” of young leaves thus strongly enhancing the apparent severity of Zn deficiency. Zn deficiency strongly enhanced P toxicity at the middle and high levels of P application apparently by limiting plant growth and concentrating the available P in plant tissues.
The enhancement of P toxicity by Zn deficiency explains reports that high P increases Zn requirements of plant tissues. It will occur more easily in plants grown in sand and water culture than in plants grown on soils with some capacity to react with P.