PHOSPHORUS TOXICITY IN TOMATO PLANTS - WHEN AND HOW DOES IT OCCUR

For many years and in most agricultural production systems, phosphorus (P) has been identified as the most frequently occurring essential el ement deficiency limiting crop yields, and it is still an element that continues to receive considerable research attention. However, little has been researched or written about the effects of high P on plants other than that associated mainly with zinc (Zn) nutrition, and iron ( Fe) to some degree, as high P levels are known to interfere with their normal metabolism. Also, P is known to promote manganese (Mn) uptake. At what levels P would be considered toxic has not been clearly defin ed for most crops. The author has observed the occurrence of nutrition al stress in tomato plants when the P level in leaves exceeds 1.00% of its dry matter, mainly occurring for container-grown plants and those being grown hydroponically. This and other data will be presented tha t suggests that 1.00% P may indeed be the critical level between suffi ciency and toxicity for tomato plants. Visual symptoms of P toxicity w ill also be described.