DETECTION AND CORRECTION OF MIDSEASON P DEFICIENCY IN IRRIGATED POTATOES

Abstract

Three experiments were performed in an automatic rainshelter and two in the field to determine the role of soil moisture management and phosphorus (P) fertilization in controlling P nutrition of potatoes (Solanum tuberosum L. ’Russet Burbank’). The rainshelter experiments indicated that permitting the upper layer (25 cm) of soil to remain dry during the early part of the growing season depressed the total P concentrations in the leaf blades at the 10% bloom stage to well below the sufficiency range of 0.45 – 0.50%, in spite of high P application rates at planting. Relieving stress at 10% bloom and maintaining soil water potential between −60 and −20 kPa until harvest significantly increased P concentrations. Tuber yields were only slightly less than on those soils without water stress throughout the growing period, provided ample P had been applied. By delaying stress relief in the upper soil layer for 3 wk, 6 wk, or until maturity, tuber yields were reduced 28, 47 and 49% respectively. Without P fertilization of this P-deficient soil at planting, leaf-P levels at 10% bloom were very low (0.26%), but application of P at this stage (banded, broadcast, or in solution) increased leaf-P concentrations and yields were similar to treatments receiving P at planting. Trimetaphosphate was particularly effective in increasing P concentrations in the leaves. In the two field experiments, tuber yields were high on all plots and treatment differences were small, even though leaf-P concentrations were relatively low. However, in the highest-yielding treatment (banded at planting) leaf-P levels averaged from 0.40 to 0.49% which is close to that considered as a sufficiency range (0.45 – 0.50), reported previously. From the practical standpoint, leaf analysis at the early bloom stage can be used to detect P deficiency, which may be caused by inadequate P fertilization or early season soil water stress. If soil and fertilizer P are insufficient, immediate application of fertilizer P will correct deficiencies and enhance yields if adequate soil water is also provided. Soil moisture stress from early bloom to near maturity should be avoided with this crop to obtain efficient use of applied P and maximum yields of tubers.Key words: Leaf analysis, P fertilizers, trimetaphosphate, soil water stress, automated rainshelter.