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Greenhouse and field experiments were conducted with corn (Zea mays L.) and soybeans (Glycine max L. Merr.) to evaluate the growth response of foliar sprays with several condensed phosphates and P‐N compounds. The maximum concentration of P tolerated in solutions of tri‐ and tetrapolyphosphates applied as sprays in the greenhouse was 1.3% with corn and 1.1% with soybeans. The maximum concentration of P tolerated as orthophosphate was 0.5% with corn and 0.4% with soybeans. With soybeans, the yields of plants sprayed with various condensed P compounds significantly exceeded the yields of the unsprayed control, which would not be classed as P‐deficient on the basis of the P content of the leaves. Spraying the plants, however, increased the P concentration in the leaves. Phosphoryl triamide produced the highest yields of above‐ground dry matter of corn plants in an experiment in which several P‐N compounds and condensed phosphates were brushed on the leaves. Several different condensed phosphates were sprayed on corn and soybeans in a field experiment on a fine loamy, mixed, mesic typic haplaquoll (Webster silt loam). An increase in yield of corn that was statistically significant at the 5% level was obtained from spraying. The yields with tri‐ and tetrapolyphosphate were, respectively, 760 and 754 kg/ha above the control yield of 10,234 kg/ha. With soybeans, the increase in yield due to treatment was significant at the 18% level, and the increase was equivalent to 256 kg/ha above the control yield of 3747 kg/ha.

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The suitability of 32 different P compounds for foliar application was tested by using a new technique in which predetermined quantities of P were applied to a fixed leaf area. This made it possible to obtain quantitative data on absorption of added P and on translocation of absorbed P out of the treated area. Presently foliar applications of P are used less than N, one reason being that no P compound is available which can be sprayed on plants in large enough quantities without damaging the leaves. Several representatives of the condensed phosphates and some compounds containing P‐N bonds or P‐N‐P linkages proved promising. Both group of compounds hydrolyze spontaneously to release orthophosphate, and all share the quality of delayed action. These P sources could be applied at 2.5 to 3 times the quantity of P that could be applied as orthophosphate in a greenhouse experiment without causing leaf damage. The most successful compound for corn (Zea mays L.) was ammonium tripolyphosphate, which could be applied at 370 µg of P/cm2 without causing leaf damage; 66% of the applied P was absorbed within 10 days, and 87% of the absorbed P was translocated out of the area of application in the same period. Other successful compounds for corn were ammonium tetrapolyphosphate and phosphoryl triamide. Soybeans (Glycine max L. Merr.) proved more sensitive than corn and could, in general, tolerate only two‐thirds to three‐fourths of the quantity of the various compounds that could be applied to corn, with the exception of phosphonitrilic hexaamide, which could be applied at 340 µg of P/cm2. Ammonium tri‐ and tetrapolyphosphate could be applied at 220 µg of P/cm2 to soybeans. Rapid intake is suspected to be an important reason for the damage observed with orthophosphate. Several organic P compounds were tested, but none of them was superior to the condensed P sources.