Foliar - Applied Phosphorus Fertilization

Foliar fertilization with phosphorus is generally not practiced to the extent that it is done with nitrogen and micronutrient fertilizers although a limited amount of fertilizer phosphorus can be absorbed by plant foliage. Silberstein and Witwer (108) tested various organic and inorganic phosphorus-containing compounds on vegetable crops. They generally observed small responses in plant growth, but some compounds caused injury at phosphorus concentrations as low as 0.16%. They concluded that orthophosphoric acid was the most effective foliar phosphorus fertilizer evaluated. Barrel and Black (109,110) reported that several condensed phosphates and some phosphate fertilizers containing phosphorus and nitrogen could be applied at 2.5 to 3 times the quantity of orthophosphate without causing leaf damage. Yields of corn and soybeans (Glycine max Merr.) were higher with tri-polyphosphate and tetra-polyphosphate than with orthophosphate.

Teubner (111) reported that although about 12% of the phosphorus in the harvested plant parts of some field-grown vegetable crops could be supplied through multiple foliar sprays, foliar phosphorus fertilization did not increase total phosphorus absorbed or crop yields. Upadhyay (112) reported that the yield of soybeans were highest when all fertilizer phosphorus was soil-applied, intermediate where 50% of the phosphorus was soil-applied and 50% foliar-applied, and lowest where all the phosphorus was foliar-applied.

Some research suggests that phosphorus in combination with other nutrients might delay senescence and increase yields, but results are inconsistent. Garcia and Hanway (113) reported that foliar applications of N, P, K, and S mixtures during seed filling seemed to delay senescence and increase yield in soybean and the complete mixture produced greater yields than foliar sprays where the mixture was incomplete.

Subsequent work with soybeans by others ranged from no-yield response (114) to yield reduction (115) for foliar mixtures containing phosphorus. Similar negative responses have been obtained with other crops. Harder et al. (116,117) observed temporary decrease in photosynthesis and a decrease in grain yield of corn (Zea mays L.) receiving foliar N, P, K, and S. Batten and Wardlaw (118) reported that applying monobasic ammonium phosphate to the flag-leaf of phosphate- deficient wheat (Triticum aestivum L.) delayed senescence but failed to increase grain yield.

Because only a modest portion of the crop's total phosphorus requirement can be met by foliar application and foliar fertilization does not produce consistent positive responses where residual soil phosphorus or soil-applied fertilizer phosphorus is sufficient, foliar fertilization with phosphorus is seldom recommended as a substitute for soil fertilization practices.