AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 1997, No. 7
Starter fertilizer jump-starts grain sorghum. One of the most consistently observed effects of starter fertilizer is increased early season sorghum growth. Increases can be dramatic. Studies conducted throughout the U.S. have shown that early growth dry matter yields of starter-fertilized sorghum range from one to 10 times that of sorghum with no starter fertilizer. This increased early growth often leads to earlier maturity, lower grain moisture, and higher yields. The magnitude of harvest benefits depends on several factors.
Tillage and planting date affect the magnitude of yield response to starter fertilizer. Crop growth in the southeastern U.S. is slower when sorghum is planted early in the season under reduced-tillage systems. Starter fertilizer containing nitrogen and phosphorus helps overcome the cool, wet conditions encountered under these circumstances. Research showed a 31 bushel per acre yield increase in a no-tillage system compared to an 11 bushel per acre increase in a conventional-tillage system. In the Great Plains, studies show that preplant subsurface band applications of nitrogen, phosphorus, and potassium produce better nitrogen uptake efficiency under reduced and conventional tillage systems and better phosphorus uptake efficiency in reduced tillage systems.
Nutrient soil test levels affect sorghum response to starter fertilizer. Sorghum grain yield responses to starter fertilizers containing phosphorus and potassium are most likely when soils test low to medium in these nutrients; however, responses to phosphorus have occurred even at very high soil test levels. A two-year Kansas study demonstrated significant yield increases and maturity advancement from applications of starter fertilizer containing both nitrogen and phosphorus on a soil with 45 parts per million (ppm) Bray-1 phosphorus (very high). Nitrogen and sulfur in starter fertilizer are more likely to produce yield responses on sandy soils low in organic matter. Zinc may be needed when the zinc level in the soil is insufficient to meet early season plant demand.
Sorghum grown in acid soils responds to banded P. In acid soils, aluminum becomes soluble and is taken up by the plant. Under very acid conditions, solution aluminum can reach levels that adversely affect sorghum growth. The best way to correct this situation is to raise soil pH with lime. However, in situations where lime applications are not feasible, band applications of phosphate may be used as an alternative. Phosphate reduces aluminum toxicity by complexing the aluminum. Data from a two-year Kansas State University study showed a 6 to 34 bushel per acre grain yield increase from banded P. This response occurred on a soil testing high in extractable aluminum and very high in soil test phosphorus.
Sorghum hybrids respond differently to starter fertilizer. A two-year study in Kansas investigated the effects of applying starter 2 inches to the side and 2 inches below the seed at a rate of 30 pounds nitrogen per acre and 30 pounds per acre P2O5. Only one of the four early maturing hybrids showed a significant yield increase (14 to 27 bushels per acre) to starter fertilizer. All four of the medium-maturing group hybrids and three of the four late-maturing group hybrids showed significant responses to starter. The yield increases by these responsive hybrids ranged from 9 to 29 bushels per acre.
More emphasis is being placed upon earlier planting and conservation tillage practices. Research has shown dramatic responses to starter fertilizer under these conditions. Increases in grain sorghum yield and earlier maturity make starter fertilizer applications a profitable management practice.