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Fertilizing Grain Sorghum
DR. BRENT BEAN
LUBBOCK, TEXAS
The cost of planting grain sorghum is much less expensive than other crops, primarily because of seed price. As a result, many growers assume they can save on other input costs as well. The best example of this involves the use of fertilizer. On a per bushel basis, grain sorghum requires a similar amount of most nutrients as corn. If a grower wants to produce a 100-bushel sorghum crop, the grower must support this yield goal with a supply of adequate nutrients, including soil pH, phosphorus, nitrogen, sulfur and potassium.
Soil pH
This critical component enables the soil to provide nutrients to plants. Optimum nutrient availability occurs within a pH range of 6.5 to 7.5. When pH drops below 5.6, liming is warranted to reduce acidity and improve fertility.
Phosphorus
Phosphorus should remain in a pH range between 6 and 8. When pH falls outside the optimum range, agronomists often recommend applying phosphorus in or close to the seed furrow or in a narrow band a few inches to the side and a couple of inches deep into the soil. By concentrating the phosphorus, the nutrient has less opportunity to become “tied up” by the soil. In addition, common phosphorus fertilizer sources contain nitrogen and sometimes sulfur, which tend to lower the pH within the narrow band to allow greater phosphorus availability. Growers should base the amount of phosphorus fertilizer needed on soil test estimates and yield goal.
Nitrogen
In most farming environments, crops most need nitrogen as a nutrient. Sorghum is no exception, requiring 1.12 pounds per bushel of grain produced, which is very similar to the recommendation for corn. As shown in the graph below, yield becomes less responsive to nitrogen as yield approaches 150 bushels per acre because yield becomes limited by other factors. Growers must base the amount of nitrogen applied on factors including yield goal, the amount of residual nitrogen in the soil profile and the expected nitrogen amount mineralized from soil organic matter. Soil sampling and analysis are critical for this determination and will prevent both the overapplication and underapplication of fertilizer.
Going into the growing season, growers often find it difficult to set a yield goal and, in turn, struggle with calculating the proper amount of nitrogen needed. Since growers do not want to overapply nitrogen, they can manage nitrogen with a multiple-application plan. Growers should apply no more than 50 percent of the anticipated needed nitrogen at preplanting and then adjust the remainder of the nitrogen applied to meet the anticipated yield potential as the season progresses. A multiple-application plan is particularly important on sandy soils where preplant nitrogen can leach out of the sorghum root zone following heavy rains.
Once sorghum is established, two critical growth stages occur when lack of nitrogen can significantly impact yield. The first stage occurs approximately 30 days after emergence, when sorghum enters a rapid growth period and the potential number of grain kernels start to form. Adequate nitrogen must be present to support this rapid growth period. The second stage is at or just prior to the boot stage, which occurs approximately one week prior to heading, when nitrogen encourages good pollination, grain set and grain fill.
Sulfur
An often neglected nutrient in grain sorghum is sulfur, which improves the nitrogen use efficiency and is especially important as yield increases. Growers should strive to keep the nitrogen to sulfur ratio at approximately 15-to-1.
Potassium
In most U.S. soils, potassium is available in adequate supply for the needs of sorghum. However, growers may need to add potassium in very shallow soils with reduced rooting depths and on sandy soils. ∆
DR. BRENT BEAN: Sorghum Checkoff Agronomist
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