Planning a soil fertility strategy for grain sorghum has many of the same requirements as corn. Although many producers view grain sorghum as a low maintenance crop, with its deep fibrous root system, sorghum responds well to nutrient applications, especially in lower testing soils. Table 1 shows the typical nutrient removal for a 100-bushel per acre sorghum crop. Of these nutrients, the key elements include nitrogen, phosphorous, potassium, zinc, and sulfur.
Table 1. Approximate quantity of nutrients in a 100 bushels per acre sorghum crop
|Nutrient||GRAIN (lbs.)||STOVER (lbs.)|
Source: Adopted from National Plant Food Institute
Soil testing is the basis for determining which of these nutrients will likely limit production. Soil test results will allow you to develop and fine-tune a sound fertility management plan. Balanced fertility programs improve water use efficiency (drought tolerance) and grain yield, at the lowest possible cost. Fertilizer has a significant influence on water use, as illustrated in Table 2.
Table 2. Optimum Fertility Improves Water Use Efficiency
|Not Fertilized||Fertilized||Increase Due
|Grain yield / inch of water||2.99 cwt. or 5.3 bushels||3.81 cwt. or 6.8 bushels||28%|
|Grain yield / acre||33.5 cwt. or 60 bushels||46.0 cwt. or 82 bushels||36%|
Properly fertilized sorghum used an average of one inch more water than unfertilized sorghum but produced 12.5 hundred weight (22 bushels) more grain per acre.
Row applied starter fertilizer can maximize uptake efficiency for nutrients such as phosphate, zinc and sulfur in low testing soils. The most dramatic visual response to starter fertilizer occurs when soils are cool at planting time. Sorghum planted under cool soil conditions can show a significant early growth response when starter fertilizer is properly applied.
The benefits of rapid early growth include more uniform stand establishment and plant size. Early growth response does not always increase grain yield but may result in earlier maturity of the crop. Earlier flowering can improve yield in years of early frost. The effect of starter fertilizer is most often observed on grain sorghum in areas where nights are cooler. Earlier maturity may also result in slightly drier grain at harvest (one or two percentage points lower grain moisture).
The rate of starter fertilizer depends on the salt content, or index, of the fertilizer, the distance between the fertilizer and the seed, and the soil texture. Use of pop-up fertilizer placed in direct contact with the sorghum seed is more risky, but can be done successfully by precisely metering a lower rate. Do not place urea or ammonium thiosulfate in direct contact with the seed.
The salt index of a fertilizer can be determined by adding the rate of nitrogen (N), the rate of potassium (K20), and one half the rate of sulfur (S) applied. For example, if nine gallons per acre of 7-21-7 fertilizer, weighing 11.0 pounds per gallon is used, then 99 pounds of material is applied per acre. At 100 pounds per acre, a total of seven pounds of nitrogen (.07 x 100) and seven pounds of potassium (0.7 x 100) would be applied per acre. The estimated salt index would be 14 pounds per acre. The phosphorus content of the fertilizer is not considered when the salt index is calculated. Salt index limits are listed in Table 3. These salt limits are designed to provide safe conditions for all environments, with rare exceptions. In general, the application of excessive amounts of N, K20 and S too close to the seed will delay grain sorghum emergence and reduce stand.
Table 3. Salt Index (lb./acre)
|Placement||Sandy Soils - Salt Index||Non-Sandy Soils- Salt Index|
|With seed (pop-up)||5||5|
|¼ to ½" from seed||10||10|
|1". from seed||20||40|
|2" from seed||20+||40+|
Source: Univ. of Nebraska-Lincoln
Bill McClure holds a Bachelor of Science degree in agronomy from the University of Nebraska. He currently serves as a Technical Product Manager and is involved in sorghum production with growers in Kansas, Oklahoma, and Texas. Prior to his current role, he was a Field Sales Agronomist involved in sorghum production with growers in Nebraska. He has been with Pioneer Hi-Bred since 1990.