Sorghum Fertility Management
By Bill McClure
Introduction
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.) |
|---|---|---|
| Nitrogen (N) | 84 | 95 |
| Phosphorus (P205) | 42 | 20 |
| Potassium (K20) | 22 | 107 |
| Sulfur (S) | 8 | 13 |
| Magnesium (Mg) | 7 | 10 |
| Calcium (Ca) | 1.4 | 18.9 |
| Copper (Cu) | .01 | .02 |
| Manganese (Mn) | .06 | .11 |
| Zinc (Zn) | .07 | .14 |
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 To Fertilizer |
|
|---|---|---|---|
| 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.
Starter Fertilizer (2X2)
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
Nitrogen
Nitrogen is the nutrient most often limiting in sorghum production. Under low rainfall conditions with low yield potential, 30 to 60 pounds of nitrogen per acre may be adequate. In productive irrigated situations, up to 200 pounds of nitrogen will be required. In all situations, a nitrogen credit should be given for residual nitrogen in the soil (from soil test), nitrogen content in irrigation water, and nitrogen contribution from a previous legume crop or applied manure. The actual nitrogen rate is based on the expected yield goal for the specific field, multiplied by a factor of 1.2 units per bushel (or 2.1units per hundred weight or .021 units per pound) minus any nitrogen credits. The yield goal needs to be realistic, taking into account moisture, soil type and cropping sequence.
Example: Yield goal bushel per acre = 110 bu/acre, carryover nitrogen is 40 lb/acre from soil test. Other credits include 15 lb. nitrogen/acre from previous manure application.
Actual rate = 110 x 1.2 - 40 -15 = 77 pounds actual nitrogen needed.
Example: Yield goal pounds per acre = 6200 lb/ac, carryover nitrogen is 40 lb/ac from soil test. Other credits include 15 lb. nitrogen/acre from previous manure application.
Actual rate = 6200 x .021 - 40 -15 = 75 pounds actual nitrogen needed.
Nitrogen can be applied at varying times with good results. Sorghum utilizes nitrogen rapidly after the plants reach the five-leaf stage. Applications should be timed such that nitrogen is in place and available for this rapid growth phase, as yield potential is being established at this time. Use of starter and sidedress applications should be considered when the potential for nitrate leaching is high. Sidedress nitrogen should be applied by the time sorghum reaches the five-leaf stage. At boot stage, 65 to 70 percent of the total nitrogen has been taken up.
Phosphorus and Potassium
Both phosphorus and potassium are immobile nutrients in the soil and are generally safe from leaching. Application methods include preplant broadcast or banded as a starter at planting time. Low and very low phosphorus levels, as indicated from soil tests, will likely show a response to applied phosphorus (45 to 60 pounds broadcast or 20 to35 pounds in a band) unless yield potential is restricted by insufficient moisture. Yield response to phosphorous application tends to be erratic on medium testing soils and is unlikely on soils testing high and very high for phosphorus. Soils testing in a medium or higher range for potassium (K) generally do not show a yield response to added potassium fertilizer. The likelihood for deficiency will be greater on sandy soils when compared to fine textured soils. Potassium is sometimes promoted as a stalk strength or standability-enhancing nutrient. Adequate potassium levels are necessary for strong stalks, but high potassium rates alone will not provide total protection from stalk lodging. Proper fertilization for adequate levels of all nutrients is the best way to maximize standability.
Figure 1: Phosphorus deficiency shown by purpling is often aggravated by other conditions, such as cool wet soils and slow root growth.
Other Nutrients
Secondary and micronutrients are required only in certain areas and on certain soils. Rely on experience and local recommendations to determine the need to supplement these nutrients where a grain sorghum crop is planned. Soil tests for some of these nutrients are difficult to interpret. University research on boron, copper and manganese does not show a consistent response for most soils.
Sulfur can be lacking on sandy soils low in organic matter. Under irrigated conditions, irrigation water can supply much of a crop's sulfur needs if irrigation water contains sulfates. A water analysis is helpful to determine sulfur content of the water. For production on sandy soils with low organic matter, it is suggested to try a 10 to 15 pounds per acre sulfur application to ascertain the likelihood of a sulfur response. Sulfur incorporated in a starter mix is a good method of application.
Zinc is a nutrient that is often overlooked, but is a crucial element for optimum sorghum production. Soil tests are a good tool for predicting zinc needs. Zinc is most likely deficient in areas where topsoil has been removed and under high yield conditions. Zinc is usually applied along with phosphorous and potassium. Manure applications, if available, are a good source for many nutrients, including zinc.
Liming
A soil pH range of 6.0 to 7.5 is ideal for sorghum production, with pH of 6.5 considered optimum. Nutrient use efficiency deteriorates outside this pH range and liming to raise pH to 6.0 or above is effective. Adjusting high pH downward is usually impractical. Kansas State University research suggests that on acid soils, banding 35 pounds per acre P2O5 at planting can increase yield to the same level as liming at a rate of 5000-10,000 pounds. ECC (effective calcium carbonate) at a significantly lower cost. The concentration of aluminum in the soil increases as soil pH decreases, eventually becoming toxic to crop growth. Phosphate combines with the toxic aluminum, effectively reducing aluminum concentrations in the soil. Kansas State cautions that the use of phosphate for aluminum toxicity reduction is a temporary solution, since it does not change the pH of the soil. The soil will become more acidic each year and liming will eventually be necessary. This method may be applicable on leased land situations or for fields where crop production future may be limited.
Figure 2: High pH induced iron deficiency chlorosis. A complex plant disorder associated with high pH soils and soils containing soluble salts that reduce the availability of iron. Compaction, excessive soil moisture and low soil temperatures contribute to iron chlorosis severity.
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.