no. 0.540

Fertilizing Grain and Forage Sorghums

Quick Facts...

• Nitrogen is the most limiting nutrient for grain sorghum and forage sorghum production.
• Apply nitrogen fertilizers at rates based on expected crop yields minus credits for residual soil nitrates, estimated nitrogen mineralized from soil organic matter, manure and previous legume Crops, and nitrogen present in irrigation water.
• Apply phosphate and zinc fertilizers at rates based on soil test results.
• Most Colorado soils contain sufficient available potassium and sulfur for grain and forage sorghum production.

Adequate soil fertility is one of the requirements for profitable grain and forage sorghum production. Nitrogen (N) is the most yield-limiting nutrient, unless high N fertilizer rates or manure applied to the previous crop have left high residual NO₃-N levels in the soil. Phosphorus (P) is the next most limiting nutrient, while zinc (Zn) and iron (Fe) also may be limiting in some soils.

Soil Sampling

The value of a soil test in predicting nutrient availability during the growing season is directly related to how well the sample collected represents the area sampled. Take surface samples from the 1-foot soil depth. If the field has been in no-till, reduce the sampling depth of the tillage layer to 4 to 6 inches. Take subsoil samples to a depth of 2 feet for determination of available NO₃-N. A good sample is a composite of 15 to 20 soil cores taken from an area uniform in soil type. Sample separately areas with major differences in soil properties or management practices.

Thoroughly air dry all soil samples within 12 hours after sampling by spreading the soil on any surface where the soil will not be contaminated. Do not oven dry the soil because this can change the soil test results. Do not ship field moist samples to the laboratory because soil test values will continue to change during shipping time. Place the air-dried soil in a clean sample container for shipment to the soil test laboratory.

Submit a carefully completed information form with the soil sample. This form provides information so fertilizer application suggestions can be tailored to your specific situation. Take soil samples for NO₃-N analysis every year for optimum N fertilization of Crops. Soil analyses for availability of the other nutrients, pH, and organic matter content every three or four years may be sufficient.

More detailed explanations of the importance of taking proper soil samples are found in fact sheets 0.500, Soil Sampling, 0.501, Soil Testing, and 0.502, Soil Test Explanation. These fact sheets are available at your Colorado State University Extension county office or from the Extension Resource Center, 115 General Services Building, Colorado State University, Fort Collins, CO 80523; (970) 491-6198.

The Colorado State University Soil, Water and Plant Testing Laboratory is located in Room A319, Natural and Environmental Sciences Building, Colorado State University, Fort Collins, CO 80523; (970) 491-5061.

Nitrogen Suggestions

Base N rates for grain sorghum, forage sorghum, sorghum x sudan hybrids, and sudan on the expected yields for each field. Nearly all sorghum Crops will require some N fertilizer, unless there is a substantial N carryover. High N rates in excess of crop needs can result in potential groundwater contamination by NO₃-N under irrigated conditions and potentially high forage NO₃-N contents if not managed properly.

Give credit for the amount of residual NO₃-N in the soil. The suggested N rate is reduced 8 pounds per acre for each ppm of NO₃-N (average concentration in the soil sample depth) in the soil for a 2-foot sampling depth. The method to calculate a depth-weighted NO₃-N concentration in soil in the root zone where surface and subsoil samples are taken is as follows:

Other credits include the amounts of N estimated to become available during the season from mineralization of soil organic matter (20 lb N/A per percent organic matter), and manure or previous legume crop residues after incorporation into the soil, as well as NO₃-N in irrigation water. Table 1 shows credits for N in manure and previous legume Crops. These credits are subtracted from the total crop needs to determine the suggested N fertilizer rate for the expected yield.

Dryland Grain Sorghum

The basis for suggested N rates is an algorithm (equation), developed by Kansas State University. Nitrogen rate is determined as follows:

N rate (lb/A) = [1.6 x EY (bu/A)]
- (8 x average ppm NO₃-N in the soil)
- other N credits (lb N/A)
where EY = expected yield.

For example, if your expected yield of dryland grain sorghum is 40 bu/A, with the top 2 feet of soil containing an average 1.5 ppm NO₃-N, and 0.5 percent organic matter (OM) in the tillage layer, the suggested N rate would be:

Suggested N rates for dryland grain sorghum are lower than those for irrigated grain sorghum because of lower plant populations and expected yields, and less than optimum soil moisture conditions. Table 2 suggests N rates for dryland grain sorghum at an expected yield of 40 bushels per acre. Fertilizer N rates decrease with increasing levels of NO₃-N in the top 2 feet of soil and increasing soil organic matter content.

When a soil test result for organic matter content is not available, assume a level of 1.5 percent organic matter for eastern Colorado soils. Suggested N rates in this table do not account for other N credits (Table 1). These credits should be subtracted from the N rates in Table 2 to determine the N rate for the field.

Dryland Forage Sorghum, Sudan, and Sorghum x Hybrids

The basis for suggested N rates is an algorithm (equation), developed by Kansas State University. Nitrogen rate is determined as follows:

N rate (lb/A) = [10.67 x EY (tons/A)]
- (8 x average ppm NO₃-N in the soil
- other N credits (lb N/A)
where EY = expected yield.

For example, if your expected yield of forage sorghum, sorghum x sudan or sudan silage is 15 tons/A, with the top 2 feet of soil containing an average 5 ppm NO₃-N, and 1.5 percent organic matter (OM) in the tillage layer, the suggested N rate would be:

Suggested N rates for dryland forage sorghum, sudan and sorghum x sudan hybrids are lower than those for irrigated forages because of lower plant populations and expected yields. Table 3 suggests N rates for expected silage yields of 15 tons per acre. Add or subtract 10.67 pounds of N per acre for each ton per acre difference in silage yield. Excessive N rates may result in high NO₃-N forage if the crop is not managed properly.

Fertilizer N rates decrease with increasing NO₃-N in the top 2 feet of soil and increasing soil organic matter content. When a soil test result for organic matter content is not available, assume a level of 1.5 percent organic matter for eastern Colorado soils. Suggested N rates in this table do not account for other N credits (Table 1). Subtract these credits from the N rates in Table 3 to determine the N rate for the field.

Irrigated Grain Sorghum

Suggested N rates for irrigated grain sorghum are calculated by the same algorithm as for dryland grain sorghum.

N rate (lb/A) = [1.6 x EY (bu/A)]
- (8 x average ppm NO₃-N in the soil)
- other N credits (lb N/A)
where EY = expected yield.

For example, if your expected yield of dryland grain sorghum is 80 bu/A, with the top 2 feet of soil containing an average 5 ppm NO₃-N, and 0.5 percent organic matter (OM) in the tillage layer, the suggested N rate would be:

Table 4 suggests N rates for irrigated grain sorghum at an expected yield of 80 bushels per acre. Fertilizer N rates decrease with increasing levels of NO₃-N in the top 2 feet of soil and increasing soil organic matter content. As with dryland grain sorghum, when a soil test result for organic matter content is not available, assume a level of 1.5 percent organic matter for eastern Colorado soils. Suggested N rates in this table do not account for other N credits (Table 1). Subtract these credits from the N rates in Table 4 to determine the N rate for the field.

Irrigated Forage Sorghum, Sudan, and Sorghum x Sudan Hybrids

Suggested N rates for irrigated sorghum forages are calculated by the same algorithm as for dryland forages, except the factor is 9 lb of N/ton of silage:

N rate (lb/A) = [9 x EY (tons/A)]
- (8 x average ppm NO₃-N in the top 2 feet of soil)
- other N credits (lb N/A)
where EY = expected yield.

For example, if your expected yield of silage is 30 tons/A, with the top 2 feet of soil containing an average 9 ppm NO₃-N and 1.5 % organic matter (OM) in the tillage layer, the suggested N rate would be:

Table 5 suggests N rates for expected silage yields of 30 tons per acre. Fertilizer N rates decrease with increasing levels of NO₃-N in the top 2 feet of soil and increasing soil organic matter content. When a soil test result for organic matter content is not available, assume a level of 1.5 percent organic matter for eastern Colorado soils. Suggested N rates in this table do not account for other N credits. Subtract these credits (Table 1) from the N rates in Table 5 to determine the N rate for the field. Add or subtract 9 pounds of N per acre for each ton per acre difference in silage yield. Excessive N rates may result in high NO₃-N forage if not managed properly.

Methods and Timing of N Applications

Apply N before preplant or sidedressing. Complete sidedressing by growth stage 3 (about 30 days after planting) to maximize N use efficiency. Fall application of N is not recommended on most soils.

Some N may be band-applied in combination with starter fertilizers, but the rate should be less than 20 pounds of N per acre. Use of planter attachments with the standard 2 x 2-inch placement (2 inches below and beside the seed row) is preferred for starter fertilizers. Use popup fertilizer placement (directly with the seed) with caution because seedling emergence may be decreased in dry soil, especially at rates supplying more than 10 pounds of N per acre.

Sorghum roots quickly grow into the soil between the rows. Sidedress N fertilizers early in the growing season to avoid root pruning.

For irrigated grain sorghum, application of N fertilizers with irrigation water is a convenient method and allows split applications to improve N use efficiency. Use in-season soil or plant analysis to determine the N status of the crop and apply additional N with the next irrigation if needed. Apply N fertilizers in furrow irrigation systems only on fields where a tailwater recovery and reuse system is in place. Urea-ammonium nitrate solution is the most efficient N fertilizer to apply through sprinkler systems. Anhydrous ammonia is not recommended for application through sprinkler systems because of N losses as ammonia and possible problems due to formation of solids in the water. Refer to 0.512, Fertigation.

Phosphorus Suggestions

Grain and forage sorghum, sorghum x sudan hybrid, and sudan responses to applied P are most likely on soils with low or medium levels of extractable P. Suggested P fertilizer rates (Table 6) are for band and broadcast applications related to soil test levels. The main tests for extractable P in Colorado soils are the AB-DTPA and sodium bicarbonate (NaHCO₃) tests. Values for both tests are given in Table 6.

Placement of P fertilizers in the root zone is important because P is not mobile in soil. Band application at planting (starter fertilizer) is the most efficient placement method for P. Suggested rates for broadcast application in Table 6 are about double those for band application. Incorporate broadcast applications of P fertilizers into the soil prior to planting. Subsurface placement of P may be especially important for reduced tillage cropping systems. Use popup placement (directly with the seed) with caution because seedling emergence may be decreased in dry soil, especially at rates supplying more than 10 pounds of N per acre. Monoammonium phosphate (MAP, 11-52-0), diammonium phosphate (DAP, 18-46-0) and ammonium polyphosphate (10-34-0) are equally effective per unit of P if properly applied. Choose a product based on availability, equipment available, and cost per unit of P.

Potassium Suggestions

Most Colorado soils are relatively high in extractable potassium (K), and few crop responses to K fertilizers are reported. See Table 7 for suggested K rates related to soil test values (AB-DTPA or NH₄OAc). The main K fertilizer is KCl (potash). Broadcast application incorporated into the soil prior to planting is the usual method.

Zinc Suggestions

Zinc deficiencies are common on soils where the subsoil is exposed, or on soils with high levels of free lime. Zinc availability decreases with increasing soil pH, and most Zn deficiencies are reported on soils with pH levels higher than 7.0. Incorporation of manure in eroded soils may correct Zn deficiencies, as well as improve soil structure.

Suggested fertilizer rates in Table 8 for band and broadcast applications are based on use of ZnSO₄. Apply effective Zn chelates at about one-third of the rate of Zn as ZnSO₄. Band application is more effective than broadcast application; thus suggested rates are lower for band application. Soil test values for available Zn by the DTPA soil test are similar to those by the AB-DTPA soil test (see Table 8).

Zinc deficiencies also may be corrected by foliar sprays of a 0.5 percent ZnSO₄ solution applied at a rate of 20 to 30 gallons per acre, but several applications may be necessary. ZnSO₄ solutions are difficult to prepare in the field so ZnEDTA or other soluble Zn sources can be used. A surfactant (wetting agent) increases plant absorption of the applied Zn.

Iron Suggestions

Iron deficiencies (chlorosis) of sorghum often occur on high pH soils. Iron deficiencies are most likely to occur on highly calcareous soils (pH higher than 7.8) or on soils leveled for irrigation where the subsoil has been exposed. Visual symptoms of Fe chlorosis are yellow striping of younger leaves. Entire leaves may become yellow, or even white with severe deficiencies.

Foliar spray applications of a 1 to 2 percent FeSO₄ solution at a rate of 20 to 30 gallons per acre may partially correct the chlorosis. Several foliar applications may be needed to completely correct the chlorosis, which may not be economical. FeSO₄ solutions are difficult to prepare in the field and other Fe sources may be used. Soil applications of most Fe fertilizers generally are not effective. However, application of manure often is the best method to help correct Fe deficiencies of Crops. It is usually advisable to plant a more Fe-tolerant crop on high pH, Fe-deficient soils than to try to correct severe Fe chlorosis of sorghum.

Other Nutrients

Most Colorado soils contain adequate levels of available sulfur (S), and soil tests for available S are not routinely performed. Irrigation water from wells often contains appreciable SO₄-S, so irrigated soils usually are adequately supplied with S. However, some deep well waters are low in S. Analyze water samples for SO₄-S if soils are low in organic matter and you suspect S deficiency.

There have been no confirmed deficiencies of boron (B), copper (Cu), manganese (Mn) or molybdenum (Mo) in grain sorghum, forage sorghum, sorghum x sudan hybrids, and sudan in Colorado.

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