Soybean fertilizer recommendations
Soybean is an important crop in Minnesota and provides a significant return in many farm enterprises. Yields of the soybean crop will decrease when essential nutrients are deficient. Read on for recommendations on developing a soybean fertilizer program to maximize yields.
The soybean is a legume and if properly inoculated, can use the nitrogen in the atmosphere (N2) for plant growth. Therefore, nitrogen fertilizer is not needed for soybean production in most situations.
The amount of fixation that takes place depends to the amount of nitrate-nitrogen (NO3--N) in the soil. In general, the amount of N fixed in the symbiotic relationship increases as the amount of NO3--N in the soil decreases. If soil NO3--N is suddenly depleted, the soybean plant will increase N fixation rapidly to meet the N demand.
Some growers will apply livestock manures to soybean fields, instead of corn. Manure is an excellent source of phosphorus (P), potassium (K), all secondary nutrients and micronutrients. For producers wondering about the effect of N in the manure on nodule development, recent research gives us an answer. It shows that the soybean crop will remove more N than corn, which means you should adjust the amount of applied manure based on crop removal. If manure-N rates are lower than what the crop removed, nodulation will resume mid-season.
The application of manure to soybean fields had a consistent positive effect on grain yield. This management practice also increased vegetative growth which led to more lodging of some varieties. The increased vegetative growth also provided a more favorable environment for white mold growth and development. The effect of manure on production was the same for several soybean varieties. Therefore, we don't recommend changing your decisions about variety selection with manure.
Should you apply in-season N to soybeans? University of Minnesota research shows that in-season fertilizer-N has no effect on soybean yield. The research, conducted throughout Minnesota, evaluated the effect of various N sources applied in-season. In-season application of fertilizer-N is not suggested for soybean production in Minnesota.
Research in the Red River Valley has shown that fertilizer N can increase soybean yields under certain conditions. 1) Producers have had problems with nodulation; 2) NO3--N at a 24-inch depth is less than 75 lb./acre. For these situations, use of some N (50 to 75 lb./acre) in a fertilizer program may be beneficial. Measure the amount of NO3--N to a depth of 24 inches before applying nitrogen fertilizer.
Table 1 lists phosphate fertilizer guidelines for soybean production. Table 2 provides suggestions for potash use. The suggested rates of phosphate and potash are not adjusted for placement. Research shows that neither banded nor broadcast placement is consistently superior for phosphate and potash. Soybean seeds are very sensitive to fertilizer placed on or near the seed row, so do not band fertilizer with the seed as a "popup" fertilizer application at planting.
Apply phosphate in the spring before planting. This will reduce the time for contact between soil and fertilizer. Timing becomes especially important with the soil pH is 7.4 or higher.
Phosphate fertilizer can produce substantial increases in soybean yields if soil test values for phosphorus are in the low and very low ranges.
If moisture is adequate, soybean yields have usually been slightly higher if the suggested rates of phosphate or potash are broadcast and incorporated before planting. The use of air seeders for planting soybeans is increasing in popularity. There are several options for placement of seed and fertilizer with this seeding method. One option involves mixing fertilizers and soybean seed in the same band. However, research shows that contact between fertilizer and seed in a narrow band reduces soybean stands. The soybean seed is very sensitive to salt injury. Therefore, placement of fertilizer in contact with soybean seed is a risky practice. There is, as yet, no firm and consistent evidence that this placement is superior to broadcast applications. Any method of application that places soil between fertilizer and seed is satisfactory.
Because phosphorus and potassium are not mobile in soils, many people wonder about broadcast applications in no-till systems. A substitute would be to band phosphate or potash fertilizers below the soil surface, then plant on top of the band. Research at the West-Central Research and Outreach Center at Morris show that yield responses to phosphate fertilization in no-till production systems are the same for both banded and broadcast applications. The roots of the soybean plant that are actively involved in nutrient uptake are located near the soil surface. Fertilizer incorporation is adequate in many no-till planting systems.
Table 1 summarizes phosphate recommendations for soybean production. Use one of the following equations if you want a phosphate guideline for a specific soil test and a specific expected yield:
- Recommended P2O5 = [1.752 - (0.0836) (Bray P in ppm)] (Expected yield)
- Recommended P2O5 = [1.752 - (0.1114) (Olsen P in ppm)] (Expected yield)
Table 1: Phosphate fertilizer guidelines
|Expected yield||P soil test: 0-5 parts per million (ppm) Bray and 0-3 ppm Olsen||P soil test: 6-10 ppm Bray and 4-7 ppm Olsen||P soil test: 11-15 ppm Bray and 8-11 ppm Olsen||P soil test: 16-20 ppm Bray and 12-15 ppm Olsen||P soil test: 21+ ppm Bray and 16+ ppm Olsen|
|< 30 (bu/acre)||50 (lbs P2O5 per acre)||30 (lbs P2O5 per acre)||0 (lbs P2O5 per acre)||0 (lbs P2O5 per acre)||0 (lbs P2O5 per acre)|
Table 2 summarizes potash recommendations for soybean production. Use the following equation if you want a potash guideline for a specific soil test and a specific expected yield:
- Recommended K2O = [2.0 - (0.0088) (Soil test K in ppm)] (Expected yield)
Table 2: Potash fertilizer guidelines
|Expected yield||K soil test: 0-50 ppm||K soil test: 51-100 ppm||K soil test: 101-150 ppm||K soil test: 151-200 ppm||K soil test: 200+ ppm|
|< 30 (bu/acre)||55 (lbs K2O per acre)||35 (lbs K2O per acre)||20 (lbs K2O per acre)||15 (lbs K2O per acre)||0 (lbs K2O per acre)|
*Use the following equation to calculate potash fertilizer suggestions for specific yield goals and specific soil test values for K:
K2O Suggested = [2.0 - (0.0088) (K Soil Test, ppm)](Yield Goal)
No potash fertilizer is suggested if the soil test for K is 200 ppm or greater.
Impact of cation exchange on soybean K guidelines
Potassium fertilizer guidelines for soybean were revised based on recent research on medium- and fine-textured soils in Minnesota. Currently, these guideline rates are not adjusted based on a soils ability to hold potassium on cation exchange sites of clays. Coarse-textured soils, such as sands and loamy sands, have very little clay and low cation exchange capacity (CEC). Potassium can leach on low CEC soils potentially wasting K fertilizer and reducing economic return to fertilizer. Research in Minnesota is on-going to determine if K guidelines need to vary based on soil CEC. Recent research on sandy soils with a CEC around 5 meq per 100 grams showed sandy soils needed less potassium fertilizer than medium- and fine-textured soils with the same soil test K level and had a lower critical soil test level. Due to the K leaching potential and a lower critical level of low CEC soils, it is not recommended to build soil test K greater than 120 ppm. Until more research data are available, K fertilizer could be applied on low CEC soils using the equation below. However, use of this equation will reduce K fertilizer application rates on low CEC soils and should be done on a trial basis to ensure K is not limiting yield on irrigated soybean grown on low CEC sandy soils.
K2Osuggested = [2.0 - 0.0146 (Soil Test K, ppm)] (expected yield)
Source of potassium for soybean production
Potassium chloride (muriate of potash) is the primary source of potassium fertilizer sold and used in Minnesota crop production. Potash fertilizer contains equal amounts of potassium and chloride. Research in the southern U.S. has shown negative effects of high rates of chloride applied to soybean. The effect of chloride on soybean production is not known in Minnesota. The impact of chloride on soybean is more likely to be an issue when high rates (more than 200 lbs) of potash are applied ahead of the soybean crop or in areas of central or western Minnesota where soluble salts, and chloride, can build in the soil. Reduced grain yield due to excessive chloride build-up in the soil has been identified at Lamberton, Morris, and Crookston in abnormally dry years regardless of when the potash was applied in fall or spring ahead of the soybean crop. If build-up of chloride is suspected no more than 100 pounds of potash should be applied directly ahead of the soybean crop. If high rates of potash must be applied, research has shown that application ahead of corn or wheat may is a better option as corn and wheat can tolerate higher potash application rates. Research is underway to assess alternative fertilizer sources, however, potash is currently the most cost-effective source of K in Minnesota.
Research in Minnesota has shown that soybeans do not respond to the application of other nutrients in a fertilizer program. Though manganese deficiency has been reported in glyphosate resistant soybeans in areas of the Eastern Corn Belt, Minnesota research has not shown a benefit to soil or foliar-applied manganese on soybean.
Several research studies have evaluated the use of sulfur for soybean in Minnesota. Soybeans may respond to sulfur application by increasing plant growth, but yields almost never increased and decreased in some circumstances. We only suggest sulfur in fields with a history of low yields for crops like alfalfa and corn, which are susceptible to deficiency. You can also consider sulfur when soil organic matter in the top six inches is 2.0% or less, and sulfur has not been applied for many years. These limited circumstances can warrant an application of 10 to 15 lbs of S. In most cases, sulfate-sulfur from previous application or mineralized from the soil will be enough.
Reviewed in 2018