Fertilizing Soybeans in Minnesota

Introduction

Soybean is an important crop in Minnesota and provides a significant return in many farm enterprises. The fertilizer needs of the crop are frequently neglected while most of the attention is directed at fertilizing other crops in the rotation. Yields of the soybean crop will decrease when essential nutrients are deficient. Therefore, profitable fertilizer programs must be developed to maximize yields for this crop. This bulletin will discuss fertilizer recommendations that are a key component of profitable production.

Nitrogen Considerations

The soybean is a legume and, if properly inoculated, can use the nitrogen gas (N₂) in the atmosphere for plant growth via fixation in the nodules. The amount of fixation that takes place is related 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. Although the amount of N fixed in the nodules is small when soil NO₃-N is high, some fixation still occurs and, if soil NO₃-N is suddenly depleted, N fixation by the soybean plant will increase rapidly in order to meet the N demand.

Concerns for environmental quality have encouraged the practice of applying livestock manures to fields planted to the soybean crop rather than applying excessive amounts on acres where corn is grown. Manure is an excellent source of phosphorus (P), potassium (K), all secondary nutrients, and the micronutrients. However, producers have been concerned about the effect of N in the manure on nodule development. During the 1990’s, research was conducted at 10 sites throughout Minnesota to evaluate the effect of manure application on soybean production. That research effort produced several conclusions about the use of manure for soybeans. The soybean crop removed significantly higher amounts of N when compared to corn leading to the conclusion that rate of manure applied should be limited to the amount of N removed by this crop. The results of the study also showed that if manure-N was applied at rates to supply less N than was removed, nodulation quickly resumed in mid-season and the final N removal was similar for both manured and non-manured fields.

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, decisions about variety selection should not be changed when manure is used.

In recent years, some scientific speculation has questioned the ability of the soybean nodule to supply adequate amounts of N late in the growing season -- a situation that could limit soybean yields. This speculation leads to recommendations, by some, for in-season fertilizer N application for the crop. In the late 1990’s, University of Minnesota research was conducted at many locations throughout the soybean growing areas to evaluate the effect of in-season application of various N sources during the growing season on soybean yield. Results of the study were conclusive. In-season application of fertilizer-N had no effect on soybean yield.

The effect of nitrogen fertilizer use on soybean yield at one site is summarized in Table 1. Foliar application of nitrogen during the growing season can decrease yields (see Table 2). In-season application of fertilizer-N is not recommended for soybean production in Minnesota.

 

Table 2. Yield of irrigated soybeans as affected by time and method of application of urea fertilizer.

Nitrogen fertilizer use for soybean production in the Red River Valley deserves special consideration. Research in the region has shown that use of fertilizer N may increase yields where producers have experienced problems in getting good nodulation and the amount of NO₃-N to a depth of 24 inches is less than 75 lb./acre. The use of some N in a fertilizer program (50 to 75 lb. per acre) could be beneficial for some soybean fields in the Red River Valley. Soybean growers in northwestern Minnesota are advised to measure carryover NO₃-N before they decide to apply fertilizer N.

Phosphate and Potash Use

The use of phosphate fertilizer can produce substantial increases in soybean yields if soil test values for phosphorus are in the low and very low ranges. The magnitude of the increase is shown in Table 3.

Phosphate fertilizer recommendations for soybean production are listed in Table 4. The suggestions for potash use are listed in Table 5.

Table 3. The effect of rate of broadcast phosphate on soybean yield.

 

Table 4. Phosphate fertilizer suggestions for soybean production in Minnesota

 

Table 5. Potash fertilizer suggestions for soybean production in Minnesota

 

The suggested rates of phosphate and potash are not adjusted for placement. A summary of research in Minnesota and neighboring states leads to the conclusion that neither banded nor broadcast placement is consistently superior if adequate rates of phosphate and/or potash are used. If moisture is adequate, soybean yields have usually been slightly higher if the suggested rates of phosphate and/or potash are broadcast and incorporated before planting in many research trials where banded and broadcast placements have been compared.

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. Results of trials with this placement, however, have shown that placement of any fertilizer in contact with the seed when both are in a narrow band reduces stand establishment of soybeans. 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.

No-till planting of soybeans raises special questions with respect to phosphate and potash fertilization. There’s general agreement that phosphorus and potassium are not mobile in soils. So, broadcast applications in no-till systems can be questioned. A substitute would be to band phosphate and/or potash fertilizers below the soil surface, then plant on top of the band. Results of research conducted at the West-Central Research and Outreach Center at Morris show that responses to phosphate fertilization in no-till production systems are the same for both banded and broadcast applications. Apparently, the roots of the soybean plant that are actively involved in nutrient uptake are located near the soil surface. The fertilizer incorporation that takes place in the planting operation is apparently adequate in many no-till planting systems.

Timing of the phosphate application is an important consideration when fertilizing soybeans. If phosphate is recommended for fields having a pH of 7.4 or higher, the fertilizer should be applied in the spring before planting. This practice will reduce the time interval for contact between soil and fertilizer thereby reducing tie-up of phosphorus and the soybean plant will make more efficient use of the applied phosphate.

Iron Chlorosis

Frequently, soybeans that are grown on fields which have a pH of 7.4 or greater turn yellow, and, in some cases, die. This condition is described as iron chlorosis. There is no deficiency or shortage of iron in the soil. Because of soil and/or environmental conditions, the soybean plant is not able to absorb or take up the amount of iron that is needed for normal growth and development.

There is no easy solution to the iron chlorosis problem. There are, however, several management practices that can be used to reduce the severity.

Careful variety selection is of major importance. The University of Minnesota publication, “Varietal Trials for Farm Crops,” has chlorosis scores for many varieties. The majority of the companies that market soybean seed also provide chlorosis scores for their varieties.

As more is learned about iron chlorosis, it is apparent that damage can be reduced if stress on the soybean plant is at a minimum. There are several factors that can stress soybean plants. Some that are easy to identify are: 1) use of some post-emergence herbicides, 2) soils with a high “salt” content, 3) root damage from excessively deep cultivation, 4) soil compaction, and 5) seedling diseases. It is important to eliminate or manage as much as possible the factors that place stress on the soybean plant.

Other Possible Nutrient Needs

Research trials conducted at several sites in Minnesota over a number of years have shown that the soybean crop does not respond to the application of magnesium, sulfur, boron, zinc, manganese, and copper. Therefore, additions of these nutrients to a fertilizer program are not suggested.

Additional information about nutrient management in all crops can be found in related publications. These are listed below.

Related Publications

FO-00794-Sulfur for Minnesota Soils
FO-00792-Phosphorus for Minnesota Soils
FO-00723-Boron for Minnesota Soils
FO-00725-Magnesium for Crop Production in Minnesota
FO-06288-Understanding Phosphorus Fertilizer
FO-06794-Potassium for Crop Production
FO-00720-Zinc for Crop Production

 

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