Sulfur Fertilization for Soybeans
Sulfur (S) nutrition for plants has become a hot topic because of the 1) reduction in atmospheric S deposition resulting from reduced S emissions from factories and power plants, 2) increased crop removal of S because of higher yields, and 3) long-term decreases in soil organic matter. Click here for a good summary of S fertility for soybeans. Pertinent points to consider for S nutrition in soybeans follow.
• Sulfur is a component of the amino acids methionine and cysteine, and these two essential amino acids are often insufficient in protein derived from soybeans.
• Sulfur is essential for protein synthesis as well as for nodulation and nitrogen fixation in soybeans.
• Soybean grain removes about 1.7 lb of S per 10 bushels of grain, or about 12 lb/acre for a 70 bu/acre yield.
• The sulfate ion (SO4) is the form primarily absorbed by plants. Sulfate-S is mobile in most soils; thus, it is subject to leaching, especially from sandy soils.
• Organic matter is the main source of soil S in Mississippi soils. Thus, if fertilizer S in not applied to the soil, the main source of S will be the mineralization of organic matter by soil microbes. Coarse-textured and low-organic matter soils are those most likely to be sulfur-deficient.
• Sulfur levels in soil are generally considered low at 0-5 ppm, moderate at 6-8 ppm, and sufficient at >10 ppm.
• Traditional soil testing is not a good predictor of S deficiency because of the transient nature of S availability, its mobility away from the crop root zone prior to crop need, and the lack of calibration of soil tests to predict S deficiency in plants. Also, commonly-used soil tests likely will identify fields that have sufficient sulfur, but are not very good at identifying those fields that will benefit from added sulfur.
• Since the plant-available form of S is SO4, apply a SO4-containing fertilizer for an immediate crop response. These products should be applied close to peak crop demand to reduce loss by leaching. If a fertilizer containing elemental S is used, it must be applied well in advance of the crop’s need.
• Ammonium sulfate (21-0-0-24S), ammonium thiosulfate (12-0-0-26S), and calcium sulfate or gypsum (0-0-0-17S) are the most commonly used sulfate fertilizers. Potassium sulfate (0-0-50-18S) is a fertilizer that contains both K and S.
• Tissue testing of the appropriate plant part at the appropriate time is recommended to definitively identify an S deficiency. For soybeans that are < 12 in. tall, sample the whole plant; for larger plants, sample the most recently matured trifoliate.
• There is anecdotal evidence that sulfur fertilization in the late growing season will result in higher seed protein content.
• Preliminary data indicate that a 1 to 1.5 bu/acre yield increase resulting from S fertilization will pay for the fertilizer amount that will normally be applied to S-deficient soils.
Research conducted in West Tenn. provides new information about S fertility for soybeans in the Midsouth. Details of and results from that research are reported in an article titled “Corn and soybean response to sulfur fertilizer in West Tennessee” that appears in the online journal Crop Forage & Turfgrass Mgmt. (https://doi.org/10.1002/cft2.20092). A summary of the activity associated with that research follows.
• The objectives of the study were to 1) identify optimal at-planting S for corn and soybean yield, and 2) evaluate the effect of S rate on crop growth and yield, leaf nutrient level, and seed S content. Soil pH at the site averaged 6.7 and soil organic matter (SOM) averaged 1.8%.
• No-till experiments were conducted without irrigation in 2015 and 2016 on a site with silt loam soil. Each year, the corn crop was preceded by soybean, and the soybean crop was preceded by corn. The site used in the study was chosen because it had a history of visual S deficiency in corn.
• Ammonium sulfate was broadcast-applied at 0, 10, 20, and 30 lb S/acre at planting.
• Soybean canopy color did not differ among S rates at R1, but the S fertility treatments did result in a visibly greener canopy later in the season at the R5 stage. Soybean leaf S concentrations did not differ among S treatments at early bloom.
• Soybean seed weight did not differ among S fertility treatments. Soybean seed S levels increased from 0.23% for the no S fertilizer treatment to 0.27%, 0.29%, and 0.29% for the three treatments that had added S fertility . Thus, S fertility resulted in a significant seed S increase.
• Soybean yield in the four S fertility treatments ranged from 51 to 54 bu/acre in 2015 and from 62 to 64 bu/acre in 2016; the differences were not significant in either year.
• From their results, the authors concluded that: 1) an S application to soybean grown on a site with characteristics similar to those in this study is not warranted from a yield increase standpoint, but may be considered if increased seed S is desired; 2) plant tissue analysis did not indicate an S deficiency in soybean, even where visual S-deficiency symptoms were observed; and 3) tissue sampling at R1 may be too early and not useful for predicting later-season S deficiency/sufficiency in soybean.
The above results from West Tenn. research support the following guidelines for S fertilization of soybeans in the Midsouth.
• Consider S fertilizer application to coarse-textured soils with low organic matter since this is likely where an S deficiency in soybean will occur.
• Both traditional soil testing and visual difference in greenness of the soybean canopy are poor indicators of potential/actual S deficiency in soybean that is associated with seed yield. Rather, use tissue testing during critical growth stages (likely at about R5) to determine S deficiency/sufficiency in appropriate soybean plant tissues (Click here for S sufficiency level in soybean tissue).
• Sulfur fertilization of soybeans may be warranted on S-sufficient soils if an increase in seed S is the goal.
Composed by Larry G. Heatherly, June 2021, larryh91746@gmail.com