Sulfur for Minnesota soils
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Sulfur (S) is an essential element in the life processes of all living things including micro-organisms, higher plants, animals, and humans. Sulfur plays a major role in the formation of the proteins needed to sustain life in all biological organisms.
Sources of sulfur in plants
The S necessary for crop growth in Minnesota can come from one or more sources. The major ones are:
Table 1. Daily Mineralization rates of SO4-S and total S content for representative surface soils (0-6 inches) of Minnesota incubated at 90°F
|Soil||Lb/ac/day||SOM1||Total S content|
SOIL ORGANIC MATTER
Approximately 95 percent of the total amount of S in soils is found in the organic matter. As this soil organic matter is broken down, the S in the organic forms is converted (mineralized) to sulfate-sulfur (written as SO4-S). This SO4-S is the only form of S that is absorbed by plant roots. Mineralization of all S in organic matter does not take place in one year. This breakdown process is continuous and takes a considerable amount of time and is dependent on soil temperature and moisture. Different soils release different amounts of SO4-S from the mineralization process (Table 1). A typical rule of thumb is that 3-5 lbs of sulfur are mineralized per year for each percent organic matter in the 0-6" soil depth.
Plant residues left from the previous crop can affect the supply of sulfur. Soil microbes need S in order to breakdown C in the soil. Plant material with a C:S ratio of 200:1 or less will mineralize SO4-S while a ratio of 400:1 or greater will immobilize SO4-S in the soil. A summary of average C:S ratios from field studies conducted in Minnesota is given in Table 2.
Table 2. C:S Ratio of non-harvested portion of crops
Several minerals found in soils contain S in some form. The S in the various minerals is transformed to available SO4-S as these minerals are weathered or broken down during the growing season. As would be expected, there is a wide range in the total S content of Minnesota soils (Table 1).
Most fuels that are burned for heat, power, and transportation contain some S. When fuels are burned, the S escapes as sulfur dioxide gas (SO2). The SO2 is absorbed in rainfall and reaches the soil as SO4-S. The SO2 content of the atmosphere is high near many industrialized cities; however, in the Twin Cities the concentration of SO2 in the air is quite low.
Some pesticides contain S. However, their contribution to the total amount of S in the soil system is quite low.
In the past, commercial fertilizers supplied considerable S in addition to the usual N, P2O5, and K2O. Today, as the fertilizer products become more concentrated and the analysis increases, S contents are less.
This can be an important source of S for limited areas in Minnesota. Sulfur is present in irrigation water as SO4-S. The amount of SO4-S can vary and can supply most of the needed S for a growing crop.
Removal of sulfur from the soil system
Sulfur is removed from the soil system by two major mechanisms. These are:
CROP UPTAKE AND REMOVAL
Uptake of S from the soil varies with crop and the yield of that crop. In general, relatively small amounts of S are absorbed (taken up) by crops. For forage crops like alfalfa and corn silage, crop uptake equals removal. Alfalfa will remove approximately 5.4 lb. S per acre for each ton of hay produced. Corn harvested for silage can remove between 25 and 30 lb. S per acre; whereas, corn harvested for grain (200 bu. per acre corn crop) will remove about 16 lb. S per acre. Wheat (80 bu per acre crop) will remove about 8 lb. S per acre in the grain. In Minnesota, fertilizer recommendations for S are not adjusted for yield goal. Therefore, there is no real need to remember the exact amounts of S removed by the various crops.
Like nitrate, SO4-S is mobile in soils and can be moved out of the root zone by leaching. The SO4-S does not leach as rapidly as nitrate-nitrogen (NO3-N). But, excessive rainfall or irrigation water can move SO4-S below the root zone, especially on well or excessively drained (coarse textured) soils. Leaching of SO4-S from the root zone of poorly drained (medium and fine-textured) soils in Minnesota is unlikely.
When S is deficient, growth is reduced and maturity is delayed. With inadequate supplies of S, there is a reduction in protein formation with a subsequent yellowing of the foliage. With alfalfa and red clover, the entire leaf area has a light green color (Figure 1). The leaves on S deficient corn become light green. This is accompanied by a distinct striping of the leaves. Young S deficient corn plants are shown in figure 2. Plant analysis can be used as a management tool for detecting shortages of S in crops. Deficient, marginal, and sufficient levels of S in plant tissue are summarized in Table 3.
Deficiency symptoms in crops
When S is deficient, growth is reduced and maturity is delayed. With inadequate supplies of S, there is a reduction in protein formation with a subsequent yellowing of the foliage. With alfalfa and red clover, the entire leaf area has a light green color (Figure 1.). The leaves on S deficient corn become light green. This is accompanied by a distinct striping of the leaves. An S deficient young corn plant is shown in figure 2.
Plant analysis is also a management tool that can be used to detect shortages of S in crop production. Deficient, marginal, and sufficient levels of S in plant tissue are summarized in table 3.
Table 3. Sufficient levels of S in tissue of several crops
|Crop||Sample||S sufficiency range|
|Alfalfa||Top 6" growth||0.26-0.50|
|Canola||5th leaf from the top||0.17-1.04|
|Corn||Whole plant <12" tall||0.15-0.40|
|Corn||Ear leaf at silking||0.15-0.40|
|Soybean||Trifoliate at early bloom||0.25-0.60|
|Sugar beet||Recently matured leaf 50-80 days after planting||0.21-0.50|
|Wheat||Whole plant at boot||0.15-0.40|
Soil testing procedures for several nutrients have been widely accepted and utilized throughout the Midwest. In Minnesota, soil testing for sulfur has been found to correlated and effective only for sandy soils with low soil organic matter concentrations in the soil surface. The area of the state where sulfur soil testing is suggested is given in Figure 3. Table 4 summarizes sulfur soil test interpretations for alfalfa and corn on sandy soils.
Sulfur soil tests are not suggested for use on medium- and fine-textured soils in Minnesota. On medium- and fine-textured soils, alfalfa and corn yield responses to sulfur have been best related to the concentration of soil organic matter at the soil surface (0-6 inch depth). Spring weather, soil parent material and previous crop can affect S responsiveness in crops. An interpretation of potential response for corn and alfalfa is presented in Table 5.
Table 4. Interpretation of the S soil test for sandy soils.
|Soil test value||Relative value||Expected response to S|
Table 5. Likelihood of a response to sulfur for corn and alfalfa grown on medium and fine-textured soils based on concentration of SOM in the top 6 inches of the soil
|Soil test value||Relative value|
|Soil organic matter concentration||Expected response to S|
Recommendations and method of application
Annual applications are suggested where a response to S is anticipated or predicted from the results of a soil test or tissue analysis. Minnesota research shows that an annual rate of 25 lb. S per acre is adequate for corn production on sandy soils, sulfur can either be broadcast and incorporated before planting or applied in a starter fertilizer at planting. Use 25 lb. S per acre for broadcast applications. The rate can be reduced to 10-12 lb. per acre if S is applied in a starter fertilizer. Liquid fertilizer materials containing sulfur can be damaging to emerging crops and should not be applied directly on the seed.
For medium and fine textured soils, suggested broadcast application rates of S range from between 10-15 lb. S per acre for both corn and alfalfa when soil organic matter concentration is less than 4.0%. Application rate should be increased up to 25 lbs of S per acre annually for intensively managed alfalfa.
When organic matter concentration exceeds 4.0% and corn follows soybean, the probability of a yield response to sulfur fertilization is low. Research has shown a response is possible on glacial till soils, especially in cool springs on poorly drained soils. In these situations a low rate of ammonium thiosulfate (2-3.5 gal per acre, 6- 10 lb. S) banded on the soil surface to the side of the seed row or 10 lbs of S broadcast may be warranted.
When organic matter exceeds 4.0% and corn follows corn with large amounts of surface residue present, research has shown yield responses to S are also possible. In these instances a band of 6-10 lb. S per acre or a broadcast application of 10-15 lb. S per acre is suggested.
Small grains may respond to S fertilization when grown on very sandy soils. For these soils, S can be broadcast at a rate of 20 lb. per acre and incorporated before planting or used at a rate of 10 lb. per acre in the row close to the seed.
There are several fertilizer materials that can be used to supply S when it is needed in a fertilizer program. These materials are listed in Table 6. The choice of the S source is highly dependent on the crop to be grown.
For alfalfa, red clover, and other forage legume production, all sources of S have had an equal effect on yield. Fertilizers which supply S in the sulfate form (SO4) are preferred for corn and small grain production. Plants absorb S in the SO4 form. Elemental S must be converted to SO4-S before it is available to plants. This conversion takes time and is slowed by cool spring temperatures.
Table 6. The sulfur content of some common fertilizers
|Sulfur fertilizer||Relative S level (%)|
|Ammonium Sulfate (21-0-0-24)||24|
|Ammonium Thiosulfate (12-0-0-26)||26|
|Epsom Salt or Magnesium Sulfate||14|
|Potassium Chloride (0-0-60)||0.4|
|Potassium Sulfate (0-0-51-18)||18|
|Sul-Po-Mag or K-Mag||22|
When needed, S fertilization can produce dramatic and profitable increases in yield for some crops. Sulfur is not needed for all soils in Minnesota. Those who farm sandy soils or soils with soil organic matter concentration <4.0% in the surface horizon should be most concerned about the use of S in a fertilizer program.