SULFUR

During the early years of commercial fertilizer use, nearly all fertilizer elements were in the sulfate form. Although not specifically planned for, the amount applied often exceeded the total amount of the three other elements combined. Also, sulfates were supplied to the soil by rain, snow and dust. Near industrial centers, where coal was burned for fuel, as much as 100 lbs. of sulfur per acre per year was measured, with a computed average of 15 lbs. to 30 lbs. sulfur per acre for most farms near northern industrial areas. Non-industrial areas received 3 lbs. to 5 lbs. sulfur per acre annually. Thus it was hard to visualize that sulfur deficiency would be a problem in many soils for some time.

However, the use of highly concentrated fertilizers containing little or no sulfur, less use of manure, and conversion from coal to gas and electricity for fuel by industries and homes has drastically reduced the amount of sulfur supplied to soils; increased crop yields with great residue removal from the field and sometimes increased leach loss from irrigation have all combined to alter soil sulfur supplies.

An additional reason for sulfur testing in some areas is the high and increasing sulfur content of water used for irrigation. This may have an influence on soil maintenance and reclamation problems, especially as regards amendments for routine use.

The Sulfur Test Values

It should be remembered that sulfur, like phosphorus, is usually found in relatively small amounts in soils. Whereas, the phosphorus content is largely represented in the inorganic mineral form, sulfur is largely found in the organic form. As in the case of nitrogen, sulfur transformations are largely biological. They go on readily in most soils and the transformation may be indicated in a general way as follows:

The last stages of sulfur oxidation are brought about largely by certain types of soil bacteria. The sulfate compounds that result become the available source for plant uptake. Because most sulfur-containing mineral materials are highly soluble and the sulfate portion subject to leaching, the best way of building sulfur reserves in soils is by adding all available organic materials and maintaining an adequate organic matter content.

Where satisfactory organic sulfur reserves cannot be maintained, certain fertilizers will have to be depended upon to supply the crop with its sulfate requirements.

Our laboratory has been analyzing soils for sulfur determinations. Approximate levels for most crops have been established with the following amounts of available sulfur offered as general guidelines:

Sulfur deficiencies, like other nutrient deficiencies, depend to a large extent on the crop yield. For high yield, intensive cropping, the test values need to be in the high reading range for the particular crop type; for lower yields a smaller supply of sulfur is adequate. The ratio of nitrogen to sulfur in the plant tissue may be a better indication of sulfur deficiency; but this can only be used to correct a deficiency on the current crop already growing or on next yearn crop. Obviously it cannot be used for fertilization at the previous planting time.

On the following page is a chart giving some rough guidelines for sulfur needs and amounts to add according to the soil test. You should also consider the distribution of sulfur in the profile to a depth of 3 feet. If the lower 2 feet have relatively high sulfur, the recommended amounts can be lowered, and only the seedling establishment needs consideration. If the lower depth is very low, you should consider slight increases in recommended amounts.

Sulfur deficiencies should be corrected on an approximate pound deficit to pound added basis. For ease and convenience in blending we suggest rounding a recommendation upward in units of 5 Ibs. per acre, rather than attempting application of an exact poundage per acre as shown to be needed by the test.

To convert ppm S to pounds per acre S multiply the ppm by 3.2 for sandy loam soils, by 3.5 for loamy sand soils, and by 4.0 for sandy soils.

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