Optimizing Production with Sulfur

Fine-tuning potato nutrient management

Published in the January 2013 Issue Published online: Jan 25, 2013 Wes Haun, H.J. Baker Research Agronomist
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Optimizing potato production requires attention to details in all phases of management. Soil fertility is a primary area where a return on investment can be quickly realized. An understanding of potato nutrient requirements and the plant's uptake pattern greatly enhances the opportunity for maximum economic yields.

All essential plant nutrients need to be available for plant uptake throughout the growing season. Potatoes follow a growth response curve such that significant nutrient uptake continues until maturation. When potato bulking begins, about 40-50 percent of nitrogen (N) and potassium (K) have been taken up, and about 30-40 percent of phosphorus (P) and sulfur (S) have been taken up.

Generally, the three major nutrients N, P and K receive attention for plant nutrients and other essential plant nutrients are over looked. Once N, P, K are available at optimum levels, attention needs to be directed to the secondary and micronutrients. Oftentimes potato yields are limited due to lack of sulfur and/or micronutrients. Potatoes require 20 lbs sulfur, 3 lbs iron (Fe), 0.3 lbs. manganese (Mn), 0.2 lbs. zinc (Zn) per acre for 450 cwt yield.

Importance of Sulfur

Sulfur (S) is a secondary plant nutrient required by all plant biological systems. Plant sulfur nutrition and the role of sulfur within the plant's biological and chemical processes have contributed to increased awareness of sulfur deficiencies in crop production. It contributes to an increase in crop yields: 1) provides direct plant nutrient value; 2) provides indirect plant value as soil amendment, especially for calcareous and saline/alkali soils; and 3) it improves the use efficiency of other essential plant nutrients. Research has indicated that sulfur deficiencies not only reduce yields, but adversely affect quality. Determining the supply of sulfur to potatoes is more comprehensive than for most other nutrients.

There are natural and "man-made" S sources that perpetually fluctuate in their reactivity. Three primary forms of sulfur available for field application include elemental sulfur, ammonium thiosulfate and ammonium sulfate. The remainder of this discussion will focus on elemental sulfur, as it has the highest concentration of nutrients and is the most economical. Sulfur is a unique plant nutrient, as it must undergo microbial breakdown in soil prior to uptake by plants. This microbial process is sulfur oxidation, which converts elemental sulfur to plant available sulphate (SO4)-sulfur.

Sulfur Oxidation

Sulfur oxidation is a function of several factors, including soil temperature, soil moisture, population of micro-organisms (Thiobacillus) and sulfur particle size. The first three factors mentioned are environmental or naturally occurring, of which there is minimal managerial control. Optimum conditions for greatest sulfur oxidation rates include soil temperatures 75-100 degrees F, soil moisture slightly less than field capacity and population of Thiobacillus bacteria increases with repeated applications of sulfur. When sulfur is part of a planned nutrient application program and applied annually or semi-annually, it provides ample food supply for the Thiobacillus bacteria. Therefore, the Thiobacillus bacteria become more active and reproduce as food supply supports the population. Greater bacteria population results in faster oxidation of S to SO4.

Sulfur particle size can be influenced; hence, the addition of bentonite clay to elemental sulfur. Bentonite clay, upon exposure to water, has a capacity to swell up to 15 times its dry volume. Sulfur bentonite contains 10-15 percent clay. When field-applied, the clay in the sulfur pastilles absorbs soil moisture and swells, resulting in the sulfur pastilles fracturing into very small particles. These particle sizes can range from 800 to 60 microns.

Smaller sulfur particles provide greater surface area for Thiobacillus bacteria to feed on the sulfur and oxidize it to sulfate. There is an inverse relationship between particle size and surface area. The oxidation rate increases exponentially as sulfur particle size decreases. Tiger-Sul Products produces a sulfur product, Tiger-90CR, which contains 90 percent elemental sulfur and 10 percent bentonite clay. This product accomplishes the aforementioned benefits with the clay.

Potatoes have shown significant yield responses to sulfur and two micronutrients, Zn and Mn (Fig. 1).
The treatment containing a combination of sulfur, zinc and manganese had the greatest yield of the U.S. No. 1 grade tubers with the lowest U.S. No. 2 yield, as well as lowest yield of cull tubers. Generally, it is more common to observe an increase in both categories if a treatment results in larger tubers. This combination seemed to be able to increase tuber size without having typical problems with abnormal shape of tubers.

Sulfur with Micronutrients

The micronutrients are added to the sulfur matrix as extremely fine oxide particles. This fertilizer material (Tiger Micronutrient Potato Mix II), having micronutrients blended in with the elemental sulfur, represents a significant potential advantage for two reasons. First, having the micronutrients blended with the elemental sulfur results in a better spread pattern. When micronutrients are spread at low rates with high-analysis materials, it is not uncommon to have a high percentage of plants not being in close proximity to a fertilizer pastille, which is the common situation. Secondly, the solubility of zinc and manganese is very low in alkaline, calcareous soils common in much of the irrigated regions. Combining these nutrients with elemental sulfur results in an increase in solubility (data not shown) as the sulfur oxidizes and lowers the pH/dissolves lime in the micro-site and acidulate the oxide to plant available nutrients around the fertilizer pastille (Fig. 2).

Tiger-90CR sulfur and Tiger Micronutrients are products that provide the opportunity for you to fine-tune your nutrient management program that will enhance your potato quality and yield.