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| CROP and SOIL NEWS/NOTES OSU Extension Service |
March, 1998 Vol. 12, No. 3 |
John Hart
Welcome to spring and to the nutrient question of the moment—sulfur. Questions about fertilization with S are generated by the appearance of two new/different S fertilizers and the increase in price of our principal S source, ammonium sulfate. This article is written to provide a brief overview of S fertilization issues.
Sulfur accumulation by the aerial portion of a cross section of crops grown in Oregon is presented in Table 1. The amount of S in these crops ranges from a low of 5 lb/a for subclover to a high of 60 lb/a for a vigorous stand of winter rape. Most crops contain between 10 and 30 lb S/a.
Table 1. Sulfur content of selected crops
| Crop | S in aerial portion of crop at harvest |
| lb/a | |
| Peppermint (Willamette Valley) | 15 to 30 |
| Grass grown for seed | 10 to 25 |
| Sub clover forage | 5 to 15 |
| Alfalfa | 4 to 6 lb/t |
| Winter rape | 45 to 60 |
Should we expect a fertilizer application to supply the total S requirement of a crop? No. The soil will supply S to the crop. The next logical question is "how much S will be supplied by the soil?" In soils with an argillic (B) horizon or moderate clay content, the entire S need of the crop is the answer. Some conditions exist for the response to be true. First, a fertilizer program supplying 20 to 30 lb S/a for the past 3 to 4 years is necessary. This rate of S application should leave sufficient S in the soil for a crop this season.
We have been taught that SO4-S is an anion and that it will leach with the high rainfall in western Oregon. Sulfate-S will leach, but not as rapidly as NO3-N. Sulfate-S is adsorbed by clay. A single season’s rainfall should not deplete the soil of sulfate-S as shown in Figure 1 (Castellano). The sulfate from the calcium sulfate treatment illustrates sulfate movement occurs but all sulfate is not removed or leached in a two year period.
Further evidence that an annual application of S is not necessary can be found in Table 2 (Kiemnec). The 88 and 176 kg S/ha as gypsum were applied in 1974 only. Forage yield in 1978 was not different than annual applications of gypsum.
Table 2. Total dry matter forage yield over a four year period at the Roseburg site as influenced by rate, source and particle size of sulfur applied annually or as a single application.
| Source | Rate1 | 1975 | 1976 | 1977 | 1978 | cumulative |
| Kg S/ha | metric tons/ha | |||||
| Check | 0 | 5.11 | 5.46 | 3.68 | 4.88 | 19.13 |
| gypsum | 11 | 5.80 | 7.52 | 4.99 | 6.77 | 25.08 |
| 22 | 5.19 | 6.78 | 4.38 | 6.95 | 23.30 | |
| 44 | 5.87 | 7.94 | 5.05 | 7.11 | 25.97 | |
| 88 | 5.97 | 6.41 | 5.73 | 6.13 | 24.23 | |
| 176 | 5.66 | 7.14 | 4.99 | 6.28 | 24.07 | |
| S(f) | 11 | 4.81 | 7.06 | 5.52 | 6.17 | 23.55 |
| 22 | 5.30 | 6.88 | 4.15 | 6.33 | 22.67 | |
| 44 | 5.68 | 8.11 | 5.43 | 7.62 | 26.84 | |
| 88 | 5.52 | 7.35 | 5.21 | 5.27 | 23.35 | |
| 176 | 5.68 | 7.56 | 5.39 | 5.97 | 24.60 | |
| S(Mf) | 88 | 4.68 | 7.28 | 5.29 | 6.18 | 23.43 |
| 176 | 5.09 | 7.29 | 4.66 | 6.89 | 23.92 | |
| S(mc) | 88 | 5.28 | 6.79 | 4.98 | 5.95 | 23.00 |
| 176 | 5.18 | 6.76 | 5.46 | 5.91 | 23.31 | |
| Sİ | 88 | 5.45 | 7.37 | 4.97 | 5.22 | 23.01 |
| 176 | 5.43 | 6.47 | 5.70 | 6.89 | 24.48 | |
| LSD(.05) | 0.37 | 0.25 | 0.26 | NS | 2.99 | |
111, 22 and 44 kg S/ha applied annually; 88 and 176 kg S/ha applied in Sept., 1974 only.
If you are faced with the need to reduce fertilizer cost because of the increase in ammonium sulfate prices, three choices exist. Don’t apply any S this year if S has been a part of the fertilizer program for the last three to four years. The second choice is to reduce the amount of S applied. Instead of applying 30 to 40 lb of S/a apply 10 to 15 lb/a or a correspondingly lower rate. The third choice is to look for alternative sources. Check the price of 16-20-0-14, potassium sulfate, ammonium thiosulfate, magnesium sulfate, potassium magnesium sulfate, or elemental S. Suggesting that elemental S be used in an annual cropping situation or where S will be required in the season of application usually elicits a response that elemental S will not be oxidized to the sulfate form quickly enough for the current season crop.
Table 2 provides support for the statement that elemental S provides sufficient S for a crop in the season of application if the particle size is sufficiently small. The application of 44 kg/ha of fine S, S(f), produced a yield higher than the check and no different from any of the rates of sulfate –S as gypsum in the first harvest year, 1975. Fine S, s(f) was described by Kiemnec as particles that passed through a 60 mesh sieve. The openings in a 60 mesh sieve are 250 microns. Several elemental S products currently available consist of S particles 50 microns and smaller bound together with a water soluble binder to form a normal fertilizer size granule.
Willamette Valley spring soil temperatures should be adequate for oxidation of elemental S to sulfate if the particle size is sufficiently small. A sufficiently small particle size is surely 50 microns and probably as large as 250 microns.
Don’t stress over S applications this spring. Reduce S application, choose a less expensive alternative, or skip applying S this year if S has been part of the fertilizer program for the last 3 to 4 years.
References
Castellano, S.D. 1990. Influence of cropping and sulfur fertilization on sulfur transformations in soil. M.S. Thesis, Oregon State University, Corvallis.
Kiemnec, G.L. 1984. Diagnosis of sulfur deficiency and sulfur fertilization of Trifolium subterraneum L. Ph.D. Thesis, Oregon State University, Corvallis.
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