Apr. 13, 2009
Over 40 years of prior research in Iowa had rarely noted improved crop yield with sulfur (S) fertilization and S deficiency was not considered an issue for crop production. Statewide and regional studies conducted in Iowa during that time period with corn and soybean found yield increase from S fertilizer application only three times out of nearly 200 trials, with one multi-year study having a small average yield decrease. Research in the early 1980s had also documented sufficient plant available S in the soil profile for crop production on most Iowa soil associations. Results of recent studies in corn and soybean production in areas of Iowa outside of northeast Iowa (2000-2005) were consistent with results of the historical research.
However, over the past decade alfalfa grown on some silt loam and loam soils in northeast Iowa exhibited a slowly worsening problem with areas in fields of stunted growth and poor coloration. Investigations determined the growth problems were largely due to S deficiency, with the most prominent symptoms in field areas with low soil organic matter and side-slope landscape position. On similar soils and on coarse textured soils, early corn growth has also recently been exhibiting strong visual S deficiency symptoms.
On-farm research trials were conducted to determine alfalfa and corn response to S fertilization and evaluate specific soils and extent of northeast Iowa affected by S deficiency. The following provides a summary of research conducted in northeast Iowa alfalfa and corn production fields, methods to identify potential S deficiency, and S fertilization guidelines.
Alfalfa Response to Sulfur Fertilization
In 2005, on-farm trials were conducted on established alfalfa fields near Elgin, Gunder and West Union, Iowa. These sites were selected because there were large areas in the fields with both poor and good alfalfa plant coloration and growth. Within identified poor and good coloration/growth areas, three fertilizer treatments were established. The treatments consisted of a no-S application, 40 pound S/acre as ammonium sulfate, and 40 pound S/acre as calcium sulfate (gypsum). Treatments were applied after the first cut. Alfalfa harvests included second cut and third cut in 2005 at all three sites, and first cut in 2006 at the Elgin and Gunder sites.
Dry matter yields with applied S on the good areas were not different from that of the unfertilized no-S control. However, S applied on the poor areas more than doubled yields in 2005 and nearly double yields in 2006. Plant analysis from the untreated poor areas was 0.14% S, clearly well below the suggested sufficiency level of 0.25% S. Plant analysis for the untreated good areas was also considered deficient at 0.22% S, but by a very small margin.
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