Brian Lang, Crops Field Specialist, Northeast Area
For the last few years, many alfalfa fields in northeast Iowa exhibited a light green to dark green mottling. Farmers were concerned about poor alfalfa production in the lighter areas. Plant disease analysis and soil tests were not able to explain the majority of these situations.
Sulfur fertilization has always been suggested for alfalfa grown on sandier, low organic matter soils, however many of the fields exhibiting these symptoms have finer textured, higher organic matter soils. Potential contributing factors to the problem include: 1) Atmospheric sulfur deposition in air pollution has decreased at least 50% over the last 20 years (Data from Wisconsin DNR). 2) The sulfur soil test is not accurate at predicting sulfur needs for crops, however, many farmers and fertilizer dealers are unaware of this. 3) Plant analysis is the preferred method for determining the need for sulfur fertilization, but it is not very convenient or well understood.
ISU Extension Crops Specialist investigated many of these fields and noted reduced nodulation by alfalfa in the lighter areas. Factors that can influence reduced nodulation include plant disease, nematodes, clover root curculio, wet/seep areas, droughty soils, low soil pH, and sulfur deficiency. The Crop Specialist decided to use on-farm field demonstrations to help trouble-shoot the problem of these lighter colored areas. Six on-farm demonstrations were established in the summer of 2004. Plant and soil samples were collected for disease analyses, nutrient analyses, and soil tests. Field plots were also flagged and fertilized with sulfur fertilizer.
On-farm demonstrations pointed to two significant problems. One was a fairly high prevalence of Aphanomyces root rot. The other was a highly significant response to sulfur fertilizer, with at least a doubling of dry matter yield in the previously sulfur deficient areas.
This data and photographs of the sites were shared with approximately 1,000 alfalfa growers during winter meetings. Approximately 50% of the farmers commented on similar visual symptoms in a least part of their farm operation. Fields that received manure applications usually did not show this problem, probably because manure contains significant amounts of sulfur.
Many of these farmers will experiment with sulfur fertilizer applications this year because of the evidence presented at the meetings. ISU Extension will also initiate a more detailed study in 2005 to further pursue and pinpoint the potential need for sulfur fertilization. The preliminary anticipated response to the farmers use of sulfur fertilizer on fields identified with this problem is for an average net profit of $40 per acre.
April 15, 2005
103 -- Nutrient Management
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