Sulfur Diagnostic Criteria as Affected by Age and Defoliation of Subclover
- M. B. Jones2,
- J. E. Ruckman3,
- W. A. Williams3 and
- R. L. Koenigs2
Sulfur deficiencies are widespread in California annual grasslands and in other areas of the world where subclover (Trifolium subterraneum L.) is an important legume. Intensive grazing is often practiced where subclover is grown. Therefore, to determine the need for S fertilization it is important to know how defoliation and plant age influence the diagnostic tissue tests used to assay subclover S status.
Subclover was grown on S−deficient soil (Josephine, fine−loamy, mixed mesic, Typic Haplozerults) with six levels of applied S and four clipping treatments in a greenhouse. Four criteria for assaying S status were used: total S, sulfate S, sulfate S/total S ratio, and N/S ratio in the three youngest fully expanded leaves required to give 90% of maximum yield (critical levels based on Mitscherlich functions).
The critical levels of sulfate S/total S and N/S ratios were the most stable over plant age and clipping frequency. Both assays accounted for about three−quarters of the variation on the average.
The critical levels of total S and sulfate S although associated with over 90% of the variation were not stable individually, decreasing from 0.20 to 0.08% S and from 220 to 140 pprn sulfate S at 60 and 133 days from planting, respectively. Clipping frequency caused considerable variation in the critical levels for total S and sulfate S assays, making them more difficult to interpret than the sulfate S/total S and N/S ratio assays.
It was concluded that the sulfate S/total S ratio has some advantages over the other assays, such as stability through the growing season, being little affected by defoliation, and resulting from a single analytical regime.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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