Assessing the Sulfur Status of Field‐Grown Wheat by Plant Analysis1

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Abstract
Diagnostic methods developed under laboratory or greenhouse conditions require field confirmation. This paper reports the relative field performance of four indexes for the diagnosis of S deficiency in wheat (Triticum aestivum L.) which were previously assessed in the greenhouse. Sulfate S (as percent total S), sulfate S concentration (as ppm dry matter), total S concentration (as ppm dry matter), and total N: total S ratio in the aboveground parts of a soft white wheat cultivar, ‘Olympic’, were compared as diagnostic indexes in a factorial experiment with five rates of gypsum (0 to 50 kg S/ha) and three rates of ammonium nitrate (0 to 100 kg N/ha). A strong relationship was found between final grain yield and each index at the following stages of growth; fourth leaf (61 days from sowing), fully‐tillered (92 days), and jointing (134 days). However, as the critical value for sulfate S (as percent total S) was least affected by the age of the plant or N supply, it is considered to be the most convenient index. The overall relationship between yield and sulfate S (as percent total S) was strong (r = 0.88) and the critical value at 90% of maximum yield was 13%. This compares with r values of 0.87, 0.96, and 0.91 and critical values of 14, 12, and 11% for the plant tissue at the individual growth stages examined. Where the adequacy of the S supply to a wheat crop is in question, the proportion of total S as sulfate S in the plants is indicated as a useful guide to the need for S fertilizer in securing optimal grain yield.

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