Crop Scene Investigation - 37 Solved: What caused the yellow spots in that winter wheat?

Monday, March 5, 2012

by BERNARD TOBIN

The yellow spots identified in wheat fields last spring by Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) cereals specialist Peter Johnson were caused by sulphur deficiency.

Tissue sampling revealed that green, healthier plants in other areas of the field tested much higher for sulphur levels than the yellow plants. The diagnosis was confirmed when Johnson applied sulphur to test strips of yellow plants.

Johnson explains that sulphur deficiency has rarely been seen in wheat, mainly due to the impact of acid rain, which would typically deposit up to 30 pounds of sulphur per acre in the early 1990s, almost double the requirements of a wheat crop. Johnson quips that acid rain "should have been called foliar fertilizer."

As environmental measures have reduced acid rain, the amount of sulphur available to wheat crops has also been cut significantly. "By 2005, we were seeing less than 15 pounds per acre and in 2010 we were getting down to the 10 pounds per acre range," says Johnson.

With those numbers in mind, Johnson is now telling growers it's time to take a look at sulphur in their wheat crop. In 2011, he conducted 26 sulphur trials in winter wheat with an average increase of 5.5 bushels per acre. Twenty of the trials showed a positive economic return, while three were neutral. One trial north of London delivered a nine-bushel increase.

Johnson pegs the cost of the additional sulphur at about one bushel per acre. "If you had seen a bit of discolouration in your fields last year, maybe you should just go to the field with five or 10 pounds of sulphur per acre – and leave some strips to test," he says. Congratulations to Ashley Mosburger, Bright, Ont. for his correct answer. BF