Crop Scene Investigation - 25 Solved: The case of the 20-foot header

Wednesday, May 5, 2010

by BERNARD TOBIN

The majority of Allan's soybean field turned yellow due to a potash deficiency. However, the strips that could be found every 20 feet across the field were not potash-deficient and, as a result, produced healthy, green soybeans which led to this strange pattern in the field.

"When the soil tests came back, they revealed that the potash levels in the yellow areas were down in the 60s and the potash in the green areas was in the high 80s and 90s," explains Keith Reid, soil fertility specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs.

"The case really came together when we linked the pattern in the field to Allan's combine and the fact that he left the straw on the field for six weeks," says Reid.

Basically, all the potash taken up by the wheat accumulated in the straw and chaff which was sitting in those swaths. With six weeks of wet weather, the potash leached out of the swaths and into the ground below before Allan had a chance to bale the straw. That left the strips with sufficient potash to produce a healthy plant, while the rest of the field was deficient.

Reid notes that potash deficiency tends to show up more in dry years, especially in Harriston loam. These soils tend to be more susceptible to drought stress, which reduces potash availability.

"The message here is that growers need to use soil tests to manage soil nutrient levels before deficiencies show up," says Reid.

"This will be a really important consideration as we move into bio crops and removing residues. If they are harvested fresh, they remove a huge amount of potash from the field. If they sit and dry and weather in the field before they are harvested, most of that potash gets returned back to the system."

Congratulations to Michael Strang, Exeter, for his correct answer. BF