The perfect soil pH for growing field crops ranges between 6.2 and 6.8. If you get much above 7.3, the soil pH is considered high. This causes a reduction in nutrient availability and makes plants more sensitivity to certain herbicides -  either the chemicals work too well, or not at all. 

Robert Hoeft is an emeritus professor of soil fertility at the University of Illinois. He says particularly in soybeans, the two nutrients that are affected the most by high pH are iron and manganese.

"Both of them have very similar symptoms. They both have what we call “interveinal chlorosis”. The veins stay green, in between the veins they get chlorotic or yellowed. With iron, the chlorotic area often times will go to a white, and the leaves will just look like they’re bleaching out," says Hoeft. "On manganese instead of getting white, they tend to get a rusty appearance."

He says if you’re east of the Mississippi river, you’re likely to have a manganese deficiency.  West of the river, it’s usually iron that’s lacking. 

It isn’t practical to correct a high pH problem in the soil itself, so foliar sprays are needed.

 

"If it’s manganese deficiency you can spray them, they’ll green up," he says. "If it’s iron deficiency, it’s more difficult to correct the problem with a foliar spray. With iron, you’re better off selecting the varieties that are more resistant to the problem."

Hoeft says don’t spray the entire field, just spray the affected areas. If other parts of the field aren’t showing symptoms, it’s not a problem.