When soil goes sour

Ammonia based fertilizer, which provides nitrogen, can offer a great boost to even an otherwise not so healthy soil. But ammonia fertilizer, which depends on petroleum for its manufacture, is becoming very expensive. The consistent high yields of wheat on the Palouse depend on applying about 100 pounds of fertilizer per acre, with that fertilizer currently costing $50-80/ton. More significant, however, is not the cost, but the long-term effect of applying so much fertilizer.

Soils on the Palouse before farming were generally neutral, with a pH of 7, says Rich Koenig. Since then, the pH of the soil has dropped in some cases as much as two full pH units because of ammonia-based fertilizer use.

Two pH units represent a 100-fold change, so those soils are 100-fold more acidic than they were 30 years ago.

“It’s a trend that we’re going to have to deal with at some point,” says Koenig. “We’re already seeing some pretty major crop-yield implications due to soil acidity.”

Many things change in the process of acidification, he says. “Minerals dissolve, minerals form, there are a lot of secondary byproducts, and implications in phosphorous nutrition, for example.”

The major concern is the effect of acidity on legumes, which are an integral to Palouse crop rotations and through their rhizobial activity contribute nitrogen to the soil. Excessive soil acidity inhibits the rhizobial activity.

The most extreme problem with low pH is around the Rockford, Washington, area, south of Spokane. Farmers there have long grown grass seed, which requires twice the amount of fertilizer as wheat.

“Twenty years ago, they were able to rotate legumes in,” says Koenig. But no more. The soil is simply too acidic for legumes. The pH of some soil in the area is below 4.

“Essentially they’re in a wheat-grass seed rotation,” he says. “The last five years’ wheat yields have been miserable.”

The traditional remediation for low soil pH is to add lime. Unfortunately, lime in the Pacific Northwest is not readily available. Shipping in sufficient lime to treat the fields would be astronomically expensive.

Koenig’s research group is exploring some of what he considers short-term solutions.

It is possible to genetically induce tolerance in wheat. But not legumes. “The problem with legumes is not the plant,” he says, “but the bacteria.”

“Ultimately,” he says, “we’re going to have to lime.”