Rains Boosts Fertilizer Needs

BOZEMAN - Jun 9/05 - SNS -- Recent heavy rainfall in western Canada and parts of the United States mean the fertilizer needs of this year's crops may be considerably higher than the amount growers applied last fall or this spring, according to a Montana State University soil scientist.

Recent rains in Montana that have increased soil moisture to well above five-year averages may warrant additional nitrogen fertilizer on crops to increase yield and quality, said Blain Jones, MS Extension soil fertility specialist.

He said that the percentage of Montana fields with adequate or surplus topsoil moisture has risen from 37% in early April to 83% for the week ending June 5, based on Montana Agricultural Statistics Service data. By comparison, the five-year average for June 5 is 54% with adequate or surplus moisture. The percentage of subsoil with adequate or surplus moisture hasn't increased as much, but is still above five year averages.

The favorable moisture levels have translated into 68% of the spring wheat crop being rated good to excellent, compared to 48% for the five-year average. "Because many nitrogen fertilizer decisions were made in early spring after a dry winter, many fields probably don't have enough nitrogen for their new, higher potential yields.", Jones said. In addition, he said some nitrate leaching may have occurred on shallow, coarse-textured soils. Therefore, the amount of nitrogen needed to maximize both yield and quality is probably higher than growers would have estimated two months ago.

Winter wheat is probably too tall to be top-dressed with nitrogen fertilizer, except by plane. However, both spring grains and grass can be top-dressed with fertilizer now if the soil is dry enough to get equipment into the field.

The difficult decisions that producers and their consultants may now want to make are first whether a crop should be top-dressed with nitrogen, and if so, with how much. "Predicting the weather for the remainder of the growing season is impossible, so all that growers can do is to make a best estimate of nitrogen needs with what is known," said Jones.

The total fertilizer nitrogen need in areas that now have near normal soil moisture can be based on historical average yields and using the booklet "Fertilizer Guidelines for Montana Crops" that can be ordered through or obtained at your local Extension office. A soil nitrate-nitrogen test will be invaluable in calculating top-dress nitrogen needs. Nitrogen is then calculated as the total nitrogen need minus the sum of pre-plant nitrogen and soil nitrogen, Jones said.

Alternatively, potential yields for spring wheat at seeding can be adjusted by using the rule of thumb of 4-5 bushels per inch of available water (soil water plus growing season moisture).

For instance, in an area that received two inches more rain than anticipated when fertilizer decisions were made, the spring wheat yield potential should have increased by 8-10 bushels, which requires about 30 units of top-dress nitrogen, assuming other plant needs are met.