Farmers already pressed by rising production costs may be spending many millions of dollars on unneeded fertilizers because of widely inconsistent and inaccurate recommendations by the country's major soil-testing laboratories.

These and other findings from a study of commercial and public university labs that do 75 percent of the nation's soil testing are reported in the January issue of New Farm magazine, a publication of the Rodale Press at Emmaus, Pa.

Dr. William C. Liebhardt, a soil scientist who headed the study by the Rodale Research Center, said that inaccurate recommendations by many of the laboratories mean farmers could be spending between $16 and $25 per acre for fertilizers their land does not need.

He said that rough calculation was based on the recommendations from labs that tested identical samples from six different crop rotations, sent to them ostensibly as soil from a farm in the Pennsylvania Dutch country--not identified as a Rodale project--seeking a 125 bushel-per- acre corn yield.

As an example, Virginia Polytechnic Institute, at Blacksburg, recommended across-the-board that the Pennsylvania farmer should apply 138 pounds of nitrogen per acre, at an average cost of $20.70, on each of the six test plots. Some of the plots had been planted with alfalfa and soybeans, which actually add nitrogen to the soil.

In contrast, the University of Maryland laboratory at College Park recommended considerably lower amounts of nitrogen for the same soil, which would have cost the farmer $15.49 per acre--$5 less than the VPI recommendation.

Similar variations in recommended fertilization levels showed up in the results from the 67 other private and public labs. Suggested nitrogen rates for the same soils ranged from zero to 210 pounds per acre, apparently without regard to previous legume crops, cropping history and soil organic content.

American farmers last year spent more than $8 billion on fertilizer, an important element in maintaining high production levels. The prospective deregulation of natural gas, vital to nitrogen manufacture, is expected to push fertilizer prices still higher. Nitrogen is the principal fertilizer used by U.S. farmers.

The Rodale findings on nitrogen, certain to stir controversy in agribusiness circles, are the first in a series that New Farm will publish this year on other major fertilizers--phosphorus, potassium, lime and micro-nutrients (trace elements such as manganese).

Liebhardt said that the laboratory recommendations for these other fertilizers roughly paralleled the inconsistencies and obvious overdosages found in the nitrogen portion of the study.

"Soil testing is not an exact science. It is a series of approximations," Liebhardt said yesterday. "But I was flabbergasted . . . .A number of labs eliminated nitrogen completely; some took into account past cropping history. There were variations all over the board."

"One of the results of this is that the farmer who follows a certain laboratory's recommendations may spend up to $25 an acre for fertilizers that don't add any more corn to his crib," Liebhardt said. "We feel that if the farmer has the right information, he'll begin to make more judgments about his needs."

Liebhardt, who formerly taught soil science at the University of Delaware, said it is "fairly standard" for fertilizer companies to pay the costs of a farmer's soil tests as an incentive for making a sale. "We can't say there is collusion," Liebhardt said, "but a farmer puts himself in a vulnerable position if he lets a company pay for his tests."

As an example of savings from more accurate tests, Liebhardt cited a Maryland farmer who recently sought fertilizer advice from the Rodale center. The farmer reported a saving of $25,000 on potassium alone, with no apparent change in crop yield, by following a different prescription.