You know the old saying about what happens when you assume something. Well, that saying could apply if you assume the phosphorus (P) and potassium (K) levels in your fields are high enough not to limit yields.

At the same time, if you assume you need more P and K without checking a current soil test, you may be wasting your money.

Conventional wisdom among farmers and agronomists says that many North American fields have enough P and K for crop production and won't benefit from any more fertilizer. However, a review of 1.8 million soil samples says that assumption is not always correct.

The Potash and Phosphate Institute (PPI) is a non-profit organization of potash and phosphate fertilizer producers. PPI summarized soil tests and found almost half of the samples - one of every two acres - tested medium or below in P and K. Those are levels where most agronomists would expect a significant yield jump in the year you applied fertilizer.

The summary reveals 46% of the samples tested medium or below in P and 44% tested medium or lower in K. The highest frequency of tests medium or below P, 60-80%, was found in the Northern Great Plains. East of the Mississippi, 16 out of 23 states had half or more of the K tests in medium or lower results. Eastern states had more soils in the medium to lower range for K than Western states because Western states had less weathered soils.

The summary of soil tests pulled in the fall of 1996 and the spring of 1997 indicate the nutrient-supplying capacity available in soils in North America for the '97 cropping season.

Many fields, farms and areas within the fields would grow crops more profitably with more P and K, says Paul Fixen, a PPI senior vice president based in Brookings, SD. Yet, he strongly cautions against application on those acres that don't need P and K.

With the current low corn and bean prices, you may be reluctant to spend any more cash. But it takes time to change P and K levels. High soil test levels mean you can probably hold off applications without any yield-limiting consequences. However, the summary shows that farmers in some areas don't have that flexibility because their soil test levels are low.

"We have gone from surplus nutrient budgets that build soil fertility to deficit budgets that really are nothing more than a loan from future productivity," he says.

Fixen isn't advocating that you base fertilizer decisions solely on the soil test summary, which he claims isn't perfect. The PPIexecutive says thesummary's results argue for a closer review of soil test data and maybe even mor e soil testing.

"We see a gap between application and what we see in soil tests. The acres are suboptimal in P and K because suboptimal levels have been applied. Maintenance may be right for some farmers, but not all," he points out.

George Rehm, a University of Minnesota extension soil fertility specialist, agrees that a critical look at soil tests can help save some money next spring. Your fields or areas of fields with medium or higher levels can do without P and K, letting you instead target your fertilizer dollars to fields with medium or low levels.

Soybeans, in particular, benefit from higher phosphorus. Rehm says it pays to add P when its readings are less that 10 ppm for the Bray test and less than 8 ppm for the Olsen test. For example, he cites research figures from '97 when beans without P yielded 23 bu/acre and those with 46 lbs of phosphate, which cost $11/acre, yielded 35 bu/acre.

"Those 12 bu - however you price them - are worth more than the $11 an acre for phosphate," he points out. "However, once you get above 10 ppm the potential for yield increase in soybeans is very small."

Soil tests taken a couple of years ago can still help. He says, "Soil test values for P and K don't change rapidly. Tests two or three years old will tell me where I am in the ballpark so I can make decisions. Don't ignore the possible need to fertilize for soybeans."

Brad Joern at Purdue University says that the summary shows farmers have done a good job improving fertility over the years.

"Very few soils were adequate in the '50s," he says. However, the soil fertility trends still need to be viewed on a field-by-field basis for farmers. "For individual producers, this summary isn't very meaningful. They (farmers) need to assess the situations on their farms," he adds.