NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

December 3, 1998

Soil Test Key to Phosphate Management

Crop producers coping with tough financial times might be thinking about cutting back on inputs like fertilizer to reduce cash outlay. There might be some opportunity for many growers to cut back on phosphate fertilizer, but an accurate soil test is essential to determine if cutting back fertilizer will raise or lower production costs, according to Joe Giles, soil science researcher at North Dakota State University.

Phosphate is one of the three macronutrients, along with nitrogen and potassium, essential for plant growth, and is the one of the three that moves very little in the soil. Nitrogen, for example, will move with soil water and can leach out of the soil profile, but phosphate will not. Phosphate is taken up by plants over very short distances, so it needs to be located, in available form, very close to plant roots.

Giles says growers tend to follow one of three general management philosophies to assure their crops have adequate phosphate available. One is to apply the same amount of fertilizer every year. A second is to maintain the fertility level, basing the season's fertilizer application on how much phosphate was removed by the crop the previous year. A third is deficiency correction, applying enough fertilizer to build soil phosphate levels and increase the soil test level.

Soil testing results in North Dakota and Minnesota rate 30 to 60 percent of soil samples high in phosphate. Results below this level would reveal a need to be concerned about available phosphate, Giles says.

North Dakota soil testing uses five soil test calibration categories, ranging from very low to very high, to give meaning to soil test results. In soils with a very low rating, the probability of getting a response to applied fertilizer is very high, greater than 80 percent. Crops on fields rated low will respond to fertilization 60 to 80 percent of the time. With a medium rating, the probability of getting a response is about even, from 40 to 60 percent. In the high category, crops will respond to applied fertilizer about 20 to 40 percent of the time, while on soils rated very high the probability of a response is quite low, at less than 20 percent.

The important thing, says Giles, is to get phosphate to the critical level where the crop can reach an optimum economic yield. With a high or very high soil test, there is probably no need to apply fertilizer for the current year's crop, but soil phosphate levels may rate lower in future years.

Growers need to invest in a soil test and decide from the results how much fertilizer to apply, says Giles. If a field tests low or very low, applying phosphate fertilizer is likely to pay off. If the test shows medium or higher phosphate, it would be possible to cut back, at least for one year, without significantly lower yields Adequate phosphate levels could mean cuts in production costs per bushel and increases in net return.

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Source: Joe Giles (701) 231-7858

Editor: Gary Moran (701) 231-7865