Project News:
Soil test interpretation is a key to profitable crop production
by John Rodecap, Northeast Iowa Demonstration Project Coordination

Soil test interpretation was identified by producers in recent farming practices interviews as the key to crop nutrient management. Especially important is the upward trend in soil test levels that some producers have observed over several years.

These soil fertility records have helped producers save unnecessary fertilizer expenditures, especially where manure applications have occurred for several years.

The recent trend in fertility management has been to apply crop removal rates of phosphorus and potassium for crop production. Some producers have found this practice will quickly increase soil test levels into the high and very high range especially if manure and starter fertilizer are also applied.

A field demonstration on swine finishing manure management, known as the Burrack-Kregel study and held on a farm near Garnavillo, was initiated by the Big Spring Water Quality Project and continued by the Northeast Iowa Demonstration Project. This study clearly indicated how quickly soil fertility can be affected by manure application. After eight years of the same replicated rates of manure application or commercial nitrogen application with maintenance rates of lime, phosphorus (P) and potassium (K), the soil and stalk nitrate test levels were dramatically different, as shown in table 1.

* Optimum range for cornstalk nitrate nitrogen (NO 3 ) is 700 to 2,000 ppm
Note: Soil test levels above 31 ppm for P and 190 ppm for K are considered to be very high (VH).

High application rates of manure or nitrogen had an adverse affect on soil pH which will require more frequent application of lime and greater cost of crop production. There was no increase in corn yield when the manure application rate increased from 3,500 to 7,000 gallons per acre of manure testing 67 pounds of nitrogen in each 1,000 gallons of manure. There is sufficient phosphorus buildup in the manure soil, 70 to 216 parts per million, or ppm, to support crop production for many years without additional plow down application.

The question of phosphorus-potassium ratio was addressed in 1996 and 1998 in Clayton County field demonstrations on very high testing soils for phosphorus and potassium, where each application rate was replicated three times. The corn yield results with various rates of potassium are shown in table 2.

There was no difference in corn stalk quality or stalk rot among the potassium treatments and no consistent yield response to added potassium.

A field demonstration using various rates and sources of commercial fertilizer on soybeans in 1997 indicated no advantage to using additional crop nutrients on very high testing soils, as shown in table 3.

An additional long-range considerationfor livestock producers is the possibility of rules restricting manure application on fields with excessive soil test levels for phosphorus.

If unnecessary nutrient rates are applied and buildup of phosphorus, similar to soil test levels shown in table 1, occurs it might require producers to travel longer distances with manure to locate fields not testing excessive for phosphorus. The suggestion is to begin intensive soil nutrient management now to save money and avoid possible costly manure disposal problems in the future.

A soil test history should be developed to track phosphorus and potassium levels for each field. Producers need to cut through the unnecessary numbers on their soil test report to find the phosphorus and potassium test results. A few producers have become very proficient at soil test interpretation and crop nutrient management, saving large annual investments in unnecessary fertilizer and also protecting water quality.

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