From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Spring 1996, No. 5


Farmers frequently say, “I know fertility doesn’t limit my yields because for years I’ve been soil testing and following the resulting recommendations.” However, intensive soil sampling on many fields in the last few years reveals that this frequently is untrue. It’s common to see low testing areas completely masked by very high testing areas in the same field when conventional soil sampling procedures are used. In addition to plant nutrients, soil pH can vary widely. In the western Corn Belt, fields averaging near-neutral in pH often have acid areas testing below 5.5 surrounded by soil zones testing greater that 7.5. You would never realize a problem existed without intensive soil sampling.

The general pattern of soil areas within fields with untapped yield potential varies with cropping system, soil types and field history. The hidden potential in fields with good manuring or fertilizing histories often is found in the field areas currently producing the highest yields. Intensive sampling of fields with limited nutrient applications in the past often shows that the low yielding areas have the greatest hidden potential.

For example, recent landscape position sampling in North Dakota has shown that low yielding hill tops frequently test substantially lower in several nutrients than the bottom areas in the same field. The hilltops were lower in organic matter and higher in pH due to a combination of less soil development and soil erosion. Compared to the bottom areas, test values averaged 56 percent for nitrate, 38 percent for phosphorus, 13 percent for chloride, 35 percent for sulfate, and 53 percent for zinc. In many cases fertilizer application rates based on field averages would leave several nutrients limiting for the hilltop positions. Removing these nutritional limitations will not likely make these hilltops yield as well as other areas of the field, but soil test data indicate yields could be increased substantially. In this case, the major hidden yield potential exists in the soil areas that are currently the lowest yielding.

High residue management that allows more water to soak into hilltop portions of the landscape rather than run off, combined with more intensive nutrient management approaches could produce excellent returns. Landscape position sampling of fields that haven’t been manured or fertilized extensively in the past could help develop more profitable fertilization programs even if variable rate application equipment isn’t being used. More intensive sampling allows for more accurate determination of the type and rate of nutrients to apply uniformly across the field. Variable rate equipment brings additional precision by matching crop needs and application rates.

In contrast, fields with histories of extensive fertilizer or manure use often show that areas testing low in nutrient availability coincide with high yielding areas. Decades of greater nutrient removal from these areas have resulted in soil fertility depletion when nutrients have been applied uniformly across the field based on the average yield of the field. Such fields become more variable the longer they are farmed using conventional methods. The hidden yield potential is found in the areas that are already the highest yielding but that could be even higher yielding if nutrient limitations were removed.

Intensive soil sampling is a practice that can help growers find yield robbing nutrient deficiencies. The elevated crop prices of 1996 further increase financial rewards for identifying and correcting these deficiencies.

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