May, 1999

Grid soil sampling has been used for several years to guide variable-rate fertilization. Recent studies show that soil fertility patterns may be the result of topography or other predictable reasons. Thus, a high level of information might be obtained with minimal sampling and analytical expense.

Reasons for grid sampling
Grid sampling is a systematic sampling that uses a high density of soil cores to reveal soil fertility patterns. Grid sampling is especially useful when man-made variations in soil fertility levels occur that are not revealed in plant growth differences. Agronomically, there is nothing wrong with sampling by grids if it is conducted relatively densely, such as one sample per acre. However, few growers are willing to sample densely enough to give adequate information to direct variable-rate application consistently because of the time and expense.

Topography and soil fertility levels
Soil fertility levels may be related to landscape, especially in areas where high levels of fertilizers or manure applications have not been made. Mobile nutrients move in the soil from upland positions to depressions where water may collect and move into the soil. Water that infiltrates upland positions moves downward until it reaches a discontinuity in texture (coarser over finer or fine over coarser) which causes the water to flow laterally.

Nitrate may or may not be high in depressions due to the activity of denitrification bacteria. In saturated soils, denitrification bacteria transform nitrate into gaseous forms of N which are not taken up by plants. The nitrogen is therefore lost to plant uptake. In semi-arid areas, the depressions are often high in nitrate.

However, in more humid regions, the activity of denitrification bacteria is relatively high, and low levels of nitrate may be present in depressions, along with high levels of chloride and sulfate.

Non-mobile nutrient levels may also be related to topography. Due to increased plant growth and lower mineralization, depression areas tend to have higher levels of organic matter and clay-sized particles than upland areas. Organic matter accumulates micro-nutrients and P, while clay size particles may contain more K than silt or sand-sized particles. On the other hand, higher crop yields in depressions compared to upland positions may lead to higher P levels on upland positions.

Levels of nutrients are also affected by past erosion. Upland positions are particularly subject to wind erosion in semi-arid environments. In many fields in the central North Dakota glacial till region, hilltops are bare of the original high organic matter surface layer due to the action of wind during the last one hundred years of cropping. The hilltops often contain lower levels of all nutrients because of the loss of organic matter. Depression areas may contain additional clay and organic matter from water erosion and deposition. Or, some intermediate areas within slopes may have shallower organic layers due to accelerated erosion since the onset of cropping. Sulfur availability appears to be particularly sensitive to low organic matter levels. Sulfur deficiency in canola is often seen on eroded slopes but not in areas on the same slopes with higher organic matter.