Grid Sampling Soils to Improve Understanding of Soil Variability

Jarrod O. Miller, Extension Agronomist, and James Adkins, Irrigation Engineer,

For grid sampling, prior work in other states has shown advantages of grids no larger than 2.5 acres, with increased accuracy at 1 acre. To observed how accurate grids must be on the Delmarva peninsula, the Maryland Grain Producers sponsored a project at the University of Delaware Warrington Irrigation Research farm in Harbeson, DE. Soils were sampled on 0.25 acre grids, and then down sampled to create maps based on ¾, 1 ½ , and 3 acre grids.

Grid Sampling Correlations
The densest sampling scheme (¼ acre) was strongly correlated with soil characteristics and nutrient contents at the next densest sampling of ¾ acres (Table 1). The ¼ acre sampling was not as strongly related to 1 ½ or 3 acre sampling schemes, particularly for organic matter and CEC measurements. This is important, considering that organic matter and the CEC are unlikely to change over the long term, so a more accurate map could last a longer period. In some cases, correlations were not significant between 3 and 1.5 acre grids (CEC, OM, and Mg), so the relationships to general soil characteristics are again better represented by a denser grid.

Table 1: Correlations Between Grid Densities

  1/4 Acre vs 3 Acre vs
  3/4 1.5 3 3/4 1.5
CEC 0.79 0.65 0.34 0.40 ns
OM 0.63 0.20 0.40 0.58 ns
pH 0.80 0.45 0.61 0.58 0.56
P 0.92 0.76 0.54 0.62 0.48
K 0.87 0.77 0.66 0.62 0.45
Ca 0.82 0.58 0.63 0.61 0.48
Mg 0.62 0.31 0.35 0.37 ns

Interpolated Maps of Different Soil Properties and Nutrient Concentrations
Maps for soil pH reveal more accuracy at ¼ density, but with a large loss in resolution when grids reach 1 ½ acres (Figure 1). What is particularly striking about the pH map for the ¼ grids is the blocked pattern on the lower half of the pivot, following the shape of the research plots, indicating past management. This pattern is not present in the lower density sampling. Grid sampling is often described as best at finding past management issues but may miss the smallest details above ¼ acre grids. Read the full article on the DE Agronomy blog:

Grid sampling for a) ¼ acre soil pH, b) ¾ acre soil pH, c) 1 ½ acre soil pH, d) 3-acre soil pH.

Figure 1. Grid sampling for a) ¼ acre soil pH, b) ¾ acre soil pH, c) 1 ½ acre soil pH, d) 3-acre soil pH.