Plotting and Analysis of Soil Test Results


This page was designed to illustrate a way to plot soil test results and to compare soil data with a corresponding yield map using a spreadsheet. Soil analysis has become another site-specific management practice. Soil test results may be used to detect a cause of spatial yield variability as well as to prescribe variable rate chemical application treatment according to variation of soil properties. A raw data file contains a series of soil sampling locations with associated results of soil tests (soil and buffer pH, potassium and phosphorous content, CEC (cation exchange capacity), etc.).

Plotting of Soil Test Results

Similarly to yield mapping, it is possible to create an x-y plot of locations where soil samples have been collected. This plot also may be categorized based on values of a sertain soil parameter (graph above). In order to obtain a soil map, latitude and longitude of each soil sample shall be converted into x-y coordinates (easting and northing) the same way as boundary and yield data.

Laboratory soil analysis is expensive, which limits affordable soil sampling density. Therefore, interpolation is the most common method to determine value of a soil property at an unsampled location. Although, many interpolation methods are known, none of them is suitable for every application. One of the most common approaches that can be done in a spreadsheet environment is "one nearest neighbour". In this case, a grid with defined easting and northing spacing (samller than the size of the sampling grid) is established. Each grid location is assigned a value of the soil property that corresponds to the nearest actual sample. In this case, the following soil map may be obtained:

Comparison of Yield and Soil Test Results

One way to utilize results of soil analysis is to compare them with corresponding yield results. It can be done by estimating average yield at each soil sampling location. In a spreadsheet, all yield values included in a fixed (10 m) radius circle around each sampling location are averaged, and added to soil test results table as an extra column. Then, yield may be compared to each individual soil property or their combination.


You may exercise plotting and analysis of soil data using web_soil_data.xls - raw results of soil analysis performed on samples collected from Fields V and R at Davis-Purdue Agriculture Center (Farmland, IN) in spring of 1998.
Verify your results with folowing examples:
web_soil_ph.xls - 1998 soil pH maps from Fields V and R at Davis-Purdue Agriculture Center.
web_soil_v_sq.xls and web_soil_r_sq.xls - interpolated 1998 soil pH maps from Fields V and R at Davis-Purdue Agriculture Center.
web_yield_v_comp.xls and web_yield_r_comp.xls - determination of yield at each soil sampling location in Fields V and R at Davis-Purdue Agriculture Center.

Last updated: October 31, 2000
Author: Viacheslav Adamchuk

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