Soil Variability and Sampling

Soil variability is the reason sampling procedures are so important. Soils vary continuously within fields and between farms. Surface variation may be easily seen, but nutrient variability is usually not obvious. An example of how soils might vary is shown in Figure 9.1.

Figure 9.1.  Typical variability by soil mapping unit.  

The most noticeable differences are those which are physical because they are visible. Texture, color, slope, erosion, and drainage are some obvious visible differences. For example, bottom land typically has more organic matter and is darker. Uplands, particularly the eroded slopes, have less organic matter and are lighter in color.

Until a few years ago, soil sampling was recommended for 20- to 40-acre units in a field split by some logical dividing principle, such as topography or previous management history. In practical terms, this usually meant splitting the field into quarters. Site-specific farming has introduced the idea of grid sampling and managing small field areas separately. Specific procedures for grid sampling will be discussed later in this lesson. The following is a summary of the traditional sampling procedures.