6.4 Site Characterization: Soil Horizons, Surveys & Landscape Analysis

6.4 Site Characterization: Soil Horizons, Soil Surveys & Landscape Analysis

Before making any nutrient, tillage, drainage, or conservation recommendation, a Certified Crop Adviser must characterize the site. Site characterization answers the questions: What soil is here, how was it formed, how is it drained, and how does the landscape influence management? The International CCA Performance Objectives treat this as a distinct competency area because misreading a field's soils leads directly to wrong recommendations-applying nitrogen to a poorly drained wet soil, or prescribing deep tillage on a fragipan.

Soil Horizons and the Soil Profile

A soil profile is a vertical cross-section exposing genetically distinct layers called horizons. The master horizons a CCA must differentiate are:

Parent material is the unconsolidated mineral or organic matter from which a soil develops. Glacial till, loess, alluvium, lacustrine (lake-bed) deposits, and residuum (weathered bedrock) are common parent materials. Parent material sets the initial texture, mineralogy, and background fertility of the soil-a loess-derived soil will be silty and fertile, while a residuum soil over limestone will be calcareous and shallow.

Field Area Calculations

A precise field area underpins every fertilizer, seed, and pesticide calculation. Rectangular fields use Area = length x width. Irregular fields are decomposed into triangles and rectangles; the area of a triangle is 1/2 x base x height. Because inputs are priced per acre, you must convert to acres: 1 acre = 43,560 sq ft.

For a rectangular field measuring 1,320 ft x 660 ft: area = 871,200 sq ft / 43,560 = 20 acres. For a triangular wedge with a 660 ft base and 330 ft height: area = 1/2 x 660 x 330 = 108,900 sq ft / 43,560 = 2.5 acres. GPS receivers, GIS software, and planimeter tools now calculate irregular areas automatically from mapped boundaries, but the CCA must still sanity-check the output.

Slope Determination

Slope drives erosion risk, water movement, equipment safety, and irrigation runoff. Percent slope = (rise / run) x 100. A 20-foot rise over a 200-foot horizontal run is a 10% slope. Slope is determined from:

• Contour maps and digital elevation models (DEMs): contour spacing indicates steepness (closely spaced contours = steep slopes).
• Clinometers or laser levels for field verification.
• GIS and GPS elevation data collected in the field.

Slope length and steepness together form the LS factor in RUSLE, the soil-erosion equation discussed in Section 7.2.

Soil Drainage Class

Soil drainage describes how quickly water leaves the profile and is a primary land-use determinant. Well-drained soils show bright, uniform colors (reddish-brown to yellowish-brown) throughout, indicating oxidizing, aerated conditions. Poorly drained soils exhibit redoximorphic features-gray, bluish, or greenish gleying and orange or brown mottles (iron concentrations) within the upper profile, indicating seasonal water tables and anaerobic conditions.

Surface drainage is identified by standing-water patterns, grassed waterways, and surface inlets. Subsurface drainage is identified by tile lines (drain outlets, blowouts, or wet spots in a regular pattern), county drainage maps, and the depth to redox features. Drainage class is published in the soil survey for each mapping unit.

Using the Soil Survey

The NRCS Web Soil Survey (and equivalent regional soil survey reports) is the primary tool for locating soil mapping units (a named soil series and its phase, such as "Drummer silty clay loam, 0-2% slopes") on a tract of land. Each mapping unit carries published characteristics-texture, drainage class, pH, organic matter, available water capacity, CEC, and productivity ratings.

The CCA must understand the limitations of mapping scale. A published soil survey is drawn at a set scale (commonly 1:15,840 to 1:24,000); the minimum legible delineation is roughly 2-3 acres. Soils smaller than that, or contrasting inclusions (a small pocket of sand within a clay unit), are not mapped separately. This means a CCA cannot rely on the survey alone for in-field management-ground-truthing with a soil probe and observing crop performance is essential.

Land-Use and Management Implications

Site characterization directly governs management decisions:

• Leaching potential (sandy, well-drained soils) restricts fall nitrogen applications and favors split applications and nitrification inhibitors.
• Erosion potential (steep, long slopes, silty soils) requires contouring, terraces, cover crops, and reduced tillage.
• Wetlands and proximity to sensitive areas trigger buffer requirements and restrict pesticide and manure applications.
• Runoff potential drives residue management and filter-strip placement.
• Soil drainage class dictates crop selection, drainage installation, and the timing of field operations.

A CCA who characterizes the site first-and reads the soil survey critically with its mapping-scale limits-builds every subsequent recommendation on a correct agronomic foundation.