STEP 2  Soil Sampling

Since the degree of nutrient use and nutrient loss varies with each growing season, annual soil testing is recommended or at least every three years. The three components of soil sampling are:

  1. Soil sample collection,
  2. Laboratory analysis, and
  3. Interpretation of the laboratory results.

Collection of a representative sample is critical for a successful soil-sampling program. Sampling should be based on in-field variations such as topography and soil type. Collect soil samples from the depth intervals of 0-15 cm (0-6"), 15-30 cm (6-12"), and 30-60 cm (12- 24") at 20 to 30 sites per field or field-management area. Mix samples taken from the same depth intervals. Remove about 0.5 kg (1 lb) from each depth interval, and submit each sample to a commercial laboratory for analysis, for a total of three samples per field or field-management area. Including the three depths will allow monitoring of leachable nutrients (Flaten, pers.comm. 2000).

Why Soil Sample?

1. To measure the nutrient content of the soil.
2. To identify nutrient deficiencies or excess.
3. To predict crop response to added nutrients.
4. To build a nutrient management plan.
5. To assess the present soil quality.

The following soil-sampling strategies are explained below.

  • Whole Field Composite Sampling
  • Landscape Directed Sampling
  • Benchmark Sampling
  • Grid Sampling

1. Whole Field Composite Soil Sampling: Involves taking 20 to 30 cores from representative locations throughout the field, mixing the cores together, and submitting about a 0.5 kg (1 lb) sub-sample by mixing the cores collected.

2. Landscape Directed Soil Sampling: Involves dividing fields into several sampling areas based on factors such as soil characteristics, management history or yield potential. Directed soil sampling adds a detailed understanding of the landscape, and uses additional resources to validate the sampling design, including:

3. Benchmark Soil Sampling: The basic principle of the benchmark approach is sampling at the same location each subsequent year. Each benchmark is a representative area of a field varying from 5 m2 to 1000 m2 (54 ft2 to 10,750 ft2) for sampling. Fifteen to twenty samples are bulked from each of the following depth intervals: 0-15 cm (0-6"), 15-30 cm (6-12"), and 30-60 cm (12-24"), for an accurate representation of the three depth intervals at each benchmark site.

The representative area may have more than one benchmark site selected if a complex soil association (e.g. Chernozemic - Solonetzic soils) or variable landscape (e.g. hummocky) occurs. Additional benchmark samples may be required for critical nutrient deficiencies such as sulphur (S) in canola. In this case, a sample should be taken where you might expect sulphur to be low, such as upper slope positions (Kanters 1996).

4. Grid Soil Sampling: This method has been used in the USA more extensively than in Alberta. It involves sampling and analyzing an extensive number of soil samples at regular intervals throughout a field. For most producers in Alberta, the large number of samples does not provide a justifiable economic return. There are two basic methods of grid sampling.

5. Grid point sampling. Individual cores are collected within a small radius of each grid intersection. These are bulked, mixed and sub-sampled. The interval between grid points is usually in the 60-100 m (200-440 ft) range.

6. Grid cell sampling. A number of cores are taken randomly from within each cell, and are bulked and mixed. A common grid size is 1 ha (2.5 ac). A sub-sample is taken from each bulked sample and is submitted for testing.

As with conventional fertilization, soil testing of the top 60 cm (4") is adequate for normal manure application programs. It is advisable to keep records of fields sampled for future reference and planning. According to Saskatchewan Agriculture and Food (1997b), the records should include:

Soil Test Reports

Soil test reports present soil nutrient results and recommended fertilizer rates. Soil test reports allow an increased accuracy of predication of crop nutrient requirements, based on site-specific long-term average yields and probabilities of adequate moisture conditions.

Soil test reports vary based on the analytical method used and presentation format. In addition to the soil test, other factors should be used to make a final fertilizer recommendation. These include soil and climate data, soil moisture, crop type, target yield, fertilizer prices and whether the crop is dryland or irrigated.

Nutrient concentrations in soil are often expressed in terms of kg/ha for a given depth within the soil profile. To determine the nutrient concentration, the soil depth of interest, and soil bulk density must be known or have assumed values. A common soil depth for nutrient analysis is 15 cm (6 in). For example, converting 100 ppm of P to a nutrient content in the top 15 cm (0.15 m),(6 in) of the soil would require the following steps (Howard et al. 1999):
1. Soil bulk density assumed to be 1.4 g / cm3
2. 100 ppm = 100 mg / kg
3. (100 mg / kg x 10- 6 kg / mg x 1.4 g / cm3 x 10-3 kg / g x 106 cm3/ m3 x 10 4 m2 /ha x 0.15 m depth) = 210 kg / ha

1. Soil test results: These are the results of the nutrient tests performed by the lab. Nutrient content may be reported in units of ppm, lb/acre or kg/ha. The soil test values will include the macronutrients and micronutrients chosen by the producer and may include:

Figure 1. Diagrammatic representation of soil phosphorus.

Soil test P is composed of both the dissolved P (available to plants) and a portion of the active P (labile P that will supply P over the growing season). The non-labile (fixed P) is the least soluble component and provides very little plant nutrients in the first year of application, but serves as a long-term storehouse.

There are generally three common methods for extracting available phosphorus from the soil depending on the pH. The Bray method is used for low pH (acidic) soils. The Olsen method is used for high pH (calcareous soils). In Alberta, a Modified Kelowna method is commonly used. Other soil P tests (such as degree of P saturation, iron oxide strip, water-soluble, bioavailable, etc) have been studied, but are not yet developed for routine laboratory use. Soil texture, pH and the soil test method need to be taken into consideration when interpreting soil test P results (Howard et. al. 1999). As a recommendation, when using the Kelowna or modified Kelowna extraction method, if the soil test P exceeds 200 ppm in the top 15 cm (6") there may be environmental loading of phosphorous depending on the soil and landscape.

As manure is stored, changes due to decomposition, moisture loss, and volatilization of methane and carbon dioxide make it difficult to determine the exact phosphorus (P) composition. P concentrations can vary from 3 to 16 g/kg dry-matter, dry-matter decomposition can vary from 0 to 50%, and moisture content can vary from 20 to 60% (Watts et al. 1994).

Other soil quality parameters can also be tested and they generally will include:

2. Soils and Cropping Information: The producer supplies the legal land location and the previous and intended crop information when the soil sample is submitted. This information is required in order for the laboratory to make appropriate fertilizer recommendations based on regional algorithms.

3. Fertilizer Recommendations: Recommendations are provided to the farmer based on the crop intended for the next growing season. Soil moisture is very important, as crop yield potential will vary according to moisture supply. Lab methods that include organic nutrients that are mineralized over the growing season and coupled with supplemental fertilizer recommendations and yield responses are being researched.

4. Yield Increase: This area of the report shows the yield response predicted for the recommended amount of nitrogen fertilizer. This information is based on field trial studies for specific crops grown in the region.