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Chapter 3.3

tip

Experience from the United States indicates that a sampling density of one bulk sample per acre is required to provide accurate information for variable rate fertilization. Sampling larger areas may still provide useful information about the extent of field variability.

When first using this strategy, it can be difficult to select a benchmark site that best represents a field. Therefore, in the first year, it may be necessary to sample and analyze a number of potential benchmark sites. Initially, the costs for laboratory analysis will be higher but the most representative benchmarking site will be identified. If a single site does not adequately represent a field, it may be necessary to maintain multiple benchmark areas (i.e., directed benchmark sampling, see next section).

Selecting a Benchmark Site

When selecting a benchmark site, look for features such as soil colour and landscape to identify where different soil types occur. Select a site that has characteristics similar to most of the field or the dominant soil type.

Observe crop development patterns to assist in identifying different soil conditions. At the beginning of the growing season differences in crop establishment and vigor are more apparent, making a representative location easier to identify. Potential benchmark sites can also be selected based on yield, aerial photos or topographic maps.

Benchmarking is rapidly gaining popularity in Alberta, particularly with increased use of GPS. GPS coordinates help to identify and locate the benchmark site for sampling each year.

Directed Benchmark Sampling

Directed benchmark sampling is a variation on the benchmark technique. It involves establishing multiple benchmark areas and management zones, based on topography or other characteristics (Figure 3.3.7).

This strategy can be used when major areas within fields have distinct and well-defined features related to

moisture (e.g., texture, slope). Management zones can be identified using soil surveys, detailed elevation mapping, aerial black and white photographs, yield maps or remote sensed images.

Figure 3.3.7 Landscape Directed Benchmark Sampling Pattern

Directed benchmark sampling is only warranted if distinct areas are managed individually. For example, a soil analysis from a benchmark site in a low area suggests that it might respond to higher rates of N compared to a benchmark site on an upland area. Even without variable rate application capabilities, N application could be increased by other means to optimize yield in low areas.

Grid Sampling

Grid sampling is the most intense and expensive sampling strategy (Figure 3.3.8). It uses a systematic method to reveal fertility patterns and assumes there is no topographic reason for fertility patterns to vary within a field.

For grid sampling, a field is divided into small areas or blocks. A sample location within each block (e.g., the center point) is sampled 3 to 10 times. Sampling frequency may range from one sample from each 60 m × 60 m (0.5 ac) area of the field to one sample from each 2 ha (5 ac) of the field. In general, the smaller the sampling unit, the greater the accuracy.