Soil Sampling Guidelines for Better Soil Test Results
Where, when and how to maximize soil sampling in your fields.
All potato growers in Canada are aware of the reality that these days, potato production is subject to high input costs as well as increasing demands to protect the environment. Most potato farmers are utilizing nutrient management plans to better manage their inputs at rates that can sustain crop production while at the same time conserving the environment.
NMPs require current, representative and accurate soil test results. Ultimately, the goal of soil sampling and analysis is to estimate the soil’s ability to provide adequate amounts of the nutrients needed for the next season’s crop. Growers use the results of soil tests to develop representative estimates of a particular field’s average nutrient needs so that the optimal rate of fertilizer application can be determined.
When to Sample
Most soil specialists agree that soil sampling can occur at any time as long as the results are obtained in time to develop fertilizer and manure application plans for the next season. According to Pat Toner, soil management specialist at the New Brunswick Department of Agriculture, Aquaculture and Fisheries, the best time to sample is fall.
“Early fall will allow for fall liming, if required, but late fall after harvest will allow time to test samples and provide results to plan for the next growing season,” he says. Post-harvest samples also take into account crop removal of nutrients as well as the effects of fertilizer on soil acidity.
Developing a Sample Plan
Toner says that the first step to developing a sample plan is to identify the regions of each field from which samples are to be taken. This can be done with the help of aerial photos, GPS or GIS systems. Following this, subdivide fields into additional sample areas based on the following criteria:
- soil variability within a field due to soil type and/or differences in fertility;
- areas of different production potential (low- versus high-yielding fields);
- field size—areas should not exceed 10 hectares (25 acres) in size, if samples are to be representative;
- variations in topography;
- differences in variety; and
- methods of previous fertilizer, manure or lime applications, and degree of tillage mixing.
Each sample area should be assigned a permanent number for reference and comparison of results over time.
Areas to Avoid
In order to optimize the reliability of soil sampling and enhance growers’ abilities to extrapolate test results for individual fields, growers should avoid certain areas in fields with potential sources of sample misrepresentation, or at least sample them separately. Examples of these kinds of areas are:
- eroded areas;
- poorly drained areas;
- areas with different cropping patterns;
- areas with different lime, manure or fertilizer treatments;
- headlands and line fences;
- recent fertilizer bands;
- dead furrows that expose subsoil;
- areas adjacent to roads; and
- lime, manure or crop residue piles.
Toner advises growers to keep good records of their soil samples and to maintain a consistent sampling plan to get the most out of their sampling efforts. “The cost of analysis and sample collection is very low on a per acre basis when compared to other production costs, and in the end well worth the expense and effort,” he says.
|Methods Of Sampling|
The New Brunswick Department of Agriculture, Aquaculture and Fisheries recommends three standard methods of soil sampling in its leaflet entitled “Soil Sampling—The Key to Effective Nutrient Management Planning”:
Composite Sample: The most common sample collection method is to take a random sample from the field, without any reference to topography or other field features. This method works well in fields that are uniform in soil type, production and management history.
Stratified Composite Sample: A field may be broken down into areas of similar topography, management history and/or crop performance. Each area is then sampled separately.
Systematic or Grid Sampling: Sampling points are pre-determined across a field at fixed intervals such as one per hectare. Then four to five sub-samples are taken near each point. Each point then has its own soil sample and result. With this information, a fertility map may be created for the field showing areas of similar fertility. This may have benefit for high value crops or crops sensitive to certain nutrient levels and/or pH. If soils are high testing in nutrients, this method of sampling will likely have fewer benefits. This method is related to the use of GPS equipment.