A soil sampling is the first step to developing a nutrient-management plan. But not all sampling methods are created equal.

The different sampling patterns used include grid cell, point and zone sampling as well as combinations of these methods. A well-designed sampling pattern is key to accurately representing the variability within your field.

Sampling Techniques

There are many different sampling techniques for obtaining soil test results. To avoid errors in sample collection, it is important to carefully consider the sampling technique.

The knowledge of the field should be used to select a sampling strategy. Grid sampling is the most common and accurate way to sample a field. However, if you are not familiar with the field in question, a random or zone sampling approach may be more effective.

Regardless of the sampling approach taken, it is important to ensure that the soil is sampled at a uniform depth. Variations in the depth of sampling can lead to incorrect analytical data in soils that are highly stratified. A uniform sample depth is also critical in avoiding biases in the sampling results due to the effects of the soil profile on the test result.

A soil sample should be taken using a probe which is designed to collect uniform amounts of soil at a given depth. This is usually done with bucket augers or push and hammer a probes. The probe should be clean and free of debris, and the sample should be taken from a location that has not been exposed to fertilizer or manure in the past.

Depending on the chosen sampling method, soil cores may be combined to create a composite soil sample. To obtain enough nutrients to perform a reliable soil analysis, it is usually necessary to use multiple cores.

The laboratory results are then interpolated in the map. This creates a continuous map of soil properties across the field. This representation will be used to develop a prescription rate chart to control the rate at which an amendment is applied to the field.

Comparatively, cell or point sample involves dividing a field into uniform grids of 1- to 3-acre-sized cells and identifying the locations of samples at the intersections or random points within each cell grid. Then, a single composite soil is collected for each grid area or point.

Sample Patterns

There are many ways to divide a soil sample into zones. These include Cell Sampling, Point Sampling, and Directed Sampling. The methods differ in the way the zone structure is created and where soil samples are collected within the zones. The different zone setups can be used to improve the representation of nutrient variability within the field.

If a field contains significant areas of more than one soil series, the University of Nebraska-Lincoln recommends sampling each series separately. A field with a wide range of past management techniques, such as gypsum, fertilization, or cropping systems, should also be sampled separately. Similarly, when selecting zones, differences in slopes, drainage, surface litter, and other characteristics must be taken into consideration.

The use of topographic maps or digital soil surveys may assist in the design of a sample pattern. This can make it easier to take samples where known sources of nutrient variability are present. If the topography of the field is known to affect soil test levels, grid sampling may be preferred to a cell sampling method.

Consistency is the key for reliable results regardless of the method chosen. The same procedure should be followed each time a soil sample is collected. This includes the way the area is chosen, when the soil samples are collected, and how much soil is put into each bag.

A common error is to use different amounts in each bag. Especially when moving from zone/grids to another. This can lead to inaccurate results, and should be avoided. Keeping the same amount of soil in each bag can be achieved by either quartering or compartmentalizing the sample before filling. This will ensure that the same amount of soil is tested each time and will help provide accurate nutrient maps over time.

It is important to sample the same area each time, whether you use a zig-zag pattern or a grid sampling technique. This will help to map nutrients more accurately and be useful when making decisions about variable rate applications.

Sampling Sites

A sampling site is a point or area within a field that is sampled. Sample locations can be identified by tracing field boundaries on a base map using a GPS (Global Positioning System).

The success of any sampling programme depends on the selection of sample sites. The SAP must include a representative sampling site that reflects the soil properties, exposure paths and risk assessment objectives. To meet project requirements and provide high-quality data, a combination of sampling techniques are usually required.

It is important to characterize sample sites in terms of their depth, location and nearby activities. For example, human exposures to subsurface soils through ingestion and dermal contact, migration to groundwater, ecological receptors burrowing at depth and construction activities need to be taken into consideration when selecting sample locations.

It is important that the technical rationale behind the selection of the sample locations be documented in a plan for sampling. The grain size distribution of the parent material, and the soil texture can influence the availability of naturally occurring chemical elements. Parent materials, chemical and physical weathering processes and soil management activities can all influence the composition of minerals, as well as their sorption capacity.

Before sampling, it is important to conduct a visual inspection of the field to identify differences in slopes, colors, textural characteristics, and cropping patterns. These differences can then be grouped to form distinct zones. These zones can be traversed for the purpose of collecting samples. The sampling zones should have a size that maximizes efficiency, while also balancing personnel and equipment costs, as well as the quality and accuracy of the data.

For nutrient control, sampling zones could be based on the area of the field, the production area, the boundary of adjacent fields, or similar soil and crop conditions. These zones should be surveyed carefully to ensure they are representative of a larger region and that the sample design accounts for spatial variation. Composite or ISM samples reduce variability when calculating the mean and therefore require fewer analyses and are more cost-effective than moderate- to high-density discrete sampling plans when the objective is to characterize the average. VOC compounds require special consideration when composing samples due to the risk of volatile loss during analysis.

Sampling Equipment

Many agricultural consultants and equipment dealers offer soil testing services. The cost can vary widely, and many services are offered bundled into packages along with other testing and scouting services.

If you choose to conduct your own soil sampling, the most important tools are a soil corer and a clean plastic bucket or container for taking subsamples. You may also want to have a mud-map or aerial photograph to help plan where to collect samples.

Soil corers come in sizes from 1 to 5″ and with a variety handles and extensions. They can be used for a variety of purposes, such as nutrient testing or field mapping.

For most soil tests, it is recommended that a minimum of 10 cores be taken. This allows you to accurately represent any trends in soil or nutrient properties and includes the variability within an area of management.

VSI offers a variety of disposable, easy-to-use soil samplers. These are ideal for capturing soil samples in the correct volume and for keeping them in good condition as you work to develop site-specific recommendations.

Once you have collected the samples, thoroughly mix them. Discard any rocks or roots. Keep your samples cool until you’re ready to send them out for testing. To prevent damage and ensure accurate readings, they must be kept cool – ideally at or below 40F. A cooler with an ice pack is recommended to protect the samples while in transit.