Soil Sampling: Determining Your Soil Nutrient Needs

soil sampling

It is important to sample your soil in order to determine the nutrients you need. It is important to take an accurate sample for the best results.

You can adjust your sampling pattern in order to account for known sources such as topography and past nutrient application patterns. This ensures that your test results accurately reflect the entire field.

Sampling Technique

To ensure that all horizons are represented, avoid taking samples in areas where major soil changes have occurred (abnormal plowzones, organic matter and lime or gypsum application). Avoid soil sampling where past fertilizer application was made.

soil sampling

If you are unsure which sampling approach is best for your field or property, talk to an agronomist. They can recommend a method that will yield the most accurate results.

A grid cell technique is a great choice. This involves superimposing a grid over the base map, which divides the field into uniformly sized areas called cells. In each cell, a single composite sample will be collected. This allows for the characterisation of the soil types in each individual cell. The resulting recommendations for amendments are specific to each cell.

Another method is to use a point sample. This is an efficient way to get the most accurate information from a large field. The sample locations will typically be 10 feet apart. The agronomist then uses a mathematical process known as interpolation to estimate values for unsampled points.

Regardless of the sampling method, it is important to include the depths where grasses or plants will draw most of their nutrients. The deeper soil sampling zone may be more time-consuming and expensive, but it’s crucial for the success of any fertiliser program.

It is important to prepare the soil sample by using clean tools and a container to store it. Avoid any contaminating materials, such as chemicals, fertilizers or detergents. Using chrome plated or stainless steel sampling tubes and augers is recommended. The sample must be stored in a clean bucket. Obtain a shovel or spade and remove all surface litter at the sample site. Avoid sampling areas that are wet, as the lab will need to wait until the sample is dry before analyzing it. Separate samples should be taken from areas of concern, such as bare spots, wet spots, furrows in the field, and edge lines.

Sample Pattern

The pattern used to collect the soil samples in a field has a big impact on how the results will be interpreted. It is important to choose a pattern which best represents the variability in the field, whether you are using grid or point sampling. The best solution is often a hybrid approach, which combines the advantages of both systems.

When designing the sample pattern, it is helpful to consult a topographic map or soil survey maps to help “bias” the sampling points to be closer together in areas that have more differences in the underlying soil properties. This will help to improve the accuracy of interpolation, and therefore result in a more meaningful conclusion.

Consistency in sampling depths is also crucial. The concentrations of nutrients in soil are usually stratified. The analytical results are more meaningful if samples are taken at the same level for each zone. This is important, especially in fields that have reduced or no tillage because the nutrient levels tend to be concentrated near the surface.

The number of samples per field should be based on the known variability within the field, and the number of cores taken should be based on that as well. A good practice is to take at least ten cores in each zone of the field, and to make sure that the resulting sample represents the majority of the variation within the zone.

It is best to sample soil after harvest when the field has been cleared of livestock and equipment. Avoid sampling around gates, manure or lime dumping areas and water troughs.

A common practice is dividing the field into zones according to factors that are expected have a major impact on nutrient control, such as soil series, areas eroded and previous fertilizer applications. This allows the agronomist a better understanding of the soil properties and how they can manage nutrients in these unique areas of the field.

Sampling Equipment

Equipment should be easy-to-use and durable to withstand the conditions on the field. The tools are also easily cleaned and disinfected to prevent the transmission of disease between samples.

Equipment for soil sampling ranges from small handheld augers, to large, powerful systems that can dig deep into the ground to collect a core. A variety of attachments and extensions are available to increase depth penetration capacity or to work in different soil types. Augers can be powered by hand or with gas. Some are adapted for working in organic or frozen soils.

Once the soil cores are collected, they should be thoroughly mixed to create a composite soil sample before packaging and sending it to the laboratory. The composite soil sample should include enough variation in the nutrient levels and other characteristics to reflect the broad area being tested.

Typically, a field or production area will be divided into zones with similar soils and management techniques. Each of these zones must be tested separately in order to ensure that the composite sample accurately reflects the production area. Stratified soil sampling can be achieved by subdividing entire soil cores in depth or horizonal zones or by directly sampling specific layers of soil depth from soil profile holes.

A clean bucket and plastic bag are essential to holding your soil cores for shipment to the laboratory. Metal containers can interfere with the results of analysis. If you are testing ammonium or for nitrates, it is important to keep your samples cool until they arrive at a lab. Add an ice pack in the shipping container to maintain a cool temperature.

The best time to take a soil sample is prior to planting or harvesting, when the soil is at its most representative state. Avoid sampling if a field has recently received manure, lime treatment or more than 50 kg/ha nitrogen.

Sampling Locations

Consider factors when sampling a field. These will help you determine where to take soils samples. These may include the presence of different soil series, eroded areas, previous fertilizer bands and locations, or past grazing management. In the past producers used spatial factors to determine the best places to sample for lime and fertilizer application.

Divide your field into sample zones if there are obvious differences in soil types, plant performance, or past management practices. Use information from county ASCS aerial photographs, yield maps, and field experience. This helps to ensure that the entire field is represented and that the individual management zone averages are representative of the entire field.

The number of samples required for zone sampling is higher than that needed for grid or cell sample collection. When sampling zones in a field, follow a zigzag path and collect 8 to 10 soil samples at each zone. Ideally, these soil samples should be taken to the plow depth in cultivated fields and to 6 inches in permanent pastures.

Whenever possible, try to avoid sampling near manure piles, compost areas or fertilizer bands as these soils can have very different characteristics than the surrounding field. Don’t also sample in areas that are wet or near the main bund.

Once all of the samples have been collected, place them in a clean bucket and mix them well. Place a label on the bucket, and note in your field journal the zone that you are sampling and the specific locations of the subsamples.

Mail your samples to a lab that offers soil testing services. Make sure to use a plastic bag (not paper) as this is best for protecting the integrity of the soil sample. Download the soil test submission form from the lab website, fill in the contact and agribusiness info and answer questions about the sampled field. Mail the completed form and soil in a clean, zipper-sealed plastic bag or a container that can be sealed tightly.

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