For the most accurate soil analysis and meaningful interpretation of results, samples must be taken properly. Whether you’re soil sampling with a push or hammer probe or an auger, use a clean bucket and take a uniform sample to the recommended depth.
Sampling Technique
The sampling method is crucial for obtaining useful and reliable soil test outcomes. Laboratory analytic work cannot compensate for a poorly taken sample. The goal of sampling is to estimate the ability of the soil to supply adequate amounts of nutrients required for a particular crop.
There are a number of different sampling methods available to achieve this. Most agronomists suggest sampling in a structured pattern to capture field variability, and to make the most out of the time and costs invested in obtaining soil test results. No matter what sampling method you choose, it’s important that the sampled area is representative of the whole field.
The same samples should be taken each year, at the exact same time to reduce the variations in test results due to weather conditions. Between sampling and application of sulfur, lime or fertilizer, a minimum of 30 day should pass. In addition, samples should be collected before tillage or planting to ensure that the test results reflect pre-tilling and preplanting soil conditions.
To sample soil, you can use a variety of tools depending on what type of field you are working with. These include a soil auger, garden trowels, shovels, or augers. It is recommended that sampling tools be made of stainless or chrome plated steel, rather than galvanized metal to avoid contamination. It is important to use clean buckets when collecting and storing the soil core. This will prevent contamination.
No matter which sampling method is chosen, it’s important that growers consider the history of their fields as well as the management practices used when choosing the locations for samples. Past banded manure applications and fertilizer can have a significant impact on the results. Therefore, a careful examination of prior yield maps can be very helpful when choosing sample areas in the field.
It is recommended that when using a grid-cell or zone sampling method the number of sample per zone be eight times or more the distance between bands. This will provide an accurate estimate of the average value for each zone in the field.
Sample Pattern
The sampling pattern is important, whether you are using a soil sampler, wheel, or Global Positioning System to sample soil. Several patterns are available to consider, but one that is generally well-suited to the situation is a grid pattern. Grid sampling divides the field into regions for sampling, and helps to avoid patterns caused by tillage or past fertiliser applications that may bias the sample results.
Avoid areas where livestock congregate, such as water troughs, and manure piles. Do not sample the soil immediately after applying lime or fertiliser, as it takes time for them to fully penetrate the soil. Try to avoid these areas instead, such as by walking in a “W” pattern.
Even if the fields appear uniform at first glance, they should be divided into 4 ha (10 acres) units. Each unit should then be sampled separately. This will allow greater sensitivity to minor variations in the soil. This is important, especially if the soil has had a different topography or has been manured in a different way. A composite sample will not accurately represent these differences.
Sampling points should be positioned around the edges of the field to avoid missing patches of poor growth, or bare ground. Avoid muddy or eroded areas as well as back furrows, field margins and wet or eroded surfaces. These will have a significant impact on the results. It is also a good idea to exclude areas where hay or lucerne has been stored as these can be a significant source of contamination.
It is recommended that at least 10 cores be taken from each sampling point and all the cores should be mixed together thoroughly prior to sending to the laboratory. The more cores you collect within each sampling point, the more representative the results will be. A sample too small can introduce too many variations into the analysis. On the other hand, a sample too large may reduce the sensitivity.
Sampling Density
Bulk density, sometimes referred to as dry bulk density or soil density, is the weight of the solid matter of the soil per volume of the soil. This value reflects total volume, including pore spaces that can contain air or moisture. Bulk density is an important measure of a soil’s physical state.
Students and teachers can use bulk density measurements to better understand how a soil’s condition impacts its ability to play a role in an ecosystem. In addition, soils scientists use bulk density data to calculate the pore space (porosity) available to hold water and air in a soil.
Bulk density can be used to determine the physical condition of a soil, and identify problems like excessive compaction or surface sealing. Bulk density measurements also provide information on the physical properties of soils at a certain moisture content.
To avoid biases, it’s important to use the same depth of sampling for all cores. These biases may be caused by a variety of factors, including differences in sampling techniques, field conditions, and equipment.
Another common source of error is the use of different soil sampling probes with the same sampling procedure. For example, using a different probe with a bigger cutting edge diameter from the one used in previous samples can introduce error when estimating BD or SOC stocks. This problem can be reduced by comparing BD measurements from the same field with multiple probes of the same design and by evaluating probe performance over time.
Sampling Date
If you’re planning to use soil testing for nutrient recommendations (and especially when using an unexplained fertilizer deficiency), it is important to take the samples on a day when the results will be useful. This will allow you to make timely decisions regarding the use of lime and fertilizers, and will give you enough time to implement any recommended changes.
If possible, sample fields at the same time each year, such as early spring or fall. This will help you to compare the soil test results. It will also allow for a more accurate interpretation of the data. Avoid sampling in extreme conditions such as waterlogged soils or dry soils because the results can be skewed.
MU Extension agents, regional/state agronomists and MU Extension agents can provide you with sample kits and laboratories for testing your samples. It is best to send your samples to a laboratory familiar with the soils within your area. Check with your local agronomist to find one. You can also contact your county extension office for a list of certified labs in your area. A small fee is charged to send a sample of soil for analysis. The results can take a few weeks to process and will be sent to you.
