Highlights

• This publication provides guidelines on how to properly collect a soil sample in a pasture.

• Proper soil sampling is the foundation of reliable soil test reports.

• Among fertilization practices, soil testing often delivers the greatest return on investment because it helps ensure that fertilizer dollars are spent on the nutrients most needed to optimize forage growth and productivity.

• Key considerations when soil sampling pastures: Collect a sample that is representative of the area, taken at the proper depth, avoiding “hot-spot areas.” Sample at your desired interval at the same time of the year. Clean tools to prevent contamination. Properly identify the samples. Do not dry the samples. Select a soil testing laboratory that extracts nutrients using the Mehlich-3 methodology.

Representative Soil Sampling

The area in which the sampling will be performed has to be evaluated for the following parameters:

• Size: It is not recommended to collect a single composite soil sample in an area greater than 5 to 20 acres. Larger areas should be split into smaller subsections, and a composite soil sample should be collected in each of them.

• Difference in soil type, texture and/or color: In case there is more than one soil type within the same area of interest, it is advised to split the area, and a composite soil sample should be collected in each of them.

• Difference in previous management: If the area of interest has undergone different management practices, it should be divided accordingly. For example, hay and grazing pastures should never be sampled together. In hay production, more nutrients are removed from the soil. Therefore, it is expected that there will be a difference in nutrient content between hay and grazed pastures, especially if both are subjected to the same management practices.

Composite Soil Sample

It is not advised to collect a single soil core to evaluate the soil fertility of a given area. Soils can drastically differ in a relatively small area. A composite soil sample consists of collecting 15 to 20 soil cores in the area of interest, mixing them in a clean bucket and packing around 0.5 pounds of soil to send to the lab. Increasing the number of soil cores collected reduces the likelihood of sampling error by providing a more representative sample of the field.

Soil Sample Depth

The LSU AgCenter recommendations were developed considering a soil sample depth of 6 inches. Thus, it is essential that when collecting soil samples from a pasture to not collect samples shallower than 6 inches. Soil samples when collected from soil depths shallower than 6 inches can overestimate the fertility of the soil. This is because the top soil is generally the layer that is the richest in nutrients and organic matter. Conversely, taking a soil sample too deep may underestimate the fertility of the soil.

Areas to Avoid When Soil Sampling

Certain “hot-spot areas” should be avoided when performing a soil sample in a pasture. This is especially true for grazed pastures. Do not sample near nutrient rich areas such as near fence lines, shade structures, water troughs, dung pats, feed troughs and where hay is provided. Animals normally spend more time around areas where there are available forage, water and shade, and therefore, the soil near those areas will present higher fertility. However, they do not represent the fertility of the remaining pasture.

Time of the Year That Sampling Is Performed

It is preferred that soil samples be collected at the same time of the year every year (or every desired interval). For warm-season forages, soil sampling should be performed in late fall or winter. For cool-season forages, soil sampling can be performed in the summer. Sampling at the same time every year that soil sampling is performed will allow a fair comparison between soil test reports. It is expected that the amount of nutrients will fluctuate over the year depending on if there is active growth of the forage or not.

Proper Cleaning of Tools to Prevent Contamination

A soil auger is normally used to sample soil only, so it should not be an issue. However, buckets can introduce a significant amount of contamination to the soil sample. Buckets should be washed and dried before starting soil sampling. Special care should be taken for buckets that were previously used to carry minerals or fertilizers.

Proper Identification of Soil Samples

Giving names to your soil samples that refer to the pasture from which they were collected will certainly help track which pasture a specific soil test report refers to. Make sure to list the name of your sample, your name and some kind of contact information. This information will be extremely helpful in case your submission form gets misplaced. In addition, making a map of how the areas were divided for soil sampling purposes will be helpful for planning fertilizer applications and future soil samplings.

Do Not Dry Soil Samples

There is no need to dry the soil samples before submitting them to the LSU AgCenter Soil Testing and Plant Analysis Laboratory. Once the samples arrive at the lab, they will be placed on a forced air oven, in which the drying process will occur at a medium and constant temperature (140 F). Drying samples at temperatures above 140 F will compromise them. High temperatures will change the chemistry of the soil and analysis will show values very different from the actual values of your soils. Consequently, samples exposed to temperatures above 140 F must be discarded and are no longer suitable for soil testing analyses.

Select a Soil Testing Lab That Uses Mehlich-3 Methodology

As long as the chosen laboratory employs the Mehlich-3 extractant to quantify the amount of plant-available nutrients in the soil (Mehlich, 1983), it can be any laboratory of your choice. Different extractants remove different amounts of nutrients from the soil and, therefore, they are usually not comparable. Basically, different extractants are developed for different regions and/or different soil characteristics. Mehlich-3 methodology is the one that better estimates the amount of plant-available nutrients in Louisiana’s soils and is the one used for studies of fertilizer recommendations.

Fill Out the Submission Form

The very last step on the soil sampling process is to fill out the submission form. It is crucial for the lab to be able to send back your results. Include at least the following information: your name, parish, phone number, email address and forages of interest. It is very important to select your forages of interest if you wish to receive lime and fertilizer recommendations. Especially for fertilizers, there is no one size that fits all. The fertilizer recommendations can drastically vary depending on the forage you are planning to grow and its planned purpose, such as grazing versus hay.

Submission form for soil analyses from the LSU AgCenter Soil Testing and Plant Analysis Laboratory. Illustration provided by Leandro Vieira.