By Dr. Beatrix Haggard, Agronomist – Macon Ridge Research Station
Dr. Josh Lofton, Agronomist – Macon Ridge Research Station
Why should soil tests be performed?
Obtaining soil test data is an important component of any production management system. It is especially important in Louisiana due to highly variable of our soils and weather patterns, which can affect soil nutrient content. Soil tests are important for agricultural production, pastures, as well as home gardeners. Soil testing provides scientific data for available nutrients and other soil characteristics. Location throughout the landscape produces differences in, soil texture, pH, organic matter, soil depth, slope, and elevation (low spots). All of the previously mentioned differences can affect the nutrient availability for crops. Because nutrient availability can be affected by many different factors it is very important to obtain soil samples that are well representative of the area. By spending a little more on soil testing it is possible to save in the end by not over-fertilizing the entire production system.
Where should soil tests be taken in a field?
Accurate soils samples should be taken via two different methods, grid-sampling and management zone sampling. For grid-sampling, fields should be divided into 0.5 to 10 acre sections and one soil sample should be collected per section. Grid-sampling can provide valuable, detailed information across the field; however, this sampling method can be very costly and labor intensive.
Another sampling method that can also provide valuable information is management zone sampling. Management zones should be determined in the production system before samples are collected from the field. These zones can be determined through multiple techniques, such as:
- Yield maps
- Previous farm records
- EC (electrical conductivity)
- Elevation differences (topography)
Yield maps would be preferred if data from multiple years existed. However, elevation and EC differences can also be used effectively (Figure 1). These management zones allow soil samples to be collected based on inherent soils properties or yield differences.
Within each grid or management zones, samples should be collected so that they are representative of the area. Caution should be taken to not collect samples where lime or animal manures have been stored. Additionally, samples should not be collected along fertilizer bands because these do not represent the true fertility. To ensure this point, soils should be collected in a random manner throughout the entire sampling area.
How deep and how many soil samples?
The depth of the sample is different based on each crops needs and should be focused on where the effective rooting depth will be located. In most instances, soil samples collected from 0-6 inches are adequate; however, for mobile nutrients and deep rooted crops grown in Louisiana’s deep alluvial soils 6-12 inches would be beneficial. Additionally, if it is thought that salts have accumulated; samples should be taken from 0-3 inches. This zone has the greatest influence on seed germination. Further, in some areas samples should only be taken where roots can penetrate. This is especially important in the Macon Ridge soils where hard layers limit deep root growth.
For grid sampling, one sample (making sure there will be enough for analysis) taken near the center of the grid. While for management zone sampling, 15-20 samples should be obtained from each zone. Samples can be taken with a soil probe or shovel, making sure to stay within the previously mentioned depths. These samples should be mixed in a plastic bucket and placed in a quart sized sealable plastic bag. Large pieces of plant material should be removed.
When should samples be taken?
Soil samples can be taken anytime during the course of the year; however, samples taken in the fall allow for timely applications as well as time for lime to react, thereby altering the soil pH. For mobile nutrients such as Nitrogen and Sulfur, soil samples should be taken around the same time that fertilizer will be applied. Regardless of what information is needed from the soil samples they should be taken around the same time each year. Any residue which remains should be scraped away from the surface before samples are pulled.
For questions regarding results from the soil analysis contact your local Parish Extension Agent.
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