Soil and Plant Sampling and Testing for Nutrient Status

Below is a summary of the content of articles that deal with soil and plant sampling that are used to make plant nutrient recommendations to producers. The first two articles were authored by Dr. Bryan Hopkins and colleagues, and were prepared as a contribution from the Western Region Nutrient Management Coordinating Committee [WERA-103].

Soil Testing: How Precise Is My Lab Data?–Crops & Soils Magazine [CRSO]–Sep.-Oct. 2022. Pertinent points from this article follow.

•    Producing good data to use for estimating soil nutrient additions requires 1) proper sampling, 2) accurate and precise analysis of soil in submitted samples, and 3) correct interpretation to ensure proper addition of nutrients based on test results.

•    Producers and/or consultants can control steps 1) and 3), but often have less control or even no control over step 2), the analysis portion of the process.

•    The North American Proficiency Testing [NAPT] Program is comprised of experts who are members of the Soil Science Society of America [SSSA]. The SSSA operates the NAPT to furnish analysis laboratories with quality control and quality assurance tools to assist them in generating accurate and precise analyses of soil and plant samples. The data generated by the various laboratories are available here.

•    Some important points gleaned from results from these laboratories are: 1) Soil pH was the most precise measurement by the labs; 2) Soil carbon, including both organic matter and total carbon, were ranked the next most precise measurement; 3) The primary macronutrients–nitrogen [N], phosphorus [P], and potassium [K]–were all measured with relatively good precision; 4) the secondary macronutrients–calcium, magnesium, and sulfur–were variable in measurement precision; 5) Measurement precision of macronutrients was relatively better than that of micronutrients; and 6) Micronutrients–e.g. zinc, manganese, copper, iron, boron, and chloride–were measured with the least precision and/or the measurements were highly variable. This likely is a result of errors that are inherent in the methodology used by various labs.

•    The evaluation of laboratories showed that they are collectively capable of excellent analysis, although different methodologies among labs can lead to quite different values for a particular measurement. Thus, lab results should be interpreted with this in mind.

•    Results from any of the labs that are used for soil analysis should not be interpreted to a finer scale than what is actually achieved by the lab.

Plant Tissue Testing: How Precise is My Lab Data?–CRSO–May-June 2021. Pertinent points from this article follow.

•    The first three bulleted points above apply to plant tissue testing. Methodology to determine precision of data from plant tissue samples was the same as that for the above soil sample data.

•    An analysis was performed on plant data from three unique sources that were submitted to participant labs.

•    Measurements of dry matter (DM) and total N were the most precise, followed by measurement of macronutrients. Measurement precision of micronutrients was variable and generally less precise than measurement of DM and macronutrients.

•    The analysis process found that agricultural and environmental labs were capable of excellent precision when measuring nutrient contents of plant tissue. However, methods used for measurement of some nutrients such as iron and nitrate-N have inherent errors that impact those measurements, resulting in lower measurement precision.

•     Analysis of the data suggest that researchers should request a total N analysis over a nitrate-N analysis.

•    As with soil sampling, results from any of the labs that are used for plant tissue analysis should not be interpreted to a finer scale than what is actually achieved by the lab.

In a Delta Farm Press article titled “Study shows nutrient test results can vary when soil is very dry”, results from research conducted in Arkansas are presented. Major points of interest in that article follow.

•    Soil test results can vary depending on the soil moisture status at the time of sampling. The researchers found that the optimum soil moisture content for sampling was near field capacity, which allowed proper core depth and maintenance of soil core integrity.

•    Collecting a representative sample to the proper depth for determination of soil nutrient status plays a major role in providing accurate fertilizer rate recommendations for any crop that will be grown on the sampled site.

•    The most pronounced effects of improper sampling depth for fertilizer recommendations were seen on P and K determination.

•    Dry soil makes sampling to the proper depth of 6-8 in. very difficult, which results in an inflated value of soil nutrients such as P and K because they are more heavily concentrated in the shallower soil depths.

•    The time of year a soil sample is taken for testing can affect soil pH determination, with lower pH values usually found in soil obtained in a dry summer and in soil obtained following fall harvest. Lower-than-expected soil pH measured in the fall can be checked by taking a few samples in the spring when soil moisture is nearer the optimum for sampling in order to verify that liming is actually required as indicated by pH measurement of soil sampled in the fall.

Click here for an article on this website that provides reasons for fall soil sampling. Click here and here to access soil sampling and plant tissue testing White Papers, respectively, on this website.

Composed by Larry G. Heatherly, Nov. 2022, larryh91746@gmail.com