21.2: Taking Soil Samples
- Page ID
- 25242
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The usual time to take soil samples for general fertility evaluation is in the fall or the spring, before the growing season has begun. These samples are analyzed for pH and lime requirements as well as for phosphorus, potassium, calcium and magnesium. Some labs also routinely analyze for organic matter and other selected nutrients, such as boron, zinc, sulfur and manganese, while others offer these as part of a menu you can select from. Whether you sample a particular field in the fall or in the early spring, stay consistent and repeat samples at approximately the same time of the year and use the same laboratory for analysis. Keep in mind that soils are usually sampled differently (timing and depth) for evaluating N needs (see below). As you will see below, this allows you to make better year-to-year comparisons.
- Care and consistency when taking samples are critical to obtaining accurate information. Plan when and how you are going to sample, and be sure there is sufficient time to do it correctly.
- Don’t wait until the last minute. The best time to sample for a general soil test is usually in the fall. Spring samples should be taken early enough to have the results in time to properly plan nutrient management for the crop season.
- Take cores from at least 15–20 spots randomly selected over the field (or a zone in a field) to obtain a representative sample. Taking many cores for a single sample is critical for obtaining meaningful soil test results, independent of the size of the field or zone. One sample should not represent more than 10–20 acres. For precision zone or grid fertility management, consider a sample for every 1–5 acres.
- Sample between rows. Avoid old fence rows, dead furrows and other spots that are not representative of the whole field.
- Take separate samples from problem areas if they can be treated separately.
- Soils are not homogeneous: nutrient levels can vary widely with different crop histories or topographic settings. Sometimes different colors are a clue to different nutrient contents. Consider sampling some areas separately, even if yields are not noticeably different from the rest of the field.
- For diversified vegetable farms that use blocks for grouping crops (by plant family, periods of growth, type of crop), sample by management zone block in addition to visibly different portions of fields, like strips or other portions of fields that form the basis of the rotation.
- In cultivated fields, sample to plow depth.
- Take two samples from no-till fields: one to a 6-inch depth for lime and fertilizer recommendations, and one to a 2-inch depth to monitor surface acidity.
- Sample permanent pastures to a 3- or 4-inch depth.
- Collect the samples in a clean container.
- Mix the core samplings, remove roots and stones, and allow the mixed sample to air dry.
- Fill the soil-test mailing container.
- Complete the information sheet, giving all of the information requested. Remember, the recommendations are only as good as the information supplied.
- Sample fields at least every three years and at the same season of the year each time. Annual soil tests on higher-value crops will allow you to fine-tune nutrient management and may allow you to cut down on fertilizer use.
—Modified from The Penn State Agronomy Guide (2019–2020)