21.10: Adjusting a Soil Test Recommendation
- Page ID
- 25250
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Specific recommendations must be tailored to the crops you want to grow, as well as to other characteristics of the particular soil, climate and cropping system. Most soil test reports use information that you supply about manure use and previous crops to adapt a general recommendation to your situation. However, once you feel comfortable with interpreting soil tests, you may also want to adjust the recommendations for a particular need. What happens if you decide to apply manure after you sent in the form along with the soil sample? Also, you usually don’t get credit for the nitrogen produced by legume cover crops because most forms don’t even ask about their use. The amount of available nutrients from legume cover crops and from manures is indicated in Table 21.5. If you don’t test your soil annually, and the recommendations you receive are only for the current year, you need to figure out what to apply the next year or two, until the soil is tested again.
No single recommendation, based only on the soil test, makes sense for all situations. For example, your gut might tell you that a test is too low (and fertilizer recommendations are too high). Let’s say that although you broadcast 100 pounds N per acre before planting, a high rate of N fertilizer is recommended by the in-season nitrate test (PSNT), even though there wasn’t enough rainfall to leach out nitrate or cause much loss by denitrification. In that case, you might not want to apply the full amount recommended. Another example: A low potassium level in a soil test—let’s say around 40 ppm (or 80 pounds per acre)—will certainly mean that you should apply potassium. But how much should you use? When and how should you apply it? The answer to these two questions might be quite different on a low organic matter, sandy soil where high amounts of rainfall normally occur during the growing season (in which case, potassium may leach out if applied the previous fall or early spring) versus a high organic matter, clay loam soil that has a higher CEC and will hold on to potassium added in the fall. This is the type of situation that dictates using reputable labs whose recommendations are developed for soils and cropping systems in your home state or region. It also is an indication that you may need to modify a recommendation for your specific situation.
| A. Modified Morgan’s Solution (Vermont) | |||||
|---|---|---|---|---|---|
| Category | Very low | Low | Optimum | High | Excessive |
| Probability of response to added nutrient |
Very high | High | Low | Very low | |
| Available P (ppm) | 0–2 | 2.1–4 | 4.1–7 | 7.1–20 | |
| K (ppm) | 0–50 | 51–100 | 101–130 | 131–160 | >160 |
| Mg (ppm) | 0–35 | 36–50 | 51–100 | >100 | |
| B. Mehlich 1 Solution (Alabama)* | |||||
|---|---|---|---|---|---|
| Category | Very low | Low | Optimum | High | Excessive |
| Probability of response to added nutrient |
Very high | High | Low | Very low | |
| Available P (lbs/A)** | 0–6 | 7–12 | 50 | 26–50 | >50 |
| K (lbs/A)** | 0–22 | 23–45 | 160–90 | >90 | |
| Mg (lbs/A)** | 0–25 | >50 | |||
| Ca for tomatoes (lbs/A)*** |
0–150 | 151–250 | >500 | ||
| *For loam soils (with CEC values of 4.6–9), from: Alabama Agricultural Experiment Station. 2012. Nutrient Recommendation Tables for Alabama Crops. Agronomy and Soils Departmental Series No. 324B. **For corn, legumes and vegetables on soils with CECs greater than 4.6 me/100g. ***For corn, legumes and vegetables on soils with CECs from 4.6–9 me/100g. |
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| C. Mehlich 3 Solution (North Carolina)* | |||||
|---|---|---|---|---|---|
| Category | Very low | Low | Optimum | High | Excessive |
| Probability of response to added nutrient |
Very high | High | Low | Very low | |
| Available P (ppm) | 0–12 | 13–25 | 26–50 | 51–125 | >125 |
| K (ppm) | 0–43 | 44–87 | 88–174 | >174 | |
| Mg (ppm) | 0–25 | >25 | |||
| *Source: Hanlon (1998) **Percent of CEC is also a consideration. |
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| D. Neutral Ammonium Acetate Solution for K and Mg and Olsen or Bray-1 for P (Nebraska [P and K], Minnesota [Mg]) |
|||||
|---|---|---|---|---|---|
| Category | Very low | Low | Optimum | High | Excessive |
| Probability of response to added nutrient |
Very high | High | Low | Very low | |
| P (Olson, ppm)) | 0–3 | 4–10 | 11–16 | 17–20 | >20 |
| P (Bray-1, ppm) | 0–5 | 6–15 | 16–24 | 25–30 | >30 |
| K (ppm) | 0–40 | 41–74 | 75–124 | 125–150 | >150 |
| Mg (ppm) | 0–50 | 51–100 | >101 | ||
| A. Pre-Sidedress Nitrate Test (PSNT)* | |||||
|---|---|---|---|---|---|
| Category | Very low | Low | Optimum | High | Excessive |
| Probability of response to added nutrient |
Very High | High | Low | Very low | |
| Nitrate-N (ppm) | 0-10 | 11-22 | 23-28 | 29-35 | >35 |
| *Soil sample taken to 1 foot when corn is 6–12 inches tall. | |||||
| B. Deep (4-ft) Nitrate Test (Nebraska) | |||||
|---|---|---|---|---|---|
| Category | Very low | Low | Optimum | High | Excessive |
| Probability of response to added nutrient |
Very High | High | Low | Very low | |
| Nitrate-N (ppm) | 0–6 | 7–15 | 15–18 | 19–25 | >25 |
| Legume cover crops* | N lbs/acre | ||
|---|---|---|---|
| Hairy vetch | 70–140 | ||
| Crimson clover | 40-90 | ||
| Red and white clovers | 40–90 | ||
| Medics | 30–80 | ||
| Manures** | N | P2O5 lbs per ton manure |
K2O |
| Dairy | 6 | 4 | 10 |
| Poultry | 20 | 15 | 10 |
| Hog | 6 | 3 | 9 |
| *The amount of available N varies with the amount of growth. **The amount of nutrients varies with diet, storage and application method. Note: Quantities given in this table are somewhat less than for the total amounts given in Table 12.1. |
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