Building Soils for Better Crops, Third Edition

Interpreting Soil Test Results

Overview

UNUSUAL SOIL TESTS

From time to time we’ve come across unusual soil test results. A few examples and their typical causes are given below:

  • Very high phosphorus levels—High poultry or other manure application over many years.
  • Very high salt concentration in humid region— Recent application of large amounts of poultry manure, or location immediately adjacent to road where de-icing salt was used.
  • Very high pH and high calcium levels relative to potassium and magnesium—Large amounts of lime stabilized sewage sludge used.
  • Very high calcium levels given the soil’s texture and organic matter content—Use of an acid solution, such as the Morgan, Mehlich 1, or Mehlich 3, to extract soils containing free limestone, causing some of the lime to dissolve.
  • Soil pH >7 and very low P—Use of an acid such as Mehlich I or Mehlich 3 on an alkaline, calcareous soil; the soil neutralizes much of the acid, and so little P is extracted.

Below are five soil test examples, including discussion about what they tell us and the types of practices farmers should follow to satisfy plant nutrient needs on these soils. Suggestions are provided for conventional farmers and organic producers. These are just suggestions— there are other satisfactory ways to meet the needs of crops growing on the soils sampled. The soil tests were run by different procedures, to provide examples from around the U.S. Interpretations of a number of commonly used soil tests—relating test levels to general fertility categories—are given later in the chapter (see tables 21.3 and 21.4). Many labs estimate the cation exchange capacity that would exist at pH 7 (or even higher). Because we feel that the soil’s current CEC is of most interest (see chapter 20), the CEC is estimated by summing the exchangeable bases. The more acidic a soil, the greater the difference between its current CEC and the CEC it would have near pH 7.

Following the five soil tests below is a section on modifying recommendations for particular situations.

 

—SOIL TEST #1—

(New England)

Soil Test #1 Report Summary*
Field name: North

 

Sample date: September (PSNT sample taken the following June)

Soil type: loamy sand Manure added: none Cropping history: mixed vegetables

Crop to be grown: mixed vegetables

MEASUREMENT LBS/ACRE PPM Soil Test CATEGORY Recommendation SUMMARY
P 4 2 low 50–70 lbs P2O5/acre
K 100 50 low 150–200 lbs K2O/acre
Mg 60 30 low lime (see below)
Ca 400 200 low lime (see below)
pH 5.4   2 tons dolomitic limestone/acre
CEC** 1.4 me/100g  
OM 1% add organic matter: compost, cover crops, animal manures
PSNT   5 low side-dress 80–100 lbs N/acre

*Nutrients were extracted by modified Morgan’s solution (see table 21.3A for interpretations).

**CEC by sum of bases. The estimated CEC would probably double if “exchange acidity” were determined and added to the sum of bases.

Note: ppm = parts per million; P = phosphorus; K = potassium; Mg = magnesium; Ca = calcium; OM = organic matter; me = milliequivalent; PSNT = pre-sidedress nitrate test; N = nitrogen.

What can we tell about soil #1 based on the soil test?
  • It is too acidic for most agricultural crops, so lime is needed.
  • Phosphorus is low, as are potassium, magnesium, and calcium. All should be applied.
  • This low organic matter soil is probably also low in active organic matter (indicated by the low PSNT test, see table 21.4A) and will need an application of nitrogen. (The PSNT is done during the growth of the crop, so it is difficult to use manure to supply extra N needs indicated by the test.)
  • The coarse texture of the soil is indicated by the combination of low organic matter and low CEC.

General recommendations:
  1. Apply dolomitic limestone, if available, in the fall at about 2 tons per acre (and work it into the soil and establish a cover crop if possible). This will take care of the calcium and magnesium needs at the same time the soil’s pH is increased. It will also help make soil phosphorus more available, as well as increasing the availability of any added phosphorus.
  2. Because no manure is to be used after the test is taken, broadcast significant amounts of phosphate (P2O5—probably around 50 to 70 pounds per acre) and potash (K2O—around 150 to 200 pounds per acre). Some phosphate and potash can also be applied in starter fertilizer (band-applied at planting). Usually, N is also included in starter fertilizer, so it might be reasonable to use about 300 pounds of a 10-10-10 fertilizer, which will apply 30 pounds of N, 30 pounds of phosphate, and 30 pounds of potash per acre. If that rate of starter is to be used, broadcast 400 pounds per acre of a 0-10-30 bulk blended fertilizer. The broadcast plus the starter will supply 30 pounds of N, 70 pounds of phosphate, and 150 pounds of potash per acre.
  3. If only calcitic (low-magnesium) limestone is available, use sul-po-mag as the potassium source in the bulk blend to help supply magnesium.
  4. Nitrogen should be side-dressed at around 80 to 100 (or more) pounds per acre for N-demanding crops such as corn or tomatoes. About 300 pounds of ammonium nitrate or 220 pounds of urea per acre will supply 100 pounds of N.
  5. Use various medium to long-term strategies to build up soil organic matter, including the use of cover crops and animal manures.

    Most of the nutrient needs of crops on this soil could have been met by using about 20 tons wet weight of solid cow manure per acre or its equivalent. It is best to apply it in the spring, before planting. If the manure had been applied, the PSNT test would probably have been quite a bit higher, perhaps around 25 ppm.

Recommendations for organic producers:
  1. Use dolomitic limestone to increase the pH (as recommended for the conventional farmer, above). It will also help make soil phosphorus more available, as well as increasing the availability of any added phosphorus.
  2. Apply 2 tons per acre of rock phosphate, or about 5 tons of poultry manure for phosphorus, or—better yet—a combination of 1 ton rock phosphate and 2 1/2 tons of poultry manure. If the high level of rock phosphate is applied, it should supply some phosphorus for a long time, perhaps a decade.
  3. If poultry manure is used to raise the phosphorus level, add 2 tons of compost per acre to provide some longer-lasting nutrients and humus. If rock phosphate is used to supply phosphorus, use livestock manure and compost (to add N, potassium, magnesium, and some humus).
  4. Establish a good rotation with soil-building crops and legume cover crops.
  5. Use manure with care. Although the application of uncomposted manure is allowed by organic-certifying organizations, there are restrictions. For example, four months may be needed between application of uncomposted manure and either harvest of crops with edible portions in contact with soil or planting of crops that accumulate nitrate, such as leafy greens or beets. A three-month period may be needed between uncomposted manure application and harvest of other food crops.

 

(Pennsylvania and New York)

Soil Test #2 Report Summary*
Field name: Smith upper

 

Sample date: November (no sample for PSNT will be taken)

Soil type: silt loam

Manure added: none this year (some last year)

Cropping history: legume cover crops used routinely

Crop to be grown: corn

MEASUREMENT LBS/ACRE PPM Soil Test CATEGORY Recommendation SUMMARY
P 174 87 high none
K 360 180 high none
Mg 274 137 high none
Ca 3,880 1,940 high no lime needed
pH 7.2    
CEC 11.7 me/100g  
OM 3% add organic matter: compost, cover crops, animal manures
N no soil test     little to no N needed

*Soil was sent to a commercial laboratory and extracted for P using the Bray-1 solution. This is probably the equivalent of over 20 ppm by using the Morgan or Olsen procedures. Other nutrients were extracted with pH 7 ammonium acetate (see table 21.3D).

Note: ppm = parts per million; P = phosphorus; K = potassium; Mg = magnesium; Ca = calcium; OM = organic matter; me = milliequivalent; PSNT = pre-sidedress nitrate test; N = nitrogen.

What can we tell about soil #2 based on the soil test?
  • The high pH indicates that this soil does not need any lime.
  • Phosphorus is high, as are potassium, magnesium, and calcium (see table 21.3D).
  • The organic matter is very good for a silt loam.
  • There was no test done for nitrogen, but this soil probably supplies a reasonable amount of N for crops, because the farmer uses legume cover crops and allows them to produce a large amount of dry matter.

General recommendations:
  1. Continue building soil organic matter.
  2. No phosphate, potash, or magnesium needs to be applied. The lab that ran this soil test recommended using 38 pounds of potash and 150 pounds of magnesium (MgO) per acre. However, with a high K level, 180 ppm (about 8% of the CEC) and a high Mg, 137 ppm (about 11% of the CEC), there is a very low likelihood of any increase in yield or crop quality from adding either element.
  3. Nitrogen fertilizer is probably needed in only small to moderate amounts (if at all), but we need to know more about the details of the cropping system or run a nitrogen soil test to make a more accurate recommendation.

Recommendations for organic producers:
  1. A good rotation with legumes and fall legume cover crops will provide nitrogen for other crops and prevent loss of soluble nutrients.

 

—SOIL TEST #3—

(Humid Midwest)

Soil Test #3 Report Summary*
Field name: #12

 

Sample date: December (no sample for PSNT will be taken)

Soil type: clay (somewhat poorly drained)

Manure added: none

Cropping history: continuous corn

Crop to be grown: corn

MEASUREMENT LBS/ACRE PPM Soil Test CATEGORY Recommendation SUMMARY
P 20 10 very low 30 lbs P2O5/acre
K 58 29 very low 200 lbs K2O/acre
Mg 138 69 high none
Ca 8,168 4,084 high none
pH 6.8   no lime needed
CEC 21.1 me/100g  
OM 4.3% rotate to forage legume crop
N no N soil test     100–130 lbs N/acre

*All nutrient needs were determined using the Mehlich 3 solution (see table 21.3C).

Note: ppm = parts per million; P = phosphorus; K = potassium; Mg = magnesium; Ca = calcium; OM = organic matter; me = milliequivalent; PSNT = pre-sidedress nitrate test; N = nitrogen.

What can we tell about soil #3 based on the soil test?
  • The high pH indicates that this soil does not need any lime.
  • Phosphorus and potassium are low. [Note: 20 pounds of P per acre is low, according to the soil test used (Mehlich 3). If another test, such as Morgan’s solution, was used, a result of 20 pounds of P per acre would be considered a high result.]
  • The organic matter is relatively high. However, considering that this is a somewhat poorly drained clay, it probably should be even higher.
  • About half of the CEC is probably due to the organic matter and the rest probably due to the clay.
  • Low potassium indicates that this soil has probably not received high levels of manures recently.
  • There was no test done for nitrogen, but given the field’s history of continuous corn and little manure, there is probably a need for nitrogen. A low amount of active organic matter that could have supplied nitrogen for crops is indicated by the history (the lack of rotation to perennial legume forages and lack of manure use) and the moderate percent of organic matter (considering that it is a clay soil).

General recommendations:
  1. This field should probably be rotated to a perennial forage crop.
  2. Phosphorus and potassium are needed—probably around 30 pounds of phosphate and 200 or more pounds of potash applied broadcast, preplant, if a forage crop is to be grown. If corn will be grown again, all of the phosphate and 30 to 40 pounds of the potash can be applied as starter fertilizer at planting. Although magnesium, at about 3% of the effective CEC, would be considered low by relying exclusively on a basic cation saturation ratio system recommendation, there is little likelihood of an increase in crop yield or quality by adding magnesium.
  3. Nitrogen fertilizer is probably needed in large amounts (100 to 130 pounds/acre) for high N-demanding crops, such as corn. If no in-season soil test (like the PSNT) is done, some preplant N should be applied (around 50 pounds/acre), some in the starter band at planting (about 15 pounds/acre) and some side-dressed (about 50 pounds).
  4. One way to meet the needs of the crop is as follows:
    1. broadcast 500 pounds per acre of an 11-0-44 bulk blended fertilizer;
    2. use 300 pounds per acre of a 5-10-10 starter; and
    3. side-dress with 150 pounds per acre of ammonium nitrate.

      This will supply approximately 120 pounds of N, 30 pounds of phosphate, and 210 pounds of potash.

Recommendations for organic producers:
  1. Apply 2 tons per acre of rock phosphate (to meet P needs) or about 5 to 8 tons of poultry manure (which would meet both phosphorus and nitrogen needs), or a combination of the two (1 ton rock phosphate and 3 to 4 tons of poultry manure).
  2. Apply 400 pounds of potassium sulfate per acre broadcast preplant. (If poultry manure is used to meet phosphorus and nitrogen needs, use only 200 to 300 pounds of potassium sulfate per acre.)
  3. Use manure with care. Although the application of uncomposted manure is allowed by organic-certifying organizations, there are restrictions. For example, four months may be needed between application of uncomposted manure and either harvest of crops with edible portions in contact with soil or planting of crops that accumulate nitrate, such as leafy greens or beets. A three-month period may be needed between uncomposted manure application and harvest of other food crops.

 

—SOIL TEST #4—

(Alabama)

Soil Test #4 Report Summary*
Field name: River A

 

Sample date: October

Soil type: sandy loam

Manure added: none

Cropping history: continuous cotton

Crop to be grown: cotton

MEASUREMENT LBS/ACRE PPM Soil Test CATEGORY Recommendation SUMMARY
P 102 51 very high none
K 166 83 high none
Mg 264 132 high none
Ca 1,158 579   none
pH      

no lime needed

CEC 4.2me/100g      
OM not requested     use legume cover crops, consider crop rotation
N no N soil test   70–100 lbs N/acre

*All nutrient needs were determined using the Mehlich 1 solution (see table 21.3B).

Note: ppm = parts per million; P = phosphorus; K = potassium; Mg = magnesium; Ca = calcium; OM = organic matter; me = milliequivalent; PSNT = pre-sidedress nitrate test; N = nitrogen.

What can we tell about soil #4 based on the soil test?
  • With a pH of 6.5, this soil does not need any lime.
  • Phosphorus is very high, and potassium and magnesium are sufficient.
  • Magnesium is high, compared with calcium (Mg occupies over 26% of the CEC).
  • The low CEC at pH 6.5 indicates that the organic matter content is probably around 1–1.5%.

General recommendations:
  1. No phosphate, potash, magnesium, or lime is needed.
  2. Nitrogen should be applied, probably in a split application totaling about 70 to 100 pounds N per acre.
  3. This field should be rotated to other crops and cover crops used regularly.

Recommendations for organic producer:
  1. Although poultry or dairy manure can meet the crops’ needs, that means applying phosphorus on an already high-P soil. If there is no possibility of growing an overwinter legume cover crop (see recommendation #2), about 15 to 20 tons of bedded dairy manure (wet weight) should be sufficient. Another option for supplying some of the crops’ need for N without adding more P is to use Chilean nitrate until good rotations with legume cover crops are established.
  2. If time permits, plant a high-N-producing legume cover crop, such as hairy vetch or crimson clover, to provide nitrogen to cash crops.
  3. Develop a good rotation so that all the needed nitrogen will be supplied to nonlegumes between the rotation crops and cover crops.
  4. Although the application of uncomposted manure is allowed by organic-certifying organizations, there are restrictions when growing food crops. Check with the person doing your certification to find out what restrictions apply to cotton.

 

—SOIL TEST #5—

(Semiarid Great Plains)

Soil Test #5 Report Summary*
Field name: Hill

 

Sample date: April

Soil type: silt loam

Manure added: none indicated

Cropping history: not indicated

Crop to be grown: corn

MEASUREMENT LBS/ACRE PPM Soil Test CATEGORY Recommendation SUMMARY
P 14 7 low 20–40 lbs P2O5
K 716 358 very high none
Mg 340 170 high none
Ca not determined none
pH 8.1   no lime needed
CEC not determined  
OM 1.8% use legume cover crops, consider rotation to other crops that produce large amounts of residues
N 5.8 ppm 170 lbs N/acre  

*K and Mg extracted by neutral ammonium acetate, P by the Olsen solution (see table 21.3D).

Note: ppm = parts per million; P = phosphorus; K = potassium; Mg = magnesium; Ca = calcium; OM = organic matter; me = milliequivalent; PSNT = pre-sidedress nitrate test; N = nitrogen.

What can we tell about soil #5 based on the soil test?
  • The pH of 8.1 indicates that this soil is most likely calcareous.
  • Phosphorus is low, there is sufficient magnesium, and potassium is very high.
  • Although calcium was not determined, there will be plenty in a calcareous soil.
  • The organic matter at 1.8% is low for a silt loam soil.
  • The nitrogen test indicates a low amount of residual nitrate (table 21.4B), and, given the low organic matter level, a low amount of N mineralization is expected.

General recommendations:
  1. No potash, magnesium, or lime is needed.
  2. About 170 pounds of N per acre should be applied. Because of the low amount of leaching in this region, most can be applied preplant, with perhaps 30 pounds as a starter (applied at planting). Using 300 pounds per acre of a 10-10-0 starter would supply all P needs (see recommendation #3) as well as provide some N near the developing seedling. Broadcasting and incorporating 300 pounds of urea or 420 pounds of ammonium nitrate will provide 140 pounds of N.
  3. About 20 to 40 pounds of phosphate is needed per acre. Apply the lower rate as a starter, because localized placement results in more efficient use by the plant. If phosphate is broadcast, apply at the 40-pound rate.
  4. The organic matter level of this soil should be increased. This field should be rotated to other crops and cover crops used regularly.

Recommendations for organic producers:
  1. Because rock phosphate is so insoluble in high-pH soils, it would be a poor choice for adding P. Poultry manure (about 6 tons per acre) or dairy manure (about 25 tons wet weight per acre) can be used to meet the crop’s needs for both N and P. However, that means applying more P than is needed, plus a lot of potash (which is already at very high levels). Fish meal might be a good source of N and P without adding K.
  2. A long-term strategy needs to be developed to build soil organic matter—better rotations, use of cover crops, and importing organic residues onto the farm.
  3. Use manure with care. Although the application of uncomposted manure is allowed by organic-certifying organizations, there are restrictions. For example, three months may be needed between application of uncomposted manure and either harvest of root crops or planting of crops that accumulate nitrate, such as leafy greens or beets. A two-month period may be needed between uncomposted manure application and harvest of other food crops.

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