21.9: Interpreting Soil Test Results

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Fred Magdoff & Harold van Es
University of Vermont & Cornell University via Sustainable Agriculture Research and Education (SARE) program

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Below are four soil test examples, including discussion about what they tell us and what types of practices farmers should follow to satisfy plant nutrient needs on these soils. Suggestions are provided for conventional farmers and for 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 United States. 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 CEC 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 four soil tests below is a section on modifying recommendations for particular situations.

Unusual Soil Tests

We’ve come across unusual soil test results from time to time. A few examples and their typical causes:

Very high phosphorus levels: high poultry or other manure application over many years.
Very high salt concentration in humid regions: recent application of large amounts of poultry manure in high tunnel greenhouses where rainfall is not able to leach salts, or located immediately adjacent to a road where deicing 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: the soil test used 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: the soil test used an acid such as found in Mehlich I, Bray or Mehlich 3 on an alkaline, calcareous soil. (In this case, the soil neutralizes much of the acid, so little P is extracted.)

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			liming material needed
	buffer pH***	6			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). Some labs report results in pounds per acre while others report results as ppm. The pH of a soil sample added to a buffered solution; the lower the pH, the more lime is needed. *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?

The pH indicates that the soil is too acidic for most agricultural crops, so lime is needed. The buffer pH indicates that around 2 tons per acre is needed to raise pH to 6.5.
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 crop growth, 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.

Recommendations for conventional growers

Apply dolomitic limestone, if available, in the fall at about 2 tons per acre (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 as the soil’s pH is increased. It will also help make soil phosphorus more available, as well as increase the availability of any added phosphorus.
Because no manure is to be used after the test is taken, broadcast significant amounts of phosphate (P₂O₅ —probably around 50–70 pounds per acre) and potash (K₂O—around 150–200 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.
If only calcitic (low-magnesium) limestone is available, use K-Mag (Sul-Po-Mag) as the potassium source in the bulk blend to help supply magnesium.
Nitrogen should be side dressed at around 80–100 (or more) pounds per acre for N-demanding crops such as corn or tomatoes. About 220 pounds of urea per acre will supply 100 pounds of N.
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

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 increase the availability of any added phosphorus.
Apply 2 tons of rock phosphate or a combination of 1 ton rock phosphate and 2.5 tons of poultry manure.
If poultry manure is used to raise the phosphorus level without using rock phosphate, add 2 tons of compost per acre to provide some longer lasting nutrients and humus. If rock phosphate is used to supply phosphorus and if no poultry manure is used, use livestock manure and compost (to add N, potassium, magnesium and some humus).
Establish a good rotation with soil-building crops and legume cover crops. Use manure with care. Although the application of uncomposted manure is allowed by organic certification agencies, there are restrictions. Under the Food Safety Modernization Act (FSMA), the application of uncomposted manure is now regulated for all farms growing food crops, whether organic or not. For example, four months may be needed between the time you apply uncomposted manure and either harvest crops with edible portions in contact with soil or plant 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. These FSMA rules apply to all farms with annual sales of more than $25,000.

SOIL TEST #2 (Humid Midwest) Soil Test #2 Report Summary*
Field name: #12 Sample date: December (no sample for PSNT will be taken) Soil type: clay (somewhat poorly drained) Manure applied: 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). Most university laboratories in the Midwestern United States report results as ppm while private labs may report results in pounds per acre. Note*: ppm = parts per million; P = phosphorus; K = potassium; Mg = magnesium; Ca = calcium; N = nitrogen; OM = organic matter; me = milliequivalent.

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 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 the lack of manure use) and the moderate percent of organic matter (considering that it is a clay soil).

General recommendations

This field should probably be rotated to a perennial forage crop.
Phosphorus and potassium are needed. If a forage crop is to be grown, probably around 30 pounds of phosphate and 200 or more pounds of potash should be broadcasted pre-planting. If corn will be grown again, all of the phosphate and 30–40 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.
Nitrogen fertilizer is probably needed in large amounts (100–130 pounds per acre) for high N-demanding crops, such as corn. If no in-season soil test (like the PSNT) is done, some pre-plant N should be applied (around 50 pounds per acre), some in the starter band at planting (about 15 pounds per acre) and some side dressed (about 50 pounds).
One way to meet the needs of the crop:
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 then
3. side dress with 110 pounds per acre of urea. These amounts will supply approximately 120 pounds of N, 30 pounds of phosphate and 210 pounds of potash.

Recommendations for organic producers

Apply 2 tons per acre of rock phosphate (to meet P needs) or a combination of 1 ton rock phosphate and 3–4 tons of poultry manure.
Apply 400 pounds of potassium sulfate per acre broadcast pre-plant if a combination of rock phosphate and poultry manure is applied to meet P needs.
Use manure with care. Although the application of uncomposted manure is allowed by organic certification agencies, there are restrictions. Under the Food Safety Modernization Act (FSMA), the application of uncomposted manure is now regulated for all farms growing food crops, whether organic or not. For example, four months may be needed between the time you apply uncomposted manure and either harvest crops with edible portions in contact with soil or plant 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. These FSMA rules apply to all farms with annual sales of more than $25,000.

SOIL TEST #3 (Alabama) Soil Test #3 Report Summary*
Field name: River A Sample date: October Soil type: sandy loam Manure applied: poultry manure in previous years Cropping history: continuous cotton Crop to be grown: cotton	Measurement	LBS/ACRE	PPM**	Soil Test CATEGORY	Recommendation SUMMARY
	P	60	30	high	none
	K	166	83	high	none
	Mg	264	132	high	none
	Ca	1,158	579		none
	pH	6.5			no lime needed
	CEC	4.2 me/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 (seeTable 21.3B). Alabama reports nutrients in lbs/acre. Note*: P = phosphorus; K = potassium; Mg = magnesium; Ca = calcium; N = nitrogen; OM = organic matter; me = milliequivalent.

What can we tell about soil #3 based on the soil test?

With a pH of 6.5, this soil does not need any lime.
Phosphorus, 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

No phosphate, potash, magnesium or lime is needed.
Nitrogen should be applied, probably in a split application totaling about 70–100 pounds N per acre.
This field should be in a rotation such as cotton-corn-peanuts with winter cover crops.

Recommendations for organic producers

Although poultry or dairy manure can meet the crop’s needs, that means applying phosphorus on a soil already high in P. If there is no possibility of growing an overwinter legume cover crop (see recommendation #2), about 15–20 tons of bedded dairy manure (wet weight) should be sufficient. Another option for supplying some of the crop’s need for N without adding more P is to use Chilean nitrate until good rotations with legume cover crops are established.
If time permits, plant a high-N-producing legume cover crop, such as crimson clover (or a crimson clover/oat mix), to provide nitrogen to cash crops.
Develop a good rotation so that all the needed nitrogen will be supplied to nonlegumes between the rotation crops and cover crops.
Use manure with care. Although the application of uncomposted manure is allowed by organic certification agencies, there are restrictions. Under the Food Safety Modernization Act (FSMA), the application of uncomposted manure is now regulated for all farms growing food crops, whether organic or not. For example, four months may be needed between the time you apply uncomposted manure and either harvest crops with edible portions in contact with soil or plant 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. These FSMA rules apply to all farms with annual sales of more than $25,000.

SOIL TEST #4 (Semiarid Great Plains) Soil Test #4 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 are 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; N = nitrogen, with residual nitrate determined in a surface to 2-foot sample.

What can we tell about soil #4 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

No potash, magnesium or lime is needed.
About 150 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 260 pounds of urea (or applying subsurface using a liquid formulation) will provide 120 pounds of N.
About 20–40 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.
The organic matter level of this soil should be increased. This field should be rotated to other crops, and cover crops should be used regularly.

Recommendations for organic producers

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.
A long-term strategy needs to be developed to build soil organic matter, including better rotations, use of cover crops and importing organic residues onto the farm.
Use manure with care. Although the application of uncomposted manure is allowed by organic certification agencies, there are restrictions. Under the Food Safety Modernization Act (FSMA), the application of uncomposted manure is now regulated for all farms growing food crops, whether organic or not. For example, four months may be needed between the time you apply uncomposted manure and either harvest crops with edible portions in contact with soil or plant 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. These FSMA rules apply to all farms with annual sales of more than $25,000.

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