Reviewed October 1993
Contents
Related page
Use our feedback form for questions or comments about G9111.
Publication search
To use your soil test report to the best advantage, you need to understand the information it provides. With the proper soil test interpretations and suggested fertilizer treatments, you will have a fertilizer program for maximum profitable crop production.
The soil test report provides information on:
Figure 1
Sample soil test report form.
pHs
The salt pH (pHs) of a soil indicates the level of active soil acidity. Crops require different salt pH levels for the best growth and optimum production in terms of yield and economic costs. For example, many row crops, small grains and legume-grass mixtures should be maintained in soils with a salt pH between 6.1 and 6.5.
Alfalfa requires a slightly higher salt pH of 6.6 to 7.0. Many pure grass stands do well in soils with a salt pH of 5.5 to 6.0.
Crop production may be severely reduced in soils with a salt pH at or below 5.0. As soil salt pH falls below 5.0, aluminum and manganese may increase to toxic levels. Also, phosphorus and molybdenum availability decreases as soil becomes more acid.
Maintained at a salt pH between 5.5 and 7.0, Missouri soils provide a favorable environment for root development and microorganisms (the living part of the soil). They also provide field crops with the best nutrient availability for growth.
Limestone recommendations are made to correct the problems of soil acidity. These recommendations are based on the soil tests, crops to be grown, and the region of the state the sample comes from.
Phosphorus (P)
In Missouri, the soil test for phosphorus is called the Bray and Kurtz I or Weak Bray test. The test results are expressed in pounds of elemental P per acre. This test is a measure of the relative availability of phosphorus for plant growth. The test does not measure the total amount of phosphorus in the soil.
A very low or low test indicates that crops would very likely respond to the addition of phosphorus fertilizer. Under low soil test conditions, banding a portion of the recommended phosphate may be advantageous.
A soil testing medium in phosphorus means that crops are likely to respond to phosphate, if growing conditions are favorable for high yields.
Soils testing high in phosphorus are not likely to produce economic yield increases with an application of additional phosphorus. In fact, it would cost more to fertilize than you would get back in profits from increased yield. When soils test in the high range, applying only a small quantity of phosphorus is suggested to maintain the high fertility status.
There is no economic advantage to applying phosphate to soils testing very high. The University of Missouri does not suggest applying phosphorus on soils testing very high in phosphorus.
An adequate phosphorus soil test range for row crops and small grains is about 45 to 70 pounds P per acre. Most forage crops do well in a soil testing in a range somewhat lower. An adequate range is 40 to 60 pounds P per acre. Rice, birdsfoot trefoil-grass mixtures, lespedeza-grass mixtures, and bermuda grass require soil testing in a range of only about 30 to 45 pounds of P per acre for economical production. Native warm-season grasses do well on soils testing 20 to 35 pounds of P per acre.
CEC
The Cation Exchange Capacity is a measure of the soil's ability to hold certain nutrients known as cations. The CEC on the soil test report is a calculation of the exchangeable calcium, magnesium, potassium, and hydrogen measured by the soil tests. This calculation is given in milli-equivalents (me.) per 100 grams of soil. The CEC value helps technicians make the potassium and magnesium interpretations and estimate the soil texture.
Potassium (K)
This is a measure of the exchangeable potassium in soil extracted with an ammonium acetate solution. The results are expressed as pounds of exchangeable K per acre. The soil test estimates the potassium available to the growing crop.
The exchangeable potassium test and CEC are used to interpret the need for additional potassium. In most Missouri soils, the desired level of potassium increases as the CEC increases.
The ratings of the potassium soil test levels are similar to those made for phosphorus. Very low to low soil K levels strongly indicate that crops will respond to the application of potassium. Band application of a portion of the total requirement may also be advantageous.
Medium soil K levels indicate that crops may respond to potassium if climatic conditions are favorable.
Soils with high K levels are not likely to respond to application, but they should be maintained at this level. Very high to excess soil K tests show that you can allow crops to deplete potassium until the soil test drops into the high range.
The optimum potassium level for row crops, small grains and alfalfa hay is (220 + (5 x CEC)) pounds K per acre. Table 1 gives the adequate soil K ranges for different soil textures and crops. Within these ranges, it is recommended to apply a small quantity of potassium to maintain the high fertility status.
Table 1
Adequate soil potassium ranges based on soil texture and crops grown.
| Crops | Soil texture | ||||
|---|---|---|---|---|---|
| Sand | Sandy loam | Silt loam | Clay loam | Clay | |
| Adequate potassium soil test ranges (K per acre) | |||||
| Alfalfa hay, row crops, small grains | 240 to 360 pounds | 260 to 390 pounds | 290 to 435 pounds | 320 to 480 pounds | 350 to 525 pounds |
| Alfalfa-grass pasture and all other forages | 180 to 270 pounds | 200 to 300 pounds | 230 to 345 pounds | 260 to 390 pounds | 290 to 435 pounds |
Calcium (Ca)
An ammonium acetate solution extracts exchangeable calcium from the soil. Calcium seldom limits crop growth in Missouri soils. Even very highly acid soils needing lime usually contain enough calcium as a plant nutrient for crops. The soil calcium measurement is used to calculate CEC.
The calcium rating is not based on the actual soil test calcium level. A very low to low soil salt pHs (acid soil) is rated medium in calcium. Medium or high soil pHs (near neutral soil) is rated high in calcium.
Magnesium (Mg)
An ammonium acetate solution extracts exchangeable magnesium from the soil.
Application of magnesium based on soils testing very low to low may improve yields in some forages and row crops. Application on soils testing medium is not likely to increase yields in row crops and small grains. However, application may increase magnesium levels in forages and improve forage quality. When magnesium is needed and lime is required, the use of dolomitic limestone is recommended.
Organic matter (O.M.)
The organic matter soil test is used to estimate potential nitrogen release to a crop throughout the growing season. The test also provides a basis for determining proper herbicide rates.
There is no rating system for organic matter because it is relatively stable under normal cropping and fertility practices and cannot be easily changed.
Neutralizable acidity (N.A.)
Technicians add a buffer solution to the sample used for salt pH analysis to estimate the neutralizable acidity of the soil. Neutralizable acidity is a measure of the exchangeable hydrogen within soil. It is used to calculate the pounds of effective neutralizing material necessary to lime the soil to the proper salt pH range.
Nitrogen (N)
To calculate nitrogen needs, the yield goal should be realistic, based on crop yield history of the field, irrigation, and your own managerial skills.
To calculate nitrogen needs for a crop:
Table 2
Nitrogen rate adjustments1
| Soil texture | Cation exchange capacity (me per 100 grams) | Organic matter | Cool-season crops (N per acre) | Warm-season crops (N per acre) |
|---|---|---|---|---|
| Sands -- sandy loam | less than 10 | 0.5 percent 1.0 percent 1.5 percent |
10 pounds 20 pounds 30 pounds |
20 pounds 40 pounds 60 pounds |
| Silt loams -- loam | 10 to 18 | 2.0 percent 3.0 percent 4.0 percent |
20 pounds 30 pounds 40 pounds |
40 pounds 60 pounds 80 pounds |
| Clay loams -- clays | greater than 18 | 2.0 percent 3.0 percent 4.0 percent 5.0 percent |
10 pounds 15 pounds 20 pounds 25 pounds |
20 pounds 30 pounds 40 pounds 50 pounds |
Plant population (1,000 feet per acre) = 20 x 4 = 80
Expected yield (bushels per acre) = 150 x 0.9 = 135
Total nitrogen required = 215Soil nitrogen release
Texture = Silt loam
Organic matter (from Table 2) = 2.0 percent = -(40)
Credit from previous legume crop = Soybeans - 40 bushels = -(40)
Credit from manure application = none applied = -(0)
Additional fertilizer requirement (pounds N/A) = 135
In this example, the final corn population was expected to be 20,000 plants per acre. Multiply this value by 4 pounds of nitrogen per 1,000 plants. That's only the amount of nitrogen needed to grow the crop. The expected yield is 150 bushels per acre. This yield times 0.9 pounds of nitrogen per bushel, plus the nitrogen to grow the crop, indicates the total nitrogen required (from fertilizer, previous legumes, or manures). From the total nitrogen required, subtract all credits to determine the nitrogen fertilizer requirement.
The suggested rate of application accounts for the total nitrogen required and the soil nitrogen release. You must account for additional nitrogen reductions, such as any previous legume crops or manure application. Consider these sources of nitrogen when you determine how much nitrogen to apply for a crop. For example, you may reduce nitrogen rates by about 30 pounds per acre for a crop following soybeans. A good guideline is to reduce about 1 pound of nitrogen for each bushel of the previous soybean crop. Table 3 shows reasonable nitrogen adjustments for a crop following a legume under the best growing conditions.
Manure applications also supply nitrogen. Although the pounds of available nitrogen per ton of dry manure may vary greatly, you can usually reduce nitrogen rates by about 10 to 15 pounds per ton of dry manure.
Table 3
Nitrogen added to the soil by a legume crop (optimum)
| Legume crop | Nitrogen added (N per acre) |
|---|---|
Alfalfa 80 to 100 percent stand Alfalfa 50 to 80 percent Alfalfa Less than 50 percent |
120 to 140 pounds 40 to 60 pounds 0 to 20 pounds |
| Sweet clover (green manure) | 100 to 120 pounds |
| Red clover (pure stand) | 40 to 60 pounds |
| Soybeans | 15 to 60 pounds |
Phosphorus
Suggested phosphorus rates either build up or maintain the level of phosphorus in the soil. Soils depleted of phosphorus require building up phosphorus for optimum production. If the soil already has an optimal level of phosphorus, the soil test report suggests a maintenance application. Soils testing higher than optimum do not usually respond to application, so you can allow the phosphorus level to drop to the optimal range.
Phosphorus needs are based on the level required to build up the soil to optimum availability for crop growth. Suggested fertilizer rates are also based on the amount of phosphorus removed from the soil by harvested crops. Table 4 shows the rates necessary to build up phosphorus to an optimal level with one application.
Table 4
Phosphorus rates required for build-up treatments in eight years.
| Soil test level (phosphorus per acre) |
Optimal soil test level (pounds phosphorusper per acre) | ||
|---|---|---|---|
| 30 | 40 | 45 | |
| phosphate per acre | |||
| 5 pounds | 45 pounds | 56 pounds | 61 pounds |
| 10 pounds | 32 pounds | 43 pounds | 49 pounds |
| 15 pounds | 22 pounds | 34 pounds | 39 pounds |
| 20 pounds | 14 pounds | 25 pounds | 31 pounds |
| 25 pounds | 7 pounds | 18 pounds | 24 pounds |
| 30 pounds | 12 pounds | 17 pounds | |
| 35 pounds | 6 pounds | 11 pounds | |
| 40 pounds | 5 pounds | ||
Table 5 shows how much phosphorus is removed by some crops. With this information, you can determine the suggested phosphorus rate by knowing the optimal soil test level, your actual soil test level, and expected crop yield.
Table 5
Phosphorus and potassium removed by various crops.
| Crop and yield | Crop removal | |
|---|---|---|
| phosphate | potash | |
| Soybeans at 40 bushels per acre | 34 pounds per acre | 58 pounds per acre |
| Corn (grain) at 100 bushels per acre | 45 pounds per acre | 30 pounds per acre |
| Corn (silage) at 15 tons per acre | 54 pounds per acre | 135 pounds per acre |
| Wheat at 60 bushels per acre | 36 pounds per acre | 18 pounds per acre |
| Alfalfa at 4 tons per acre | 40 pounds per acre | 180 pounds per acre |
| Fescue hay at 4 tons per acre | 36 pounds per acre | 136 pounds per acre |
Calculating a phosphorus treatment
Suppose the soil test is 20 pounds of P, and your yield goal is 150 bushels of corn an acre.
Building up your soil from 20 to 45 requires 245 pounds of phosphate fertilizer. This 245-pound buildup spread over eight years means that you would apply about 31 pounds phosphate annually (Table 4).
Growing 150 bushels of corn per acre removes about 67 pounds of phosphate. So the suggested phosphorus rate would be: 31 + 67 = 98 pounds phosphate per acre.
Potassium
Suggested application rates of potassium are based on amounts needed to build up the soil to the desired level. The rate suggested includes the amount removed by the harvested crop. Table 6 gives the rates necessary to build up soil potassium in one application.
Table 6
Potassium rates required for buildup treatments in eight years.
| Soil test level (potassium per acre) |
Optimal soil test (pounds potassium per acre) | |||||
|---|---|---|---|---|---|---|
| 200 | 240 | 260 | 280 | 300 | 340 | |
| potash per acre | ||||||
| 100 pounds | 39 pounds | 52 pounds | 58 pounds | 64 pounds | 69 pounds | 80 pounds |
| 150 | 18 pounds | 31 pounds | 36 pounds | 42 pounds | 48 pounds | 58 pounds |
| 200 | 13 pounds | 19 pounds | 24 pounds | 30 pounds | 41 pounds | |
| 250 | 3 pounds | 9 pounds | 14 pounds | 25 pounds | ||
| 300 | 11 pounds | |||||
The potassium rate to build up the soil from 250 to 300 is 114 pounds potash an acre (Table 6). This 114-pound buildup spread out over eight years means that you would apply 14 pounds potash an acre annually.
Growing 150 bushels of corn per acre removes 45 pounds of potash per acre (Table 4). So the suggested rate would be: 14 + 45 = 59 pounds potash per acre.
Zinc (Zn)
You should have the zinc soil test performed:
In Missouri, a zinc application is likely to increase corn and grain sorghum yields. Table 7 shows ratings and suggested application rates for these two crops, based on the DTPA extractable zinc soil test level.
Table 7
Suggested zinc application rates on corn and sorghum
| Soil test level (Zn) | Rating | Zinc application rate |
|---|---|---|
| Less than 0.5 ppm | Low | 10 pounds Zn per acre |
| 0.5 to 1.0 ppm | Medium | 5 pounds Zn per acre |
| Greater than 1.0 ppm | High | 0 pounds Zn per acre |
Most row crops, small grains and alfalfa are less likely to respond to zinc fertilization compared to corn or sorghum. Therefore, zinc is recommended for those crops only when the soil test is less than 0.5 ppm. Five pounds actual zinc per acre should be adequate for those crops under the low zinc test level situations.
The suggested zinc applications should correct zinc deficiency and maintain available zinc at an adequate level for at least four crop years. Zinc sulfate or zinc oxide are adequate fertilizer sources for zinc. If you plan to use zinc chelate, apply 1/3 to 1/2 of the recommended rate annually.
Sulfur (S)
Crops may respond to sulfur applied on coarse-textured soils low in organic matter that have been producing high yields. As the soil sulfate-sulfur level or cation exchange capacity increases in soils, the potential response to sulfur decreases. Table 8 outlines sulfur interpretations, based on soil sulfate-sulfur and cation exchange capacity. When these tests show a need for fertilizer sulfur, 10 to 20 pounds of sulfur is suggested for row crops, small grains and alfalfa. In Missouri, most other forages do not require sulfur, even on soils testing low in sulfur.
Table 8
Ratings of extractable soil sulfur test
| Soil sulfate-sulfur (ppm SO4-S) | Cation exchange capacity (meq per 100 grams) | |
|---|---|---|
| 0 to 6.5 | 6.5+ | |
| 0 to 7.5 | Low | Adequate |
| 7.5 + | Adequate | Adequate |
Additional micronutrients
The potential for economic response to applications of iron, manganese or copper is not likely with Missouri soils. However, many growers have questions about the level of these nutrients in their soils. Soil tests for iron, manganese and copper are available through the University of Missouri soil testing labs. Presently, field research has not shown a need for application of these micronutrients in Missouri or on similar soils in neighboring states. Table 9 outlines current DTPA soil test interpretations for these three micronutrients.
Table 9
Iron, copper and manganese soil test interpretations
| Soil test rating | Iron | Copper | Manganese |
|---|---|---|---|
| DTPA extractable soil test level | |||
| Low | 0 to 2.0 ppm | 0 to 0.2 ppm | 0 to 1.0 ppm |
| Medium | 2.0 to 4.5 ppm | ||
| High | 4.5+ ppm | 0.2+ ppm | 1.0+ ppm |
In areas of the United States where deficiencies of these micronutrients have occurred, the following treatments are the most effective:
No soil tests for boron or molybdenum are currently offered by the University of Missouri. In Missouri, apply boron annually to alfalfa at a rate of about 1 pound of boron per acre to avoid deficiency. Liming soils to a salt pH of 5.5 or greater should avoid any potential deficiency of molybdenum on any crop grown in Missouri.
G9111, reviewed October 1993