Quick Facts...
- Colorado State University routinely analyzes soil samples for pH, soluble salts, organic matter, nitrate nitrogen, phosphorus, potassium, zinc, iron, copper, manganese, lime and soil texture.
- Additional tests for gypsum and the sodium adsorption ratio (SAR) may be run in the laboratory.
- Nutrient levels are reported as parts per million (ppm) of the elemental nutrient.
- Included in a report from the Colorado State University Soil Testing Laboratory are interpretations that relate results to fertilizer and management suggestions.
Colorado State University routinely analyzes soil samples for pH, soluble
salts, organic matter, nitrate nitrogen, phosphorus, potassium, zinc,
iron, copper, manganese, lime and soil texture.
Routine Soil Tests
Soil pH, determined by the 1:1 soil:water method, indicates the acidity
or alkalinity of soil based on a scale of 0 to 14. On the pH scale, 7.0
is neutral, values below 7.0 are acid, and those above are alkaline. Most
Colorado soils are alkaline, having a pH between 7.0 and 8.0. A pH value
above 8.5 indicates that the soil may contain excess sodium.
Soluble salts are measured by the electrical conductivity
of a 1:1 soil:water mixture and are reported in mmhos/cm. Crops vary markedly
in their tolerance to soluble salts. Therefore, the values must be interpreted
in relation to the specific crop. (See Table 1.)
Organic matter (O.M.), reported as percent of total soil, contains about 95 percent of all soil nitrogen (N). About 30
pounds N per acre will be released (mineralized to nitrate) during the cropping season from each 1 percent O.M. present.
Nitrogen release rates will be slower in mountain meadow and other high elevation soils.
Nitrate nitrogen, reported in ppm NO3-N, is soluble and readily available for plant uptake and is therefore considered
equally available as fertilizer N. To determine the approximate pounds of NO3-N/acre-foot (1 acre to a depth of 1 foot),
multiply the soil test value (ppm) by 3.6. For example, 10 ppm x 3.6 = 36 pounds NO3-N/acre to a depth of one foot.
Phosphorus, potassium, zinc, iron, copper and manganese
interpretations are given in Tables 2 through 7. When the soil test is
very low to medium, fertilizer response is expected. Fertilizer recommended
for high-testing soils is for maintenance (to maintain soil fertility
at that desirable level). No fertilizer is recommended for soils testing
high for dryland production. For the micronutrients, no fertilizer is
recommended when the test indicates adequate. To date, there has been
no confirmed field crop response to copper or manganese fertilization
in Colorado. This test is an availability index. It does not measure the
total amount in the soil, but only that fraction extractable by the soil
test.
Lime (CaCO3) is reported as percent free
lime. In the routine test, values are reported as low (0 to 1 percent),
medium (1 to 2 percent), and high (above 2 percent). Specific values are
determined and reported only when a sodium evaluation is requested on
a sample. In this case, the percent freelime content is important in determining
whether elemental sulfur will be an effective amendment in sodium reclamation.
The lime content has no direct bearing on soil test interpretations for
fertilizer recommendations by the Colorado State University Soil, Water
and Plant Testing Laboratory.
Texture is estimated by the hand-feel method. Nitrogen
management suggestions are adjusted according to soil texture. It is important
on sands, loamy sands and sandy loams that nitrogen applications be split
to avoid mid- or late-season deficiency. It's also recommended that high
nitrogen rates be split for many Crops.
Additional Soil Tests
Sodium adsorption ratio (SAR) is determined by saturated paste extraction and is reported as a special ratio of sodium to calcium plus magnesium.
This test evaluates the sodium content of soil. A value of 13 or greater indicates
an excess of sodium will be adsorbed by the soil clay particles. Excess
sodium can cause soil to be hard and cloddy when dry, to crust badly,
and to take water very slowly.
The gypsum test is conducted if the SAR is greater than or
equal to 13. Total gypsum is reported in meq. (milliequivalent) CaSO4/100g.
If sufficient native gypsum is present, sodium-affected soils may be successfully
treated without addition of amendments such as gypsum or sulfur. The gypsum
supplies soluble calcium to replace the adsorbed sodium. Reclamation can
proceed if drainage of the land is possible. A gypsum recommendation is
provided if the gypsum test shows insufficient gypsum in sodic soils.
| Table 1: Tolerance levels of Crops for soluble salts. |
| Test values in mmhos/cm |
Interpretation |
|---|
| 0-2 |
Satisfactory for Crops |
| 2-4 |
Affects sensitive Crops |
| 4-8 |
High for many Crops |
| above 8 |
Very high for most Crops |
| Table 2: Available phosphorus (ammonium bicarbonate-DTPA test). |
| Test values* in ppm |
Interpretation |
|---|
| Irrigated production |
Dryland production |
| 0-3 | Very low | Low |
| 4-7 | Low | Medium |
| 8-11 | Medium | High |
| 12-15 | High |
| above 15 | Very high |
| Table 3: Available potassium (ammonium bicarbonate-DTPA test). |
| Test values* in ppm |
Interpretation |
|---|
| Irrigated production |
Dryland production |
|---|
| 0-60 | Low | Low-medium |
| 61-120 | Medium | High |
| 121-180 | High |
| above 180 | Very High |
| Table 4: Available zinc (ammonium bicarbonate-DTPA test). |
| Test values* in ppm |
Interpretation |
| Irrigated production |
Dryland production |
| 0-0.50 | Very low | Low |
| 0.5-0.99 | Low | Marginal |
| 1.0-1.50 | Marginal | Adequate |
| above 1.50 | Adequate |
| Table 5: Available iron (ammonium bicarbonate-DTPA test). |
| Test values* in ppm1 |
Irrigated and dryland production |
| 0-3.0 | Low |
| 3.1-5.0 | Marginal |
| above 5.0 | Adequate |
| 1Values below 10.0 may be deficient for turf and many ornamentals. |
| Table 6: Available manganese (ammonium bicarbonate-DTPA test). |
| Test values* in ppm |
Interpretation |
| 0-0.5 | May be low |
| above 5.0 | Adequate |
| Table 7: Available copper (ammonium bicarbonate-DTPA test). |
| Test values* in ppm |
Interpretation |
| 0-0.2 | May be low |
| above 2.0 | Adequate |
|
