Soil Sample Test Interpretation for Alfalfa
pH: Alfalfa is tolerant of a wide range of pH's. Alfalfa is more sensitive to lower pH's (below 6.5) than high pH's. The problem is at pH's greater than 7.8 especially with the presence of high amounts of lime, phosphate and Zinc become more and more unavailable as pH rises.
E.C. (Salts): Young alfalfa is not very tolerant of salts and will have trouble becoming established in fields with test values much over 1.0. Established alfalfa can tolerate salt levels up to around 3.0-4.0, but production will fall. Values under 1.0 are preferable. Fertilizers are slats in themselves and must be considered in salty field recommendations.
Ex. Lime: High levels of lime tend to make phosphorus and zinc unavailable and extra amounts of these nutrients will be needed and banding of these nutrients is a good way to counteract the tie-up potential of high lime. If high levels of sodium are present some lime in the soil is needed if one wants to use elemental sulfur or sulfuric acid to help reduce sodic soil problems (reclamation). If very high levels of lime are present iron use by the plant may be interrupted and iron chlorosis may develop in the crop that can often not be corrected by applying lime. Too much lime in the soil is difficult to correct. Leaching water, good farming, and acidifying fertilizers help reduce high levels.
Organic matter (O.M.%): The more organic matter the better is the general rule. Colorado soils tend to be very low in organic matter and it is virtually impossible to significantly affect this level. This result can be important for some pesticide rates and carryover rates in the soil.
Nitrate-Nitrogen: This level is expressed in ppm and lbs./A. To convert a ppm result to lbs./a use this formula: ppm x 0.3 x sample depth in inches. This nutrient is very mobile in the soil. Alfalfa normally needs very little nitrogen. Exceptions to this rule of thumb are: New stands of alfalfa benefit from 20-25 units/a nitrogen, soils with extremely low nitrogen, cold areas where the alfalfa does not fix nitrogen well (The bacterial in nodules on the roots does not effectively establish or thrive). If a lot of grass is in the stand and one wants good grass and alfalfa production add 10 units/a nitrogen for every ton of expected production. High levels of nitrogen will encourage weeds and especially grasses and not the alfalfa. Nitrogen can occur in the soil in nitrate and ammonium forms. Usually nitrogen is mostly in the nitrate form in the soil and even ammonium forms of nitrogen applied to the soil convert to nitrate forms in about 2 weeks. Exceptions to this rule are waterlogged soils. Alfalfa fields tend to have a bit higher levels of ammonium forms of nitrogen. This is more important in making recommendations on grain fields after an alfalfa stand is removed.
Phosphate: This result is only the available phosphate expressed in ppm. The level should be thought of as an index and not converted to lbs./a. Which test used varies the scale in which the results are expressed. Melich results are about twice the results from the bicarbonate test results. Phosphorus is often the most limiting nutrient in Colorado soils and alfalfa is a relatively heavy user of phosphorus.
| Melich III soil test Results (ppm) | Relative level | Phosphate needed |
| below 10 | very low | 100-200 |
| 10-15 | low | 75-100 |
| 15-35 | moderate | 50-75 |
| 35-50 | adequate | 0-50 |
| over 50 | high | 0-25 |
These interpretations can be modified by pH, lime, irrigation availability, salt levels in soil, stand and expected yield.
Adjustments should be made to the above recommendations as indicated below:
Potassium (K): These results are expressed in ppm and the test only reflects the available potion in the soil, often in western soils much more is unavailable at a given time, but will become available as available potassium is used by crops at varying rates. Alfalfa is a fairly high user of potassium. Potassium is important for winter hardiness. The cooler the area the greater the need for potassium.
| Soil test level (ppm) | Relative level | units needed |
| below 60 | very low | 100-200 |
| 60-150 | low | 50-100 |
| above 150 | adequate | 0 |
These interpretations need to be adjusted by the soil potassium release rate, stand, irrigation availability and expected yield.
Zinc: This nutrient is normally not deficient in western soils but low levels especially in combination with high pH's. When the level drops below 0.4 ppm addition of 1-3 units/a zinc may be needed. Add the higher amount of zinc if the pH is above 7.5.
Iron (Fe): Rarely do deficiencies occur directly in Colorado soils. Levels below 4.5 ppm are considered low and iron can be applied foliar with care since iron chelates can burn crops. Most iron problems are related to too much lime in the soil.
Manganese(Mn): levels above 1 ppm are considered adequate.
Copper(Cu): levels above 0.2 ppm are considered adequate.
Sulfate-Sulfur(S): This nutrient is very mobile in the soil. It is expressed in ppm and lbs./A. To convert to lbs./a use the same formula as nitrogen. A high yielding stand of alfalfa in Colorado needs 25-30 units/A sulfur in a season. Most soils contain very adequate levels of sulfate-sulfur. Irrigated sandy soils are the most likely to be deficient. Sulfate-sulfur usually is supplied along with other nutrients such as nitrogen and potassium. Examples include ammonium sulfate, thiosul, potassium sulfate and K-Mag.
Calcium(Ca): Colorado soils are usually high in calcium and often too high. Levels above 4000 ppm can be a problem blocking availability of other nutrients such as phosphorus.
Magnesium(Mg): levels above 50 are considered adequate.
Sodium(Na): This element is often high and toxicity is more the thing to look for in soil test results. The percentage of sodium on CEC is a better test result to determine possible sodium problems. Alfalfa is fairly tolerant of high levels of sodium and toxicity shouldn't be a problem. High levels of sodium and cause water infiltration problems.
Cation exchange Capacity(CEC): This is a measure of the sites in a unit of soil to hold cations(positively charged ions which most nutrient are except nitrate-nitrogen, sulfate-sulfur and boron). Sandy soils can have CEC's of 9-15 and loamy soils can have CEC's of 20-30. It is virtually impossible to change CEC levels in a farming situation. Organic matter can have a big effect on CEC levels.
Percentage of cations on the CEC sites:
H- hydrogen will only occur on acid soils
K - potassium usually occurs in low levels
Ca- Large levels of calcium are common and good in Colorado soils
Mg - magnesium levels are also good in most Colorado soils
Na - This is the result usually of most interest. High sodium levels on the CEC can lead to water infiltration problems, problems cultivating and growing crops and surface crusting problems. High sodium levels are changed by replacing it with calcium either by adding calcium directly in the form of gypsum or dissolving excess soil lime already present in the soil (see above) with elemental sulfur or sulfuric acid. SAR is another measure of this sodium in the soil and its results are about twice the results of ESP(exchangeable sodium percentage) found here.
| ESP value - % Sodium (Na) of CEC | Relative level |
| below 4 | low - no problem |
| 4-8 | moderate - expect some problems |
| 8-16 | high - expect problems and pursue reclamation |
| above 16 | Sodic soil problems and reclamation needed |
Note: This information should only be used as a guide. Adjustments for local conditions must always be made.