Agronomy Facts 8 Revised: Nov 30, 2017 Soil Test Interpretation Terms and Definitions By Dr. Richard Large Revised by: Dr. Oscar F. Ruiz Jr. _________________________________________ REPORT TERMS levels. However, the actual value that is best depends on many factors such as crop, yield potential, and soil type. ppm (lbs/acre) Results may be listed in ppm (parts per Rating Probability of crop response million) or pounds per acre. To convert Very low Close to 100% from ppm to pounds multiply by 2 and Low About 75% to convert from pounds per acre to Medium 50% ppm divide by 2. Optimum/High 0-25% Very High 0-10% DECISIEMENS/METER (dS/m) Monitor very high levels for excessive Electrical conductivity measurements nutrient levels that may impede uptake are often used to measure the amount of other nutrients. of soluble salts in the soil. The conductivity increases with increasing soluble salts, and the soil is considered SOIL ANALYSIS TERMS AND saline when the conductivity reading APPLICATIONS reaches 2 to 3 dS/m. (1 dS/m = 1 millimhos/cm) SOIL pH RATINGS The soil pH measures active soil acidity Most soil test readings on the report or alkalinity. A pH of 7.0 is neutral. are given a rating of very low, low, Values lower than 7.0 are acid; values medium, optimum, or very high. The higher are alkaline. Usually the most rating bars are color coded to the desirable pH range for mineral soils is rating scale across the top. 6.0 to 7.0 and for organic soils 5.0 to 5.5. The soil pH is the value that should The purpose of these ratings is to be maintained in the pH range most provide a general guideline for desirable for the crop to be grown. determining the optimum nutrient _________________________________________ www.waypointanalytical.com Agronomy Facts 8 _________________________________________ BUFFER pH soil. Sandy soils typically have low CECs therefore potassium can be lost This is an index value used for through leaching in sandy, low CEC determining the amount of lime to soils. It is recommended to split apply on acid soils to bring the pH to applications on low CEC soils to reduce the desired pH for the crop to be loss. On finer textured soils, potassium grown. The lower the buffer pH loss can occur through fixation. reading, the higher the lime requirement. For Mehlich 3 extractions, optimum levels are generally the following: PHOSPHORUS CEC 0-4.9: 95 to 115 ppm The phosphorus test measures that CEC 5-9.9: 130 to 155 ppm phosphorus that should be available to CEC 10-14.9: 155 to 190 ppm the plant. The optimum level will vary CEC 15+: 175 to 215 ppm with crop, yield and soil conditions, but for most field crops a medium to CALCIUM optimum rating is adequate. The Primarily soil type, drainage, liming and Mehlich 3 extraction is useful for a wide cropping practices affect the levels of range of alkaline to acid soil pHs. Soil calcium found in the soil. Calcium is Bray P1 is an older extraction used on closely related to soil pH. Calcium acid soils. Olsen sodium bicarbonate deficiencies are rare when soil pH is extraction is another older extraction adequate. The level for calcium will for P traditionally used on alkaline soils vary with soil type, but optimum ranges with pHs above 7.3 but has been used are normally in the 65% to 75% cation reliably down to pHs of 6.0. Excessive saturation range. Calcium saturations P levels caused by over application can above 85% may indicate a calcareous decrease micronutrient availability of or gypsiferous soil Zn, Mn, Fe, or Cu. POTASSIUM MAGNESIUM This test measures available potassium. The same factors, which affect calcium The optimum level will vary with crop, levels in the soil, also influence yield, soil type, soil physical condition, magnesium levels except magnesium and other soil related factors. Generally deficiencies are more common. higher levels of potassium are needed Adequate magnesium levels range on soils high in clay and organic matter from 30 to 70 ppm (60 to 140 lbs/ac). versus soils, which are sandy and low in The cation saturation for magnesium organic matter. Higher clay content should be 10 to 15%. Magnesium and/or organic matter increases the saturations above 20% can adversely cation exchange capacity (CEC) of the _________________________________________ www.waypointanalytical.com Agronomy Facts 8 _________________________________________ affect soil structure, water-infiltration, IRON soil drainage and aerification. Soil pH is a very important factor in interpreting iron tests. In addition, SULFUR crops vary a great deal in sensitivity to The soil test measures sulfate-sulfur. iron deficiency. Normally a medium This is a readily available form level would be adequate for most soils. preferred by most plants. Soil test If iron is needed it would be best levels should be maintained in the applied foliar. When applying to high optimum range. It’s important that pH soils, it should be applied only as a other soil factors, including organic chelated form. matter content, soil texture and drainage be taken into consideration MANGANESE when interpreting sulfur soil test and Soil pH is especially important in predicting crop response. Soil tests interpreting manganese test levels. In resulting in low sulfur levels should be addition, soil organic matter, crop and addressed with sulfate fertilizer yield levels must be considered. applications. Elemental sulfur does not Manganese will work best if applied provide readily available sulfur. foliar or banded in the soil. Chelated forms are utilized on high pH soils. BORON The readily soluble boron is extracted ZINC from the soil. Boron will most likely be Other factors, which should be deficient in sandy soils, low in organic considered in interpreting the zinc test, matter with adequate rainfall. Soil pH, include available phosphorus, pH, and organic matter level and texture should crop and yield level. For crops that be considered in interpreting the boron have a good response to zinc, the soil test, as well as the crop to be grown. test level should be optimum and should be administered in the same COPPER fashion as manganese and iron. Copper is most likely to be deficient on either low organic matter sandy soils, SODIUM or highly organic/muck soils. The crop Sodium is not an essential plant to be grown, soil texture, and organic nutrient but is usually considered in matter should be considered when light of its effect on the physical interpreting copper tests. A rating of condition of the soil. Soils high in medium to optimum should be exchangeable sodium may cause maintained. adverse physical and chemical conditions to develop in the soil. These conditions may prevent the growth of _________________________________________ www.waypointanalytical.com Agronomy Facts 8 _________________________________________ plants. Reclamation of these soils be influenced by seasonal variation in involves the replacement of the weather conditions as well as soil exchangeable sodium by calcium and physical conditions. the removal by leaching. NITRATE NITROGEN (NO3-N) SOLUBLE SALTS Nitrate nitrogen is a measure of the Excessive concentration of various nitrogen available to the plant in nitrate salts may develop in soils. This may be form. In high rainfall areas, sandy soil a natural occurrence or it may result types and areas with warm winters, this from irrigation, excessive fertilization or measurement may be of limited value contamination from various chemicals except at planting or side dress time. In or industrial wastes. One effect of high the areas with lower rainfall, the nitrate soil salt concentration is to produce test may be very beneficial. water stress in a crop to where plants may wilt or even die. The effect of A general guide to determine NO3-N salinity is negligible if the reading is levels: less than 1.0 dS/m. Readings greater Very low: 0-5 ppm than 1.0 dS/m may affect salt sensitive Low: 6-10 ppm plants and readings greater than 2.0 Medium: 11-20 ppm dS/m may require the planting of salt High: 21-35 ppm tolerant plants. Very high: 36 ppm and above ORGANIC MATTER AND ESTIMATED CATION EXCHANGE CAPACITY (CEC) NITROGEN RELEASE (ENR) CEC measures the soil’s ability to hold Percent organic matter is a nutrients such as calcium, magnesium, measurement of the amount of plant and potassium, as well as other and animal residue in the soil. The color positively charged ions such as sodium of the soil is usually closely related to and hydrogen. The CEC of a soil is its organic matter content, with darker dependent upon the amounts and soils being higher in organic matter. types of clay minerals and organic The organic matter serves as a reserve matter present. The common for many essential nutrients, especially expression for CEC is in terms of nitrogen. During the growing season, a milliequivalents per 100 grams part of this reserve nitrogen is made (meq/100g) of soil. The CEC of soil can available to the plant through bacterial range from less than 5 to 35 meq/100g activity. The ENR is an estimate of the for agricultural type soils. Soils with amount of nitrogen (lbs/acre) that will high CEC will generally have higher be released over the season. In addition levels of clay and organic matter. For to organic matter level, this figure may example, one would expect soil with a _________________________________________ www.waypointanalytical.com Agronomy Facts 8 _________________________________________ silty clay loam texture to have a lb/acre extracted. For example, a K considerably higher CEC than a sandy percent saturation of 2.5% and a Mg of loam soil. Although high CEC soils can 12% produces a ratio of 0.2.