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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 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 levels that may impede uptake are often used to measure the amount of other . 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 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 can be lost This is an index value used for through 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: 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 optimum rating is adequate. The Primarily , drainage, 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 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 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

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Agronomy Facts 8 ______affect , water-, soil drainage and aerification. Soil pH is a very important factor in

interpreting iron tests. In addition, 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, and drainage be taken into consideration 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, , crop and addressed with sulfate 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. The readily soluble boron is extracted 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 as the crop to be grown. test level should be optimum and

should be administered in the same 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 (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 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. hold more nutrients, it doesn’t necessarily mean that they are more K and Mg are nutrients that can productive. Much depends on good soil compete for uptake in various crops. In management. soils where ratios are greater than 0.33, An accurate CEC of calcareous, saline crops should be monitored for low or or sodic soils should only be performed deficient magnesium. Crops, especially with cation replacement. A calculated forages, that are fertilized with high CEC will typically be inflated with these amounts of potassium fertilizer should soil types. be monitored for magnesium deficiency. CATION SATURATION Ca and Mg are essential macronutrients Cation saturation refers to the whose levels can provide insight into proportion of the CEC occupied by a the soil’s chemical and physical given cation (an ion with a positive condition. Soils with significant and charge such as calcium, magnesium or clay content can have their structure potassium). adversely affected if magnesium levels

are disproportionate to the levels of The percentage saturation for each of calcium. An idealized Ca/Mg ratio is the base cations are commonly within typically near 6.5. the following ranges:

Calcium 40 to 80% Magnesium 10 to 40% REPORT NUMBER Potassium 1 to 5% All samples are filed by report number. When contacting Waypoint Analytical Many consider idealized base Inc. concerning a certain report, be saturation ranges to be: sure to refer to this number. Calcium 65 to 80% Magnesium 10 to 20% SAMPLE NUMBER Potassium 2 to 7% The customer’s identification number Sodium less than 1% on the information sheet for each

sample is listed on the report. RATIOS K/Mg and Ca/Mg ratios compare the LABORATORY NUMBER milliequivalents or percent saturations The identification number, which was to each other. These are not ratios assigned by the laboratory to each based on the comparison of ppm or individual soil sample, is shown. There

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Agronomy Facts 8 ______may be more than one laboratory The recommendations for N-P2O5-K2O- number or sample per report. etc. are in lbs of the actual nutrient. For example, 100 lbs of N would require GUIDELINES 294 lbs of ammonium nitrate (34-0-0) per acre fertilizer to be applied at The crop to be grown will be listed and sidedress. In the long-term, timely the recommendation units (lbs/acre or analyses can be used to adjust the lbs/1000 sq ft) will be listed on the amount of nitrogen fertilizer applied same line. The fertility guidelines are according to a field’s inherent fertility based on a season long application and and average yield. Growers whose suggested splits are normally listed in fields often test high due to manure the comments. applications, organic matter The lime application is reported in mineralization, or nutrient retention can pounds of 100% effective calcium reduce nitrogen applications by 10-15% carbonate limestone per acre. Contact of the recommended nitrogen rate. the laboratory if you have any Growers whose fields often test low questions on how the lime value is should consider split applications reported. If you want a lower pH, instead of a total application at-plant. reduce the lime application; for a Sidedressing will reduce the possibility higher soil pH, increase the lime of losing nitrogen fertilizer by heavy application. rainfall and consequently assure that

the majority of nitrogen applied is

present during the corn’s highest period of demand (V4+).

Prepared by: Dr. Richard Large June 12, 2003 Revised by: Dr. Oscar F. Ruiz Jr. November 30, 2017

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