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Understanding a Soil Analysis

The following is a reference guide to understanding your AG Laboratory soil analysis. It will provide you with guide lines along with the various ranges to allow you to agronomically adjust your amendments and fertility inputs.

Soil pH

pH is a measure of the active hydrogen in soil. The presence or absence of hydrogen determines whether the soil is acidic or alkaline. The pH values of most soils range between 4.0 to 8.5, however, slightly acid conditions usually are the most productive (6.0 to 6.9).

V Strong Strong Moderate Slight | Slight Moderate Strong V Strong
— | — — | — — | — — | — | — | — — | — — | — — | —
3 4 5 6 7 8 9 10

Because soil pH measures the active hydrogen it is quite variable from one season to the next. This variability makes it difficult to predict the amount of lime necessary to neutralize.


Buffer pH is the resulting sample pH after a liming material is added. This liming material is called the buffer solution and it acts as an extremely fast-acting lime. Each soil sample receives the same amount of buffering solution; therefore the resulting pH is different for each sample. To determine a lime recommendation, we look at the difference between the original soil pH and the ending pH after the buffering solution had reacted with the soil.

If the difference between the two pH measurements is large, it means that the soil pH is easily changed, and a low rate of lime will be sufficient. If the soil pH changes only a small amount after the buffering solution has reacted, it means that the soil pH is difficult to change and a larger lime addition is needed to reach the desired pH for the crop.


This measures the amount of free limestone in the soil. This test is reported as: Very Low, Low, Medium, High, Very High. As the rating increases so the amount of free limestone. Changing the amount of excess carbonate in the soil is difficult and economically impossible to do. However, it can be important in herbicide selection as well as selection of fertilizer application techniques.

Soluble Salts

Soluble Salt is a measure of conductivity of soil solution or the salts which are soluble in water. Soils have a wide range of salt levels, but the relative critical level is 0.60 mmhos/cm. Values greater than this can cause damage on salt sensitive crops.

Generally high salt levels are associated with soils which have poor drainage conditions. The salts accumulate at the soil surface rather than leaching throughout the profile. If irrigation water contains a medium or high amount of salt the accumulation process increases.

Exchangeable Sodium

Sodium provides information for use in reclaiming Saline-Alkali soils. Whenever the percent base saturation of sodium exceeds 5%, water infiltration rates can be reduced as well as impairing plant growth. Gypsum, Epsom salts and Elemental Sulfur are common amendments for aiding water infiltration rates.


CEC establishes the rate at which nutrients (cations) will be stored and released by a particular soil. The CEC value on the AG Laboratory report is an estimate, obtained by adding the values of the five major cations contained in agricultural soils (Potassium, Magnesium, Calcium, Sodium and Hydrogen). This CEC value can be used as an estimate of soil texture.

Estimated texturecec Value
Loamy Sand8-12
Sandy / Silty Loam13-20
Clay or Clay Loam29-40

The above table holds true about 90% of the time, however, free calcium carbonate in the soil can overestimate the CEC value. If the excess carbonate values are high or very high, a particle size analysis would need to be analyzed to accurately determine soil texture.


The five major cations in soils are; Hydrogen (H), Potassium (K), Magnesium (Mg), Calcium (Ca), Sodium (Na). The actual percentage of each cation are reported on each of the AG Laboratory reporting options along the suggested percentage. By comparing the actual percent with the suggested percentage an idea of which kind of soil amendment (Lime, Gypsum, Sulfur) may or may not be needed. The suggested cation percentages listed on each report are:

Hydrogen10 - 15%
Sodium0 - 2%
Magnesium10 - 12%
Calcium60 - 69%
Potassium5 - 7%


Organic Matter is the result of the decay process of organic residues (plant and animal). Undecomposed organic residues (wheat straw, leaf litter or thatch) is not organic matter. In most agronomically productive soil the OM content ranges from 0.5 to 10.0%. The OM range for any specific soil is determined primarily by geographic and climatic conditions.

Organic matter acts as a storehouse for plant nutrients and improves physical structure of the soil. It has a very high capacity for holding cations as well soil water. Because of its active holding sites it is also an important factor for determining herbicide selection and adjusting application rates.


Nitrate-Nitrogen is the amount of available nitrogen present in the soil at the time it was analyzed in the laboratory. Because of it’s solubility it can leach rapidly on various soil conditions. This mobility makes it difficult to predict how much nitrogen will be present throughout the growing season. However, it can be a useful tool for determining nitrogen utilization efficiencies at the end of the growing season.

To convert ppm into pounds/acre use the following formula:

Sample depth (in inches) X .333 = conversion factor (CF)
CF X NO3 ppm = NO3 pounds/acre

Soil test = 7 ppm NO3
Sample collection depth = 0 to 8 inches

8 X .333 = 2.66
7 ppm X 2.66 = 19 pounds/acre Nitrate Nitrogen

Phosphorus: (P)

Two types of phosphorus extractions are used for analysis, type is determine by the soil pH. If pH is < 7.2 a Bray I extraction is used, > than 7.2 an Olsen extraction is used. The interpretation of the two methods are different, the following table defines the two.

RatingBray P (pH<7.2) ppmOlsen P (pH>7.1) ppm
Low1 - 151 - 9
Low to Adequate15 - 2510 - 15
Adequate26 - 4016 - 24
High> 40> 24


Potassium is a cation which is held on the soil’s exchange sites. The form of potassium extracted is the readily available K. The following table provides a generalized interpretation for potassium. Soil textures have a great influence on it’s availability. With sand textures high levels of K may be difficult to obtain because of it’s ability to leach.

Low to Adequate121-190


Below are the tables which gives a general interpretation for the above micro-nutrients.

RatingZinc (PPM)Manganese (PPM)Copper (PPM)Iron (PPM)
Low1.0 - 0.81.0 - 2.51.0 - 0.41.0 - 4.5
Low to Adequate0.9 - 1.22.6 - 4.00.5 - 0.94.6 - 7.0
Adequate1.3 - 3.04.1 - 12.01.0 - 2.010 - 20.0


Below are the interpretation tables for sulfur and boron. Both of these elements are soluble in water and subject to leaching.

Low1 - 40.1 - 0.5
Low to Adequate5 - 90.6 - 0.9
Adequate10 - 251.0 - 1.5