Soil testing used as the basis for making fertilizer recommendations is, of course, a management practice that has been used by top farmers for many years.  Although most are at least somewhat familiar with looking at results of the analysis of soil samples, the reported information can be confusing.  Some Soil Testing Laboratories use standardized and frequently used procedures for the analysis of soil samples.  Others do not.  This can cause considerable confusion for those who have the responsibility of interpreting the results to provide specific fertilizer recommendations.  There is a real need to sort out some of the confusion.  Some of the information reported is important.  Some is not.  Since most Soil Testing Laboratories offer a wide variety of analytical procedures for analyzing soil, this discussion will focus on the reported information that is important.  At the same time, there will be no attempt to defend the reporting of information that has little or no value to farmers throughout Minnesota.

Soil pH and Buffer pH

This is usually the first piece of information that appears on nearly all reports from various soil testing laboratories.  The soil pH is a measure taken when the soil is mixed with water.  If the soil pH is less than 6.0, the sample is placed in a buffer solution and another pH reading is taken.  This second reading is referred to as the buffer pH.  This buffer pH value is the basis for the recommended amount of lime to apply.  The buffer solution has a pH of 7.0.  There is a recommendation of 500 lb. Effective Neutralizing Power (ENP) per acre for each 0.1 pH unit less than 7.0 when the buffer pH is considered.

Soil Nitrate Nitrogen

A measure of residual or carryover nitrogen is an important piece of information when making nitrogen recommendations for sugarbeets as well as corn and wheat in western Minnesota.  Most soil testing laboratories analyze the soil for nitrate-nitrogen.   The amount of carryover nitrogen in this form is usually reported as lb. per acre.   Soil from depths below 6 inches is needed for a measure of carryover nitrogen. Those who sample soils frequently collect samples from a variety of depths.  For this test, sampling depth is not consistent.  The lb. per acre of nitrate-nitrogen is calculated from information about depth of sampling.

Phosphorus

This has been a standard measurement in soil testing for many years.  For Minnesota, there are three analytical procedures that can be used to extract phosphorus from soils (Bray and Kurtz #1, Olsen, Mehlich III).  Most laboratories will use the Olsen procedure if the soil pH is 7.4 or higher.  At lower values, the Bray and Kurtz or the Mehlich III procedure is used.  In Minnesota, the interpretation of the results is the same for both the Bray and Mehlich III procedures.  For example, a reading of 6 ppm would be interpreted as "low" if either the Bray and Kurtz #1 or the Mehlich III procedure is used.

The Bray and Kurtz #1 procedure has also been referred to as the "weak" Bray test.  Some laboratories also report a "strong" Bray value.  These values are the result of the analysis when the more concentrated extractants used in the "weak" Bray test.  The results of the "strong" Bray test   have no relationship to crop response to phosphate fertilizer.  Therefore, the results of the "strong" Bray test should be ignored when making fertilizer recommendations.

Potassium

As with phosphorus, this is a routine analytical procedure used by all soil testing laboratories.  Most who work with soil testing and fertilizer recommendations realize that analytical results for the analysis of potassium can vary over a substantial range when results from two or more years are compared.  This variability is a consequence of soil moisture at the time of sampling rather than the analytical procedure used in the laboratory.

Sulfur

Some laboratories will analyze soil samples for sulfur and use the results as a basis for making fertilizer recommendations for this nutrient.  However, considerable research in Minnesota and neighboring states has led to the conclusion that there is no analytical procedure for sulfur that will accurately predict the need for this nutrient in a fertilizer program.  An evaluation of all of the research suggests that knowledge of soil texture is a more accurate predictor of the need for sulfur  in a fertilizer program.  Sulfur applied to soils with a loamy sand or sandy loam texture will usually produce added yield for alfalfa, corn and small grains.

There is a high risk of making a mistake if sulfur recommendations are based on the results of the analysis of a soil sample for this nutrient.

Calcium, Magnesium, and Ratios

Some laboratories will analyze soil samples for calcium and magnesium.  From this information and the information from the measurement for potassium, various ratios are calculated.  It is possible to calculate ratios for potassium:calcium, potassium:magnesium , and calcium:magnesium.  These ratios are used by some to make fertilizer recommendations.  Research throughout the North-Central region, however, has shown that these ratios have no effect on fertilizer recommendations.   Therefore, they should be ignored.

A measurement of magnesium can be important for crop producers who farm acid, sandy soil.  A measurement of magnesium is not important if soils are not sandy.

Cation Exchange Capacity (CEC)

The Cation Exchange Capacity of soils is reported by some soil testing laboratories. This value is a source of confusion for many.  It is a fixed property of soils and aside from making major changes in soil organic matter content, there are no management practices that can be used to change this value.  It is an indicator of soil texture.  The Cation Exchange Capacity of a soil has no relationship whatsoever to fertilizer recommendations in Minnesota.  This value should be ignored when making fertilizer recommendations.

Micronutrients

Most soil testing laboratories use an analytical procedure known as a DTPA extraction for the analysis of micronutrients (zinc, copper, manganese, iron).  This is a widely accepted procedure used to predict the amount of zinc needed in a fertilizer program. There is a high degree of confidence in the use of this test for predicting zinc needs.

Results of this analytical procedure, however, will not predict the severity of iron chlorosis in soybeans.  Except for the organic soils in northern Minnesota, the measure of copper extracted by the DTPA procedure has no relationship to the response to the use of this micronutrient.  Since the need for manganese in a fertilizer has not been documented in Minnesota, the amount of manganese extracted by the DTPA procedure has no value in making fertilizer recommendations.

Boron

Analysis of soil samples for this micronutrient has little value in Minnesota.  This test would only be recommended for growers who plan to grow alfalfa on the coarse textured soils in the east-central part of the state.

Soluble Salts and Calcium Carbonate

A measurement of soluble salts (conductivity) has been an option for many years.   More recently, there is some thinking that this measurement, when combined with a measure of calcium carbonate, can be used to predict the severity of iron chlorosis in soybeans.  Results of research conducted in the past three or four years indicate that these analytical procedures might be helpful.  However, there are no firm recommendations for the interpretation of the results of these analytical procedures at this time.