Interpreting Soil Sample Results - North Dakota

Soil analysis is a great tool to assess what soil amendments are needed for optimum plant function and yield potential. Soil test results list the soil test concentration for specific parameters along with an interpretation value (low, optimum, and high) and a recommendation. This Spotlight will help explain the meaning of each soil test parameter, what the optimum values are for each parameter, and general recommendations based on the soil test values.

Soil Sampling and Lab Testing

To obtain quality soil test results, the soil samples must be taken properly. Each sample must be representative of the entire field or specified sampling unit. The samples must also be taken at the proper depth during the same time frame every year. Sampling depth is 0-6 inches for phosphorus (P), potassium (K), zinc (Zn), soil pH and organic matter (OM). For nitrogen (N) sampling depth is 0-24 inches for most crops except sugar-beets. Once proper depth is chosen be consistent as this will influence the soil test result. For more detail on soil sampling refer to Agronomic Spotlight - Soil Testing.

Lab Results and Conversions

Soil lab results list the soil test interpretation, soil test result, and a recommendation. When reviewing lab results, it important to know what extraction method was used. In addition, using the same lab for annual analysis will help ensure uniformity of year-to-year comparisons.

The recommended nutrient rates shown on a soil test result are the actual amount of nutrient and not the amount of fertilizer. A lab may report these results in parts per million (ppm) or lbs/acre. To convert ppm to lbs/acre multiple ppm by 2 and to convert lbs/acre to ppm divide lbs/acre by 25:

lbs/acre = ppm x 2
or
ppm = lbs/acre ÷ 2

To determine the amount of fertilizer needed, find the fertilizer grade listed on the fertilizer bag or bill of sale, which is listed like this: N-P-K. The grade is the percent of total N, available phosphorus (P2O5) and soluble potassium (K2O). If any additional nutrients are included in the fertilizer it will be listed as a fourth value with the abbreviation of the nutrient in parentheses. Figure 1 gives an example of how to determine a fertilizer blend rate based on a phosphorus recommendation.

Determining Amount of Fertilizer 

Source: Montana State University Extension1

Soil Parameters

Although corn requires 14 mineral nutrients, North Dakota soils supply all of these naturally except for N, P, K, sulfur (S), and Zn. Other soil characteristics that play a role in nutrient availability are soil pH and soluble salts (salinity). All of these nutrients and soil factors, except for S, can be managed with the use of soil testing. More detailed information for of each soil parameter follows.

Nitrogen

An adequate supply of N is associated with high photosynthetic activity as well as the vigorous growth and dark green color of plant vegetation. There are two forms of plant available N: nitrate (NO3-) and ammonium (NH4+). Nitrate is measured most often in soil tests. Soil test results report NO3-N in lb N/ acre. Nitrate can be leached through the soil and lost to denitrification during periods of soil saturation. Nitrate is also produced by microbial decomposition of organic matter. Consequently, nitrate levels reflect what is immediately available and not what will be available in the future. Nitrogen fertilizer can be applied as a pre-plant, during planting, or as a side-dress application when corn plants are 6-12 inches tall. The test is useful for determining if significant amounts of nitrate have leached after excessive rain early in the season. The University of Minnesota reports fields with medium textured soils (glacial till or loess) and corn following corn have the greatest potential for residual NO3-N to be left for the following year’s crop2. Dr. Dave Franzen, Soil Fertility Specialist from North Dakota State University, is currently updating the N recommendations for North Dakota. He recommends using the following modified equation to determine N fertilizer recommendations = (1.1 x yield potential) - soil test NO3-N in top 2’ of soil - previous crop credit.

Phosphorus

Phosphate compounds store energy created from photosynthesis and carbohydrate metabolism that will be used for plant growth and reproductive processes. Phosphorus is not as naturally abundant in soils compared to N and K. There are two P extraction methods used for lab tests. The extraction methods are: Bray P-1 (acidic soils) and Olsen P (neutral to alkaline soils). If the entire field is acid in pH, the Bray P-1 test should be used. If part of the field has a soil pH higher than 7.0, then the Olsen test should be used. Phosphorus recommendations for corn and soybean production based on soil test results from the two extraction methods can be found in Table 1.

Phoshorus recommendations chart 

Source: 3North Dakota State University Extension

Potassium

Potassium plays a role in many functions in the plant including: activating enzymes, drawing water into the roots, producing phosphate molecules and CO2, providing energy for the translocation of sugars, and taking up and assimilating N. The K cycle is always changing and soil test K concentrations will fluctuate seasonally due to differing environmental conditions. Due to the seasonality of K availability, selecting the proper time to test can be difficult. Comparing soil tests over time is the best method of evaluating nutrient management decisions. Soil testing in the fall or spring is acceptable for determining K soil concentrations. However, sampling should occur at the same time each year for comparisons over time. Soil test K recommendations for corn and soybean production can be found in Table 2.

Potassium recommendations chart 

Source: 3North Dakota State University Extension

For more information regarding P and K recommendations Dr. Dave Franzen recommends reading this publication: North Dakota Fertilizer and Recommendation Tables and Equations,SF-882, available from both the NDSU Extension publications webpage and Dave Franzen webpage under Extension Publications.

Sulfur

Sulfur has many important functions in plant growth and metabolism. Deficiency symptoms resemble those of N: stunting, chlorosis, thin stems, and spindly plants. Sulfur deficiency is found in young tissue where as N deficiency can be found in both young or old plant parts. Sulfur deficiencies are common in North Dakota. Dr. Franzen believes the best way to determine if there is a need for S is to consider soil and weather. For loam or coarser soils, if fall rain, winter snow, and/or preseason spring rain is heavier than normal, S should be applied. Dr. Franzen recommends rates of about 10 lb/acre as a sulfate or thiosulfate form applied in the spring only to prevent a problem. Elemental S forms are not useful in this region.

Calcium and Magnesium

Calcium enhances NO3-N uptake and also regulates the uptake of cations, such as K+ and sodium (Na+). Calcium saturation results in a high pH or alkaline soil. In addition, high concentrations in the soil typically result in low concentrations of undesirable cations, such as aluminum (Al3+) in acidic soils and Na+ in saline soils. Conversely, a low Ca content in the soil can result in a low pH or acidic soil.

Magnesium is a major part of the chlorophyll molecule and without it photosynthesis cannot take place. Magnesium is also imperative in many other physiological and biochemical functions within the plant. Magnesium and Ca have some behavioral similarities in soil. Both Mg and Ca ions can easily be exchanged or taken off of negative soil colloids. One difference is that Mg can only become fixed to certain clays. Magnesium deficiencies are not widespread but can occur.

Micronutrients

The micronutrients needed in trace amounts for plant function are: Cu, Fe, Mn, Zn, B, Cl, Ni, and Mo. Although many of the micronutrients are reported on soil test reports, their levels do not currently affect fertilizer recommendations, with the exception of Zn. For soils low in Zn, North Dakota State University recommends broadcasting zinc sulfate fertilizer at the rate of 10 to 15 pounds of zinc (30 to 45 pounds of zinc sulfate) per acre3. For severe deficiencies a foliar spray can be applied as a chelate or other liquid form. Plant tissue and soil analyses should be used together to assess the need for the application of the other micronutrients.

Organic Matter

Organic matter affects many soil biological, chemical, and physical properties that influence nutrient availability. Organic matter content is related to productivity and soil tilth. Some roles of OM in the soil include: storage for nutrients, energy for microbial activity, increasing water holding capacity, and providing a buffer against changes in pH and salinity. On many soils, suitable physical properties occur at relatively low levels of OM ranging from 2 to 4%; however, increasing soil OM can generally increase productivity4. Consult your regional guidelines for a more precise influence of OM on nutrient availability.

Soil pH

Soil pH is an indicator of the level of acidity or alkalinity of the soil, ranging from 0 – 14. A reading of 7 is neutral, lower values are acidic and higher values are alkaline. Crops typically grow best when pH is between 6 (slightly acidic) and 7.5 (slightly alkaline). Results of soil pH are reported on a logarithmic scale; therefore, caution in interpretation should be made. For example, a soil with a pH of 6 is 10 times more acidic than a soil with a pH of 7, and a soil with a pH of 5 is 100 times more acidic than a pH of 7. Nutrient availability may be hindered if soil pH is not within the optimum range and can result in crop nutrient deficiencies.

Soluble Salts

High soluble salt content (or salinity) can cause water stress and nutrient imbalances in plants, as well as affect nutrient uptake. Seedlings are more sensitive to higher than normal soluble salts in soil compared to older plants. Soil labs measure electrical conductivity (ECe) of a soil extract to determine salt concentrations in soil. ECe measurements can be taken by using a saturated paste or a 1:1 by weight soil-to-water slurry method. The saturated paste method is precise but time consuming and expensive, while the 1:1 slurry method is quick and inexpensive. For corn, the threshold for a 1:1 slurry is 1.3 decisiemens/meter (dS/m) and 1.7 dS/m for a saturated paste6. The thresholds for soybean are 2.4 dS/m for a 1:1 slurry and 5.0 dS/m for a saturated paste. Decisemsens/ meter (dS/m) are equivalent to millimhos/cm (mmhos/cm) and values may be reported either way. Dr. Franzen, reports that these thresholds do not take into account other stresses. Consequently, thresholds may be lower if other stresses are present.

Cation Exchange Capacity

Although many soil tests may include cation exchange capacity (CEC) the number provided is invalid for soils with pH higher than 7. Dr. Franzen suggests that unless a more expensive, actual CEC is analyzed, the estimate provided by commercial labs is subject to huge errors due to soluble salt and lime content. CEC is a relative measure and should not be used to modify fertilizer recommendations. A proxy for actual CEC is information regarding soil texture. Soil texture can be used to help manage fertilizer N timing due to leaching potential.

Region Specific Information

The soil parameter descriptions and optimum values provided within this Spotlight can help assess your soil fertility program and help reach optimum yield potential. Due to variability in soil, lab analysis, and reporting, guidelines specific to your region may exist. A local agronomist or extension specialist can provide information specific to your area.