Interpreting Soil Sample Results - Iowa

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. Sample depth can vary by test but is usually within 6-12 inches. 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

Soil lab results list the test, the result, and may list an interpretation or 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. Labs may report the results in parts per million (ppm) or lbs/acre. To convert ppm to lbs/acre multiple ppm by 2 (lbs/acre = ppm x 2). To convert lbs/acre to ppm divide lbs/acre by 2 (ppm = lbs/acre ÷ 2).

Soil Parameters

Macronutrients are needed in large amounts by plants and are the most common fertilizers applied to the soil. Nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg) are all macronutrients. The label on a bag of fertilizer lists the most commonly applied macronutrients in this order; N-P-K. Micronutrients are needed in trace amounts for plant function. The micronutrients are: copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), boron (B), chloride (Cl), nickel (Ni), and molybdenum (Mo). Other soil characteristics that play a role in nutrient availability may also be analyzed and reported. These may include organic matter (OM), soil pH, soluble salts (salinity), and cation exchange capacity (CEC). More detailed definitions of each soil parameter follow.

Nitrogen (N)

An adequate supply of nitrogen 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. The Late Spring Nitrate Test also known as the Preside-dress Nitrate Test (PSNT), may be used in season, when corn plants are 6-12 inches tall, to determine how much N should be applied as a side-dress application. The test is useful to find out if significant amounts of nitrate have leached after excessive rain early in the season. Table 1 lists the critical nitrate levels for side dress N applications to corn, recommended by Iowa State. Since nitrate soil test thresholds are regional, consult the nitrate soil test thresholds specific to your geography.

Late Spring Nitrate test recommendations 

Source: 1Iowa State University, 2011

Phosphorus (P)

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. Phosphorus is relatively immobile in the soil because its negative charge binds to positively charged particles, such as calcium. This makes it unavailable to plants and limits movement through the soil profile. Because the amount of calcium and other cations influence soil pH, the amount of plant available P in the soil solution is also related to soil pH. The form of plant available P changes in alkaline soils; therefore, at a pH of 7.2 or higher plant uptake of P is slower. Because of this pH sensitivity, there are three P extraction methods used for lab tests. The extraction methods are: Bray P (acidic soils), Mehlich-3 P (acidic soils), and Olsen P (neutral to alkaline soils). The extracted P is then measured using a colorimetric method, or if specifically noted by an inductively coupled plasma (ICP) in the case of some Mehlich-3 ICP results. Test results may vary based on the extraction method or test used for measuring P therefore; it is important to know which extraction method and test were used, and how the results were reported, when interpreting the results. Phosphorus recommendations for corn and soybean production based on soil test results from each of the three soil extraction methods can be found in Table 2.

Phosphorus recommendations 

Source: 2 Iowa State University, 2011

  • Corn recommendations are based on 150 bu/acre yield target and soybean recommendations are based on 50 bu/acre, if yield target is higher or lower, P2O5 application rates may need to be adjusted.
  • Sub-soil P is determined based on yield history, soil series, and deeper soil sampling and testing at a 30-42 inch depth.
  • Sub-soil Bray P1 values are considered low at 8 ppm or less and high at 9 ppm, or more.
  • Application amounts were calculated based on nutrient removal throughout the season via tissue sampling and analysis.
  • If soil test values are above optimum levels (high or very high), no additional P2O5 needs to be applied.

Potassium (K)

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. Potassium is present in large quantities in most soils, although it is not always available. Some forms of K can be adsorbed to clay particles which are tightly fixed in feldspar or mica. Conversely, K+ ions bonded to negative clay minerals can easily be “knocked off” or exchanged. Potassium can also become available from manure applications or decomposition of plant residue. In addition, plants take up K + by luxury consumption, which means more can be consumed than is needed for plant function. 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, choosing 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 are 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 3.

Potassium recommendations 

Source: 2 Iowa State University, 2011

  • Corn recommendations are based on 150 bu/acre yield target and soybean recommendations are based on 50 bu/acre, if yield target is higher or lower, K2O application rates may need to be adjusted.
  • Higher concentrations of sub-soil K require less surface soil K for optimum crop production.
  • Sub-soil K is determined based on field history, soil series, and deeper soil sampling and testing at a 12-24 inch depth.
  • Sub-soil K ammonium acetate test values are considered low at 50 ppm or less and high at 51 ppm or more.
  • Application amounts were calculated based on nutrient removal throughout the season via tissue sampling and analysis.
  • If soil test values are above optimum levels (high or very high) no additional K2O needs to be applied.

Sulfur (S)

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. Only a small fraction of the total soil S is readily available to plants and that form is sulfate (SO42-). Sulfur can be mobile or immobile in soil depending on microbial activity and the quantity of carbon (C), N, and P. Sulfate may leach but quantities can also be retained in soils by adsorption to iron and aluminum oxides or clays in the soil profile. S-deficient soils have soluble SO42- concentrations less than 5 to 10 ppm5.

Calcium (Ca) and Magnesium (Mg)

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 sodium (Na+) in saline soils.

Conversely, a low calcium content in the soil can result in a low pH or acidic soil. Magnesium (Mg) is a major part of the chlorophyll molecule and without it photosynthesis cannot take place. Mg is also imperative in many other physiological and biochemical functions within the plant and is a structural component of the ribosomes.

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. Mg deficiencies are not widespread but can occur. Concentrations of Mg2+ in the soil are commonly 5-50 ppm in temperate soils but can be much higher5.

Micronutrients

The micronutrients needed in trace amounts for plant function are: copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), boron (B), chloride (Cl), nickel (Ni) and molybdenum (Mo). Although many of the micronutrients are reported on soil test reports their levels do not currently affect fertilizer recommendations, with the exception of zinc. Soil test zinc recommendations for corn are shown in Table 4. Plant tissue and soil analyses should be used together to assess the need for application of the other micronutrients.

Zinc recommendations chart 

Source: 2 Iowa State University, 2011

Organic Matter (OM)

Organic matter (OM) affects many soil biological, chemical, and physical properties that influence nutrient availability. OM content is related to productivity and soil tilth. Some roles of organic matter 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 organic matter ranging from 2 to 4%; however, increasing soil organic matter can generally increase productivity3. When OM levels are higher than 2% more N will be mineralized. A general guideline is to reduce fertilizer recommendations by 20 lb/acre for soils with >3% OM and increase N recommendations for soils with <1% OM4. 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, values lower are acidic and values higher 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 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. High soluble salt levels above 4 mmhos/cm (or ‘dS/m’) can potentially damage plants4. Salinity levels in soil can change rapidly due to leaching; therefore, sampling should take place periodically within the growing season.

Cation Exchange Capacity (CEC)

CEC is not always part of soil analysis. If it is included on a lab result, a CEC above 10 milliequivalents per 100 grams (10 meq/100g) is considered adequate4. CEC can also be reported as cmols/kg, and may be interchangeable between units reported. A high CEC is sought because it indicates a high capacity for the soil to hold cations (positively charged particles), such as, K+, NH4+, Cu2+, Fe2+, and Mn2+.

Region Specific Information

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

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