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An Overview on Soil Test Information

By Achille Correggia, B.Sc. (Agri.) Agronomy, CCA-ON

Soil can be defined as a complex medium of organic and inorganic material with several important roles in any ecosystem. Soil is a key factor in controlling water quality and losses in the hydrologic system. It functions as Mother Nature's recycling system by assimilating dead organic matter and releasing the basic elements to be reused by living organisms. Soil also provides a habitat for a wide range of living creatures from mammals and reptiles, to microscopic organisms of unbelievable numbers and diversity.

In the turf industry, the most important role for soil is providing nutrients to root systems to support proper growth and development of turfgrass. Soil nutrients required by turfgrass are situated on the surface of soil particles and organic matter called the exchange site. The exchange site is the area on soil particles and organic matter where nutrients are interchangeable with the soil solution. Once in soil solution these nutrients are available for turf grass uptake.

Occasionally, soil will develop inadequate levels of one or more nutrients required for proper turf development. Soil deficiencies, if not corrected will eventually appear as symptoms within the turf. Visual symptoms can range from yellowish-green leaf discolouration to necrotic lesions (spots of dead tissue) at the leaf margins. Visual symptoms are an excellent indication of a possible deficiency in turf, but they do not provide accurate information as to which nutrient is deficient and to what extent. The only way to accurately determine which soil nutrient is inadequate, and by how much is to take a soil sample and have it tested at a certified laboratory.

For a turf manager to correctly interpret the results of any soil test they need to understand the information it provides. This article will discuss the general components of a typical soil test; therefore helping to improve an individual's overall understanding of soil test results.

What information can be found on a soil analysis report?

Cation Exchange Capacity (CEC): The total amount of exchangeable cations (Ca⁺², Mg⁺², K⁺, Na⁺ and H⁺) that a soil can adsorb. The CEC of a soil is dependent on the amount and type of clay minerals present and the organic matter content. Soils with high CEC values will typically have higher levels of clay and organic matter. The higher the CEC, the more cations the soil is able to hold against leaching. These high CEC soils may be able to hold more nutrients, but as with all soils they require good management to make them productive.¹ Table 1 provides estimated soil textures based on CEC values.

Table 1³

Base Saturation (BS): The base saturation of a soil indicates the percent of exchange sites on a soil particle that are occupied by Hydrogen (H⁺), Sodium (Na⁺), Magnesium (Mg⁺²), Calcium (Ca⁺²) and Potassium (K⁺). If a soil sample has a calcium saturation value of 50% and a magnesium saturation value of 10%, then calcium occupies half of the cation exchange sites and magnesium occupies one-tenth of the cation exchange sites. A soil's percent base saturation is directly correlated with the soil pH; as the pH increases so does the percent base saturation. A higher base saturation corresponds to an alkaline type soil. 2 A percent base saturation of more than 80% is required for optimum plant growth. Less than 40% will create difficulty for plant growth.⁴ Table 2 provides optimum base saturation levels for most soil.

Table 2⁵

Soil pH: Soil pH is a measurement on a scale from 0-14 that indicates if a soil is acidic (below 7.0) or alkaline (above 7.0). Soil pH is very important as it affects the availability of soil nutrients for turf growth. A pH between 6.2 – 6.8 will provide the highest availability of most soil nutrients required for turf growth.⁶ Table 3 provides pH tolerance levels of different cool season turfgrass species, while figure 1 illustrates how pH affects the availability of plant nutrients.

Table 3³

Figure 1

Buffer pH: Where soil pH measures the active acidity or alkalinity of the soil solution, buffer pH measures the acidity reserve (ex. hydrogen and aluminum) on the soil particle. A buffer pH is the soil's ability to resist changes in pH. All soils differ in their ability to resist pH change; a soil with a higher CEC (more clay and organic matter) will have a greater resistance than soil with a lower CEC (higher sand content). The buffer pH is only used to determine the amount of lime required to reach a targeted soil pH for proper turf growth. Because buffer pH is only used to determine limestone application quantities, it is only analyzed when the soil pH is less than 6.5. Soils with a higher CEC generally require larger amounts of limestone to reach a desired soil pH.⁷ Table 4 provides general limestone application amounts for various buffer pH values. Please note that these values are based on a limestone with an agricultural index of 75% incorporated into the soil. Limestone application to established turf should not exceed 50 pounds in any one application³.

Table 4³

*Lime if soil pH is below 6.1 (lbs/1000ft 2)

Potassium to Magnesium Ratio: The general K/Mg ratio should be higher than 1:4 or 0.25. In other words, the percentage saturation of magnesium should be at least four times greater than the percent saturation of potassium. High potassium levels can results in reduced magnesium uptake by the plant, potentially leading to magnesium deficiencies.⁹

Soluble Salts: High levels of soluble salts in soils can be associated with excessive fertilizer application, runoff of road salt application, poor soil drainage, or significant soluble salt levels in irrigation water. Soluble salts are measured by the soil's ability to conduct electricity. The greater the concentration of water soluble salts the higher the conductivity. Soluble salts can interfere with uptake of water by turf. This reduction of water uptake is most noticeable during periods of low moisture supply (drought) and within soils with low water holding capacity (sandy soils).⁸ Table 5 provides general information on soil conductivity as a function of soluble salt concentrations.

Table 5⁸

Optimum levels of macro, micro and secondary nutrients:

Phosphorus: With proper phosphorus levels the overall quality of turf is enhanced as phosphorus is involved in many fundamental processes including photosynthesis, nitrogen fixation, protein and carbohydrate metabolism and most importantly root development¹¹. Phosphorus concentrations can be determined using four different testing methods that may be reported on a soil report. These tests include:

• Olsen Sodium Bicarbonate which measures the phosphorus levels in soils that have a soil pH value above 7.0. Using this testing method, a level of 55-65 ppm is sufficient.
• A P1 or weak Bray method measures phosphorus that is readily available to turf, a value between 20-30 ppm is typically adequate.
• A P2 or strong Bray test provides levels of readily available soil phosphorus and a small amount of the soil reserve, a value between 40-60 ppm is required.
• The Mehlich I and Mehlich III also called Double Acid, measures the readily available phosphorus in slightly acidic soil, levels from 30 to 50 ppm are sufficient.¹⁰

Potassium: Adequate potassium levels are vital for protein and starch production, turf movement (stomata opening and closing), as well as turf respiration. Potassium is especially important in helping turf adapt to environmental stresses. Good potassium nutrition is linked to improved drought tolerance, enhanced winter hardiness, better resistance to certain fungal diseases, and greater tolerance to insect pests.¹⁰ Soil test results will indicate th e level of available potassium in the soil. Soils with higher clay levels require higher potassium levels than soils with higher sand (lower clay) content. The below table indicates the optimal range at different CEC levels:³

Calcium & Magnesium: Levels of both secondary nutrients can be affected by soil type, drainage and organic matter content. Calcium is required for cell wall formation, sugar movement throughout the plant, root hair formation and overall plant tissue quality.

Magnesium is used to help regulate the uptake of other plant nutrients, assists in protein production, aids in phosphate metabolism and is essential for chlorophyll production.¹¹ The below table indicates the optimal range for both nutrients at different CEC levels:³

Boron: Boron within the plant is required for sugar transport, cell wall formation, root growth, nitrogen assimilation and respiration. Sufficient boron levels can range from 1-3 ppm. When evaluating boron levels it is important to consider soil pH, organic matter content and soil texture as all three will have an affect on boron availability. ^10&11

Copper, Manganese, Zinc & Iron: Based on the DTPA extraction method, each micronutrient has different optimal concentrations within the soil. Each of these micronutrients is required for vital turf functions:

• Copper is involved in chlorophyll formation, plant metabolism and root development.
• Manganese aids in the production of chlorophyll and the photosynthetic process.
• Zinc is involved in growth hormone production, enzyme formation and starch production.
• Iron is required for chlorophyll synthesis, plant metabolism and the formation of certain plant proteins.¹⁰

The table below provides the ideal range for each micronutrient.^3&9

What does this mean to Turf Managers?

An improved understanding of soil test information provides individuals with an increased confidence level when interpreting their results. With accurate soil test interpretation and correct fertilizer recommendations, a turf manager can develop a fertility program that maximizes the overall health of their turfgrass, allowing it to be more competitive in its surrounding environment. A highly competitive turf is less prone to insect, weed and disease invasion which would certainly decrease operating costs. Finally and most importantly is the public's enjoyment that comes from a high quality recreational turf area, any decrease in turf's quality would greatly diminish an individual's pleasure.

Reference

• Street, J.R., Watson, M.E., & Pound, W.E. (No Date). Interpreting a Soil Test for Lawns. Retrieved September 8 2006, Ohio State University, Extension Service: http://www.ohioline.osu.edu/hyg-fact/4000/4028.html
• Hardy, D.H., Tucker, M.R., Messick, J.K., & Stokes, C. (2003). Understanding the Soil Test Report. Retrieved September 10 2006, N.C. Department of Agriculture & Consumer Services Agronomic Division: http://www.ncagr.com/agronomi/ustr.htm
• No author. (No date). Understanding a Soil Analysis. Retrieved September 8 2006, AgSource Harris Laboratories: http://ag.mdsharris.com/education_train/understand.asp
• No author. (2002). Calculation of C.E.C. Retrieved September 8 2006, A&L Canada Laboratories. http://ag.mdsharris.com/education_train/understand.asp
• No author. (2002). Soil Chemical Properties: Cation Exchange, pH, Buffering Capacity, Nutrient Availability. Retrieved September 8 2006, A&L Canada Laboratories. http://www.al-labs-can.com/pdfs/SoilChemProperties.pdf
• No author. (No date). Interpreting Lawn and Garden Soil Test. Retrieved September 8 2006, Spectrum Analytic Inc: http://www.spectrumanalytic.com/support/library/ff/Interpreting_Lawn_and_Garden_Soi_Test_Results.htm
• Brady, N.C., Weil, R.R. (1996). The Nature and Properties of Soils. (11 th Edition). Upper Saddle River, New Jersey, USA: Prentice Hall.
• Ministry of Agriculture, Food and Rural Affairs Publication 611 (1998). Soil Fertility Handbook. Toronto, Ontario, Canada: Queen's Printer for Ontario.
• No author. (No date). Soil Analysis Reference Guide. Retrieved July 7 2006, A&L Canada Laboratories. http://www.al-labs-can.com/guides_soilanalysis.html
• Correggia, A., (2005). Essential Plant Nutrients and Their Role in Turf Health. Retrieved September 8 2006, Nu-Gro Ltd: http://www.nu-groturf.com/tutorials/nutrients_e.php

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