Allan Fulton2 CommentsSeptember 21, 2015Soil

Understanding and Applying Information from a Soil Test, Part 3: Secondary Plant Nutrients: Calcium (Ca), Magnesium (Mg), and Sulfur (S)

Written by Allan Fulton, Farm Advisor, Tehama County and Roland D. Meyer, Extension Soils Specialist Emeritus This article (Part 3) discusses the use of soil tests to evaluate levels of the secondary nutrients calcium (Ca), Magnesium (Mg), and sulfur (S) in orchard soils.  It is a follow up to a series of articles on intrpretation of soil sampling results. These nutrients are considered secondary because while they are essential to crop development, seasonal crop uptake is usually lower than for the primary nutrients N, P, and K but considerably higher than the micronutrients zinc (Zn), iron (Fe), Manganese (Mn), copper (Cu), boron (B), and chloride (Cl). Calcium and Magnesium Plant uptake, cation adsorption and desorption in soil, leaching from rainfall and irrigation, and weathering of minerals all contribute to the concentration of water soluble Ca and Mg available to meet tree nutritional needs.  Water soluble cations are determined from the saturated paste extract soil test procedure while the exchangeable cations are determined with the ammonium acetate procedure.  Also important are the concentrations of exchangeable (non-water soluble) Ca and Mg which help to promote favorable soil structure.  Soil chemistry is in a constant state of change attempting to reach equilibrium between the soluble and non-soluble (exchangeable and mineral) phases.  The May 2009 newsletter discussed this dynamic process.  Calcium and magnesium share similar chemical properties in soils.  Both Ca and Mg are “double positively charged (divalent) cations in the soil-water phase and on soil cation exchange sites.  Calcium is adsorbed to soil exchange sites preferentially and more strongly than Mg.  When Ca and Mg are abundant in the soluble phase tree roots absorb these nutrients by mass flow.  If Ca and/or Mg are less abundant or limited by soil moisture, uptake occurs more slowly through diffusion. Table 1 provides ranges of exchangeable

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Allan Fulton0 commentsDecember 7, 2010Soil

Understanding and Applying Information from a Soil Test: Part 2 – NPK

Part 1 of this series emphasized that soil testing is complementary to plant tissue testing and not a substitute in orchard management.  Considerations to ensure soil testing provides representative and useful information and interpretation of two common soil test parameters: 1) Saturation Percentage (SP); and 2) pH were also discussed.   This article will focus on the nutrients nitrogen (N), phosphorus (P), and potassium (K). Nitrogen Nitrogen occurs in soils as organic and inorganic forms and soil testing may be performed to measure levels of either. Nitrate nitrogen (NO3-N) is most commonly measured in standard soil tests because it is the primary form of nitrogen available to trees and, therefore, an indicator of nitrogen soil fertility.  However, soil concentrations of NO3-N depend upon the biological activity and may fluctuate with changes in soil temperature, soil moisture, and other conditions.  Nitrate is also easily leached with rainfall or irrigation so current soil tests may not reflect future levels of nitrogen soil fertility.  Table 1 provides guidelines for evaluating NO3-N soil fertility levels.  Table 1.  Guidelines for interpreting nitrate nitrogen (NO3-N) levels in soil test results. Fertility Level ppm lbs/acre1 Low <10 <36 Medium 10-20 36-72 High 20-30 72-108 Excessive >30 >108 1 Some laboratories report NO3-N as lbs/ac rather than as a concentration (ppm).  A soil bulk density is assumed in this calculation so the NO3-N fertility levels should be considered an estimate rather than an absolute level.   Ammonium  nitrogen (NH4-N) is also a plant available form of nitrogen in orchard soils and it can be determined with soil testing upon request.  In general, NH4-N is not determined and reported with a standard soil test.  Ammonium nitrogen does not usually accumulate in soil because soil temperature and moisture conditions that are suitable for tree growth are also ideal for conversion of

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Allan Fulton3 CommentsNovember 23, 2010Soil

Understanding and Applying Information from a Soil Test: pH and Saturation Percentage

This article will focus on the role of soil testing in orchard management, describe steps to acquire informative soil testing information, and discuss two of the more familiar parameters in a soil test report: pH and saturation percentage (SP). Soil testing helps understand the orchard soil environment and how to prevent or correct nutrient deficiencies, toxicities, or conditions that affect the availability of water to the trees in a cost effective manner.  Soil testing may forewarn a problem before it actually affects growth and production. Therefore, it helps anticipate possible problems and offers the earliest opportunity to manage them.  Soil testing is not a substitute for plant tissue testing, rather it is complementary.  If soil and leaf tissue analyses both indicate a deficiency or toxicity, the diagnosis is obvious. If a soil analysis indicates a deficiency or toxicity but a leaf tissue test does not, it may only be a matter of time before the deficiency or toxicity develops in the trees.  Conversely, if leaf tissue analysis indicates a deficiency or toxicity but soil testing does not, it may point out that either the soil testing does not represent how the trees’ root system integrates the soil environment or that the soil environment has changed more rapidly than the nutritional status of the tree. There are two basic philosophies for sampling soils. When marginal soil is known or suspected to exist, consider routine soil testing (at least every two or three years) to understand trends and guide long-term management. When confident that the soil is fertile, non-saline, and suitable for orchard crops such as walnut, almond, and prune, sampling is only necessary to troubleshoot problems.  Some situations do arise where a switch in sampling approach may be necessary. For example, when a change from a higher quality to lower quality

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Allan Fulton2 CommentsNovember 9, 2010Soil

Soil Testing and Analysis: What to Expect in the Report

Agricultural laboratories usually analyze and report soil salinity and fertility levels in the same report.   Saturation percentage, electrical conductivity, sodium, calcium, magnesium, chloride, bicarbonate and carbonate, sulfate, and boron are all part of salinity.  Indices such as the Sodium Adsorption Ratio (SAR) and Exchangeable Sodium Percentage (ESP) are calculated from these basic measurements of soil salinity and included in the report.  Soil testing for salinity is designed to diagnose osmotic effects, specific ion toxicities, and infiltration problems.  When the salinity (electrical conductivity) of the soil-water surrounding the root exceeds the tolerance of a salt sensitive crop like walnut, the gradient between the solute concentration in the root cells and the soil-water around the root lessens, reducing water availability to trees.  Trees influenced by osmotic effects will not grow as vigorously.   Specific ion toxicity involves the accumulation of sodium, chloride, or boron in soils to high enough levels that the risk of these elements accumulating to toxic levels in leaf tissue of trees increases.  Symptoms of ion toxicity may include death of leaf tissue along the margins or in the interveinal areas of leaves.  Soils that develop slow water infiltration and permeability rates are sometimes related to low levels of electrical conductivity and calcium, and high levels of sodium or magnesium. Fertility focuses on essential plant nutrients, which is evaluated based upon soil pH and proper quantities of nitrogen, phosphorus, potassium, zinc, iron, manganese, copper, and molybdenum in the soil to promote walnut tree growth and fruit development.  While calcium, magnesium, boron, and chloride are important to diagnose salinity, they are also of interest from the standpoint of fertility in terms of deficiency, sufficiency, and balances. Salts and nutrients exist in soils as three forms (see schematic) and this is reflected in soil test reports.  Bulk minerals and organic reserves release

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