Laboratory Methods for Soil Health Analysis, Volume 2. Группа авторов

Laboratory Methods for Soil Health Analysis, Volume 2 - Группа авторов


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K, Ca, and Mg can form complexes with SOM (Broadbent and Ott, 1957; Schnitzer and Skinner, 1963) and the capacity of soils to store mineral nutrients (i.e., CEC), increases with SOM.

      Physical Effects

      The physical benefits of SOC relate to the formation and stabilization of soil aggregates. Several studies have reported high correlation between soil aggregation and SOC (Wilson et al., 2009; McVay et al., 2006; Six et al., 2002). Degradation of organic materials by soil organisms leads to the formation of humified materials associated primarily with mucilage that surrounds and binds to clay particles, thus developing and binding the particles into microaggregates (Balesdent et al., 2000). The decomposition of protected SOC can become slow due to the clay barrier, thus promoting soil carbon sequestration. Soil health benefits of greater soil aggregation include less crusting, compaction, and bulk density (Diaz‐Zorita and Grosso, 2000); enhanced soil structure for greater water infiltration and water holding capacity (Hudson, 1994; Emerson, 1995; Gupta and Larson, 1979, Yang et al., 2014); decreased soil erosion (Schertz et al., 1994; Benito and Diaz‐Fierros, 1992); and improved aeration for root growth and microbial activity. As tillage intensity increases, soil microbial activity increases right after tillage and microaggregates are dispersed, thus releasing SOM from protection (Puget et al., 1995, 2000).

      Crop Productivity

      In addition to supporting biological control of crop pests and disease, SOM contributes to agricultural crop yields in various ways by overcoming negative soil conditions. Adequate amounts of SOM can enhance: (i) release of nutrients from decaying organic materials and thus reduce commercial fertilizer requirements; (ii) soil porosity which increases plant available water retention and aeration for root development; (iii) soil structure which reduces soil erosion potential and increases aggregation; and (iv) storage of other nutrients, including an increase in cation exchange sites for plant nutrient retention.

      Measurement of SOC

Method Advantages Disadvantages
Walkley‐Black Relatively simple, accurate, and quick High environmental pollution potential, uses hazardous chemicals
Loss‐On‐Ignition Simple, inexpensive, convenient Heating temperature and time significantly affect accuracy. It has to be calibrated.
Dry combustion by elemental analyzer Most accurate, quick Elemental analyzers are expensive to purchase and maintain.

      For this chapter, we chose to present both the LOI and dry combustion methods since they are environmentally safe, simple, and the most widely used methods in commercial laboratories.

      Methods for SOC Analysis

      Soil Sampling and Preparation

      The same general principles that apply to soil sampling for nutrient evaluation apply to soil sampling for SOM determination. Soil samples should be collected to consistent soil depth(s), a consistent number of soil cores collected per composite samples should be maintained, thatch or mulch from the soil surface must be removed prior to sampling, composite samples should be inspected after collection and any obvious pieces of crop residue should be removed. Georeferenced points should also be collected within the field when possible. For all methods described below, soil samples should be dried at 35 to 40 °C in an oven and passed through a 2000‐μm sieve.

      Dry Combustion

       Apparatus

      1 Forceps

      2 2000 μm sieve

      3 250 μm sieve

      4 Aluminum tin capsule

      5 Analytical balance with 0.001 precision

      6 C/N elemental analyzer

       Reagents

      1 Concentrated H2SO4 or HCl

      2 4N phosphoric acid solution

       Procedures

      1 Air‐dry or oven‐dry soil at 35 to 40 °C.

      2 Carefully remove all plant and animal materials from the soil using forceps.

      3 Pass the soils through a 2000‐μm sieve and grind the soil to a powder to pass through a 250‐μm sieve.

      4 Proceed to step 11 for soil with pH < 7.00.

       Pre‐treatment for soils with inorganic carbon content

      1 For soils that have a pH > 7.50

      2 Test


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