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Standard operating procedure for soil pH determination









FAO. 2021. Standard operating procedure for soil pH determination. Rome.


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    Brochure, flyer, fact-sheet
    Standard operating procedure for saturated soil paste extract 2021
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    Salt-affected soils severely limit crop growth, reduce yield, and cause plant stress and soil structure degradation. Therefore, soil salinity/alkalinity interpretation is essential to develop and apply sustainable agricultural practice and suitable irrigation techniques in semiarid and arid environments. Salinity is a soil condition characterized by a high concentration of soluble salts, generally defined as one in which the electrical conductivity (EC) of saturate soil paste extract (ECe) in the root zone exceeds 4 dS m-1 at 25 oC (Richards, 1954). Sodic soils are those with a percentage of exchangeable sodium (ESP) greater than 15. Because of the considerable experimental errors involved in ESP determination, emphasis has been placed on using the sodium adsorption ratio (SAR) of the equilibrium soil solution instead of the ESP for classification purposes. The electrical conductivity of saturation soil extracts may vary, but usually remains below 4 dS m-1 at 25 °C. The pH of saturated soil paste in sodic soils is 8.2 or higher, and in extreme cases may exceed 10.5. However, many crops exhibit a yield reduction at a lower ECe because soil salinity/alkalinity affects a number of morphological, physiological and biochemical processes and water and nutrient or toxic elements uptake (FAO, 2018; Filipovic et al, 2018). The main aim of this SOP is to prepare a saturated soil paste to measure electrical conductivity (ECe) and soluble salts of a soil from a saturated paste extract.
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    Booklet
    Standard operating procedure for soil available phosphorus - Bray I and Bray II method 2021
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    P-Bray 1 and P-Bray 2 methods are normally limited to acid soils with water pH values less than 6.8. The P-Bray 1 Method removes a fraction of the “adsorbed” phosphorus (Al-P, Fe-P, Mn-P and Ca-P but less efficient) while the P-Bray 2 Method is best suited to acid soils where rock phosphate has been the primary P fertilizer source and/or the major portion of P exists in the soil in various forms of calcium phosphate. Bray extractants should not be used on alkaline soils because the acid tends to be neutralized and/or excessive calcium phosphates may be extracted, giving a false high test for available P.
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    Standard operating procedure for soil available phosphorus - Olsen method 2021
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    This procedure is suitable for calcareous, alkaline, neutral, and slightly acidic soils containing CaPO4, since the calcium concentration in the solution is suppressed by the precipitation of CaCO3, increasing PO4 concentration in solution. Neutral and slightly acid soils (pH 6.0 to 7.0) may contain both Ca- and Al-phosphates. The NaHCO3 extractant can remove Ca-phosphates and phosphate adsorbed on the surface of calcium and magnesium carbonates along with Al-phosphates and is considered the most suitable extractant for these soils.

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