Available Phosphorus Extraction from Soil

As for phosphate, the adsorption reaction of organic phosphorus is not readily reversible, although some phosphorus can pass to the solution depending on the time of desorption, the solution-to-soil ratio, and temperature (Barrow, 1983). As solution characteristics play an important role in desorption, different approaches using free water, dilute electrolyte solutions, or chemical extractants with variable pH and ionic composition have been used to quantify the amount of phosphorus desorbable from minerals and then available to plants (Frossard et al, 1995). Desorption of inorganic and organic phosphorus was found to increase with pH (Cabrera et al, 1981 Celi et al, 2003 Martin et al, 2003), with the percentage of phosphorus saturation (Parfitt, 1979 He et a7., 1991, 1994 Martin... 

The prevalence of highly weathered soils in the tropics makes organic phosphorus potentially more important for plant availability than in temperate soils. The relationships between chemically extracted phosphorus fractions in 168 soils suggested that labile phosphate is derived mainly from stable soil phosphate fractions in slightly... 

During the last decade, information on the organic phosphorus composition of soil solutions and water extracts has been obtained by phosphatase hydrolysis. This technique not only gives structural information on filterable organic phosphorus, but also indicates its potential biological availability. In solution from Scottish upland soils, up to 64% of the filterable organic phosphorus was hydrolysed by non-specific phosphatases (Shand and Smith, 1997), while hydrolysable unreactive phosphorus in water extracts of Australian pasture soils was dominated by phosphate diesters and myo-inositol hexakisphosphate (Turner et al., 2002a). Only small concentrations of labile monoesters were detected in the latter study, possibly due to the rapid hydrolysis of labile compounds by soil phosphatase enzymes. 

Readily available phosphorus This form is present in soil pore water and the exchangeable pool. Phosphorus in this pool is continuously replenished from other stable pools at various rates, depending on the solubility of phosphate minerals and the physicochemical properties of soils. Inorganic phosphorus is extracted with neutral salts such as NaCl, KCl, NH4CI, and NaHCOj. 

Another example is a fully computerized MSFIA system for the spectrophotometric determination of available phosphorus in soil extracts. The molybdenum blue method is chosen for the colorimetric determination, using ascorbic acid as reducing agent, antimony to accelerate the reduction to the blue complex and applying the Egner-Riehm method to extract phosphorus from soil samples. It presents a hnear calibration curve between 0.75 and 15 mg/1. A determination frequency of 15/h may be achieved, with good repeatability for 12 consecutive injections of soil extracts (RSD <1.7%). Results obtained from 12 soil samples were statistically comparable to those attained by the usual batch method [102]. 

The second reason for acid-digestion is the determination of the total soil elemental content of, e.g. potassium, phosphorus or trace elements. This is seldom done for potassium in normal soil samples, mainly because the total K in soils is of no value as an index to the availability of K to plants, nor is it always of value in tracing the movement or accumulation of applied fertilizer K (Pratt, 1965). The unreactive soil phosphorus is obtained by subtracting the naturally leached reactive phosphorus from the total phosphorus, and a method for determining the latter by extraction with sulphuric acid and potassium persulphate is cited by Turner and FHaygarth (2000). They analysed... 

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