Plant-available fraction

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

EDTA (ethylenediaminetetraacetic acid) extracts of soils tend in general to correlate well with plant contents, in particular with the plant-available fraction for Cd, Cu, Ni, Pb and Zn [208-210], EDTA (0.05 mol at pH 7 was used in the certification of the two soils mentioned above [197]. This test is assumed to extract both carbonate-bound and organically-bound fractions of metals and was hence considered to be suitable for calcareous soil analysis. 

Heavy metal uptake by plants in relation to soil metal concentration has been studied in two ways from the point of view of the soil or from the point of view of the plant. Soil scientists have tried to find an extraction medium that mimics the plant-available fraction of metals in the soil. Various extractants have been tested and compared with plant uptake of a specific metals, mostly without success (e.g. Ross, 1994). It may never be possible to accurately characterize the plant-available fraction of a metal, since many plants appear to regulate metal... 

Nevertheless, the interpretation of differences between the two batches could be very difficult. How should a change in the pH value (e.g., from 7.9 to 8.2), or in the plant available fractions of nutrients in relation to their total content be interpreted A more readily applied result could be obtained from bioassays, since any decrease of the germination rate of the seed or any reduction of plant growth (production of green plant biomass) could easily be declared as a loss of compost quality, independent of the chemical composition. 

Jin Q., Wang Z., Shan X., Tu Q., Wen B., Chen B. Evaluation of plant availability of soil trace metals by chemical fractionation and multiple regression analysis. Environ Pollut 1996 91 309-315. 

Warden B.T., Reisenauer H.M. Fractionation of soil manganese forms important to plant availability. Soil Sci Soc Am J 1991 55 345-349. 

Boron exists in several forms in the soil (USEPA 1975) in soil solntion, it exists largely as the undissociated weak monobasic acid that accepts hydroxyl gronps (Gnpta and Macleod 1982). Most plant-available boron in soils is associated with soil organic matter (Gnpta and Macleod 1982), with the hot-water soluble boron fraction (Hingston 1986), and with soil solntion pH ranges of... 

Freney, J. R., Melville, G. E., and Williams, C. H. (1975). Soil organic matter fractions as sources of plant-available sulfur. Soil Biol. Biochem. 22,1163-1165. 

Canet, R., Pomares, F., Tarazona, F. and Estela, M. (1998) Sequential fractionation and plant availability of heavy metals as affected by sewage sludge applications to soil. Comm. Soil Sci. Plant Anal, 29, 697-716. 

The plant-bioavailable fraction of PTMs can be defined as the fraction of a metal total content in the soil that can be absorbed by plants via roots uptake (Kabata-Pendias and Pendias, 2001). Usually, this fraction is only a small proportion of the total element content of soils and shows much higher spatio-temporal variability than the total concentration. A plant uptakes mobile ion from the soil solution and the soil element fraction which is in solution is that which is considered immediately available. Nevertheless, the soil solid phases, inorganic as well as organic, take part in the supply and buffering of elements and allow their retention under wet conditions, which would otherwise leach all soluble elements from the soil. Therefore, the solid-bound elements take part to the available pool and this is why element concentrations in the soil solution are one to three orders of magnitude lower than those in plants (Bargagli, 1998). 

When PTMs concentration is well in excess of normal soil content, extraction method validation, in terms of direct correlation between soil extractable contents and plant contents, is less easy to achieve. In these cases, it may be adequate to develop an operational estimate of the mobile and potentially mobile metal species rather than plant-available species. It is necessary to analyse metal partitioning between such fractions as exchangeable sites, organic matter and minerals of varying solubility. 

Functionally defined speciation. Functionally defined species are exemplified by the plant-available species or chemical pools in which the function is plant availability. Available forms of trace metal cations are not necessarily associated with one particular chemical species or a specific soil component. Hence, to predict the availability of trace metals, we either have to establish the species involved and develop methods that specifically determine those forms only, or we have to establish an empirical relationship between an accepted diagnostic measure of the metal and plant growth. Both speciation in solution and fractionation of the solid phase to identify the chemical pools can affect plant uptake (phytoavailability) of trace metals and water pollution. 

A large fraction of Se in soils can be plant available and easily extractable. This is the case for nonacid soils, particularly calcareous ones, that often contain Se in the relatively soluble selenate form. 

A thorough analysis of Warynski soil samples was performed to assess the potential plant exposure to three major metal contaminants, Pb, Cd and Zn. Tables 3 and 4 describe the major metal contaminant component of the Warynski soil, displaying available fractions and soil composition differentiated by depth. 

A full description of soil Mo would include the extents of occurrence of the materials in these five categories, the transformations between them, and the plant availability of the Mo in each category. It appears that high amounts of available Mo probably result from the high Mo contents in fractions 2 and 3 thus, wet soils high in organic matter commonly yield plants with high contents of Mo (Davies, 1956 Kubota, Lemon, and Allaway, 1963). Low Mo contents may reflect a low content in the soil overall, but can also occur because of the presence or formation of the fractions 4 and 5. The relative roles of these fractions are the main concerns in the chemistry of Mo-deficient soils and in the correction of such deficiency. 

Sequential extraction or chemical fractionation techniques have been widely used in the characterization of various phosphorus fractions in soils and sediments, with an emphasis on the more bioavailable or plant-available inorganic forms (Condron et al., 2005). Early extraction procedures (Chang and Jackson, 1957 Williams et al., 1976b) focused on inorganic phosphorus associated with iron, aluminium and calcium, using various acid, base or salt extraction steps. Organic phosphorus was considered to be the residual or refractory phosphorus-containing fraction that remained after all other extractions had been performed. 

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... 

Li, R, Shan, X.-Q., Zhang, T., Zhang, S., 1998. Evaluation of plant availability of rare earth elements in soils by chemical fractionation and multiple regression analysis. Env. Poll. 102, 269-277. 

The overall effect of plant-microbe interaction shows an increase in MBM in the rhizosphere, owing to the high supply of organic carbon by roots (Lynch and Whipps, 1990). As shown in Fig. 5, enhanced microbial activity was manifested by a steady increase in MBM. Horak (1982) demonstrated that mobilization of copper in the rhizosphere of peas should be a direct outcome of low-molecular-weight root exudates and of an indirect effect via microbial activity in the rhizosphere. The results of the present study show that changes in the major soil properties, including redox potential, DOC and microbial activities, are all in favor of a transformation of metals from less available to more available fractions, leading to variations in various metal fractions in the maize rhizosphere. 

In soils and sediments, chemical fractionation or sequential extraction methods offer another approach to speciation of the forms of phosphorus. Traditionally, there has been more emphasis on bioavailable or plant available inorganic forms, e.g., Olsen s extraction used for soils. 

Phenolic acids in soils occur either in a free state in the soil solution, reversibly sorbed to soil particles, fixed (irreversibly sorbed) very tightly to soil particles (e.g., recalcitrant organic matter, and clays), and/or on and in living and dead plant tissues/residues ( free , reversibly sorbed, and fixed). Of general interest to plant-plant allelopathic interactions are the free and reversibly sorbed states frequently referred to as the available fraction. Of particular interest is the active fraction of available phenolic acids, the fraction of available phenolic acids that actually interact with seeds, roots and microbes. Unfortunately we presently do not have a means of quantifying the active fraction, thus the focus on the available fraction. 

TSUJI T. and GOH K.M. 1979. Evaluation of soil sulphur fractions as sources of plant available sulphur using radioactive sulphur. New Zealand Journal Agricultural Research, 22, 595-602. 

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