Abstract
Water soluble phosphorus ranged from 0.2 mg to 117.8mg/kg in 104 surface soil samples studied. On the average, water extracted less P from the heavy clay soils (4.8 ± 2.2mg/kg) than from the coarser clays (12.8 ± 4.6 mg/kg) and non-clay soils (13.3 + 7.2 mg/kg). Water extraction seemed to illustrate ’’the effective” P status, it is that determined by the quantity and quality of sorption components in soil, soil pH and the content of organic carbon. These factors did not affect the amounts of P dissolved in water directly but inderectly by controlling the nature of P bonding which, in turn, seems to be of decisive importance in the extractability of P into water. The P supplying power of a given fraction is obviously controlled by the quantity of corresponding sorption agent. Water extractable P correlated most closely with the molar ratio of NH4F soluble P to oxalate extractable Al (r=0.93***, n= 103). However, according to the theory presented, with progressing desorption, P starts to mobilize also from the NaOH soluble fraction, its significance being the more apparent the greater the corresponding molar ratio NaOH-P/Fe is. In addition, the role and significance of other inorganic P fractions were discussed.
Highlights
The availability of soil phosphorus to plants essentially depends on the solubility of phosphorus compounds or surface complexes
Water soluble, inorganic phosphorus is biologically immediately available and the method can be used for estimating the ability of eroded soil material to load surface waters with phosphorus or quantities of phosphorus possible to be dissolved in runoff waters
The quantities of water soluble P varied very largely; from about 0.4 kg to 235 kg per one hectare, surface layer 0-20 cm corresponding to bulk density kg/dm
Summary
The availability of soil phosphorus to plants essentially depends on the solubility of phosphorus compounds or surface complexes. A great deal of effort has been expended on trying to find suitable chemical extraction methods for the determination of phosphorus resources in soils available to plants. According to SCHACHTSCHABEL and BEYME (1980), water soluble phosphorus reflects the quantity of total phosphorus relatively dissolved and, the phosphorus supplying power of the soil. In the pot experiment made by AURA (1978) with some Finnish soils, the phosphorus extracted by water quite well correlated with the phosphorus uptake by the oats. Water soluble, inorganic phosphorus is biologically immediately available and the method can be used for estimating the ability of eroded soil material to load surface waters with phosphorus or quantities of phosphorus possible to be dissolved in runoff waters
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