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The experimental conditions and precision of alternative methods of measuring isotopically exchangeable cobalt (Coi.e) were investigated. The values of Coi.e varied from 0.16 to 5.4 p.p.m. for 25 soils and were highly correlated with the sum of cobalt extracted by ammonium acetate and subsequently by quinol, and the relationships between these forms are discussed. The amounts of cobalt sorbed in the presence of calcium chloride by different soils varied markedly between and within soil groups. The bonding energy of specific sorption of cobalt did not vary significantly between soils after the first sites were filled. Cobalt sorption capacity for all soils studied was highly correlated with cobalt content and surface area and to a lesser extent with manganese and clay content and pH, but not with organic matter. When soils of high montmorillonite content were excluded, highly significant correlations were obtained only with manganese and cobalt contents and surface area. The initial slope of isotherms of isotopically exchangeable cobalt 60Co(soi1)/60Co (solution) and Coi.e may provide measures of the intensity and capacity factors, respectively, of cobalt availability to plants.

The applicability of various techniques to measure isotopically exchangeable phosphorus for widely different soils was investigated. The E‐values obtained in the laboratory by carrier‐free (E‐value) and carrier‐methods (Ec value) were comparable to L‐values obtained in the greenhouse in low‐phosphorus fixing soils but were very much higher in high phosphorus fixing soils. In the latter group E‐values obtained by inverse‐dilution technique (EI.D. value) were more comparable to L‐values.The parameters for available phosphorus in terms of quantity, intensity and capacity factors were discussed in relation to soil properties. The E, Ec, and Q0 values were influenced primarily by total phosphorus content of soils whereas L value was significantly correlated with exchangeable Ca and Mg, soil pH, organic C and oxalate, and dithionite extractable Fe (Feo and Fed) of soils. EI.D. value was correlated with oxalate extractable Al and organic C of soils. Thus both L and EI.D. values were affected by soil characteristics that influence the availability of phosphate in soil. The intensity factor was mainly influenced by soil reaction. The parameters of capacity factor, both phosphate buffering capacity (PBC) and maximum buffering capacity (Mb) were significantly correlated with dithionate extractable Al (Ald) of soils. It is suggested that since parameters of quantity, intensity, and capacity factors were generally influenced by different soil characteristics, plant available phosphorus can be described better by consideration of more than one factor.

The importance of quantity, intensity, capacity, and rate factors of soil P availability to account quantitatively for the variation in P uptake and wheat grain yield in pot and field experiments was studied. Among the parameters of the quanitity factor (L, E, Ec, E1.0, Q0 value, NH4F‐P, NaOH‐P, H2SO4‐P, organic P, Olsen P, Colwell P, carbonate P, and anion‐exchange resin, AER‐P), L value was found to be highly correlated with P uptake at 35 and 150 days after planting (maturity) when all the soils were considered. However, the carbonate P (soil P extracted for 16 hours with 0.25M NaOH‐0.1M Na2CO3 at a soil/solution ratio of 1:100) was found to be the best parameter of the quantity factor when the soils containing high amounts of hematite/goethite (> 20%) were excluded. The parameters of the intensity factor (0.01M CaCl2 soluble P, aH2PO4i and phosphate potential) were significantly correlated with P uptake at early growth (35 days) whereas the parameter of the capacity factor (maximum buffering capacity, Mb) was bezter correlated with P uptake at the later stage (150 days). The rate factor, as measured by AER was better correlated with P uptake at 35 days than with P uptake at 150 days after planting. The quantity factor as measured by the carbonate P accounted for 75 and 93% of the variation in P uptake and grain yield, respectively. The capacity factor, Mb when combined with the quantity factor accounted for more of the variation in P uptake (150 days) from 75 to 86%. The intensity and the rate factors had a smaller effect.