The rate of Zn desorption from soil surfaces into soil solution is a dynamic factor that regulates its continuous supply to growing plants. To ascertain the pattern of Zn desorption, the soil characteristics affecting it, and whether cropping alters its rate, the kinetics of Zn desorption from the <2‐mm fraction of 14 Vertisols by diethylenetriaminepentaacetic acid (DTPA) and ethylenediaminetetraacetic acid (EDTA) were investigated using soil samples taken before and after one wheat (Triticum aestivum L.) crop. Nine kinetic models were evaluated to describe the rate of desorption of soil Zn by DTPA, which was rapid initially but gradually declined with time. The parabolic double diffusion, the two‐constant rate, and the simple Elovich equation adequately described Zn desorption from Vertisols. Rate constants for the parabolic double diffusion equation (k'σ), the two‐constant (a and b) rate equation, and the initial Zn desorption rate constant (α) from the simple Elovich equation were closely associated with clay content, soil pH, and amorphous Fe and Al contents of the soil—the soil characteristics that affect solubility, sorption and desorption, and diffusion of Zn in Vertisols. Rate constants from the latter two equations for Zn desorption by DTPA from 14 Vertisols were highly correlated with those from Zn desorption by EDTA and also with those for Zn desorption by DTPA on soil samples taken after one wheat crop. Thus, the rate constants obtained on initial samples can be used to predict Zn availability for at least two cropping seasons.

Soil solutions were obtained by a centrifugation method from 14 unfertilised and fertilised Vertisols. The soil solutions were analysed for all major cations and anions and organic carbon (C). Chemical speciation of zinc (Zn) in the soil solutions calculated with the aid of the computer program GEOCHEM showed that Zn in tile soil solution exists mainly as free Zn2+ ions in these soils. Complexation of total soluble Zn by organic and inorganic ligands constituted 40% and 50%, respectively, of total soluble Zn in fertilised and unfertilised soil solutions. The organo-Zn complexes constituted <10% of the total soluble Zn. The inorganic Zn complexes, ZnHCO3+ and ZnCO3, constituted 60–75% of the total inorganic Zn complexes. The Zn complexes with SO24- and OH- were less than or equal to 5% each of the total inorganic species in unfertilised soils; ZnSOo4 complexes were more common in fertilised soils. The activities of Zn were extremely low (0.01–0.1 µM) in unfertilised soils and were inversely related to soil solution pH. The experimentally determined solubility lines for Zn2+ in the soil solution were undersaturated with respect to the solubility of any known mineral form of Zn. Zn2+ activity was mainly determined by adsorption-desorption reactions. The weak acid ion exchangers, Chelex-100 and Bio Rex-70, retained smaller amounts of Zn front the soil solutions than the strong acid exchangers, AG 50W X2, AG 50W X4, and AG 50w X8. Soil solution pH strongly affected Zn concentrations in soil solutions. The amount of total soluble Zn present as Zn2+ ions as calculated by GEOCHEM was highly correlated with tile amount of soluble Zn retained by the cation exchange resins. In the case of Chelex-100, these amounts were equal, confirming the usefulness of Chelex-100 to estimate Zn2+ ions.

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A glasshouse experiment was conducted to study the effect of Ni on the growth and nutrients concentration in wheat (Triticum aestivum Cv. WH 291) in the presence and absence of applied N as urea. Responses to N application were observed up to 120/~g N g-~ soil. No response to Ni was observed in the dry matter yield of wheat tops (leaves + stem) in the absence of applied N while in the presence of applied N, significant yield increases were obtained at 12.5/~g Ni g-~ soil. Nickel was not toxic to wheat up to 50/.~g Ni g-i soil in the presence of 120/~g N g-I soil. Nitrogen and Ni concentration in wheat tops and roots increased with increasing levels of applied N and Ni, respectively. Applied Ni had an antagonistic effect on N concentration. Similarly, N reduced the Ni concentration in the wheat tissues. Positive growth responses to Ni were associated with 22 and 15/~g Ni g-~ in wheat tops, in the presence of applied N at 60 and 120p~g N g-~ soil, while Ni toxicity was associated with 63, 92.5 and 112.5/~g Ni g-t in wheat tops, in the absence and presence of applied N at 60 and 120~g N g-~ soil, respectively.

The soil Zn buffer capacity is an important factor that regulates the concentration of plant-available Zn in soil solution. It is measured variously as Zn buffer power, Zn sorption or Zn desorption capacity. This study was conducted to determine Zn buffer power, and Zn sorption capacity and Zn desorption capacity in Vertisols as influenced by soil properties. The Zn buffer power, defined as the slope of the line relating soil solution Zn concentration to DTPA-extractable Zn, varied from 217 to 790. Soil pH was found to be the major soil parameter responsible for the variation in Zn buffer power. The sorption of Zn by these Vertisols was satisfactorily described by the Freundlich equation. The calculated values of the Freundlich parameters were closely related to the soil pH and amorphous Al and Fe content. Desorption of Zn by a series of successive extractions with DTPA was described by the Mitscherlich equation. The calculated values of desorption capacity were negatively correlated with soil pH and positively correlated with the contents of Al and Fe oxides. Work published elsewhere showed that the parameters of both the Zn buffer power and Zn desorption capacity accounted for as much as 62% of the variation in relative yield of wheat from Zn application to Vertisols.