Soil boron : sorption behaviour in soils and uptake by plants

Abstract

Soil composition and properties, nutrient interactions, and general environmental conditions are among the factors which determine the availability of boron for uptake by plants. This thesis is directed towards studying the effects of a range of soil components on the adsorption and desorption behavior of soil boron and the effects of selected nutrient interactions on the uptake of boron by plants. The effect of pH and boron concentration on the adsorption of boron by the clay minerals (illite, montmorillonite and kaolinite); other mineral components (protoimogolite, ferrihydrite and aluminium oxide; collectively designated as soil'oxides') and an organic matter component (humic acid) was investigated for pH values of 3.0 to 7.0 and solution boron concentrations of 0 to 10 µg cm⁻³. The desorption behaviour at pH 5.0 by the same colloidal components was also studied. Boron adsorption was dependent on pH and equilibrium boron concentration. The extent of boron adsorption on the clay minerals was in the order illite > montmorillonite > kaolinite. Adsorption tended to increase at low and high pH values. The extent of adsorption for the soil 'oxides' was in the order protoimogolite > ferrihydrite > aluminium oxide. Boron adsorption by humic acid was strongly influenced by pH values of 6.0 to 7.0 due to the stability of borate esters. Boron desorption was either completely reversible or partially reversible and the extent and ease of desorption was in the order of clay minerals > humic acid> soil 'oxides'. Boron adsorption and desorption characteristics of selected New Zealand soils (Barrhill, Templeton, Temuka, Selwyn, Craigieburn, Taupo and peat) were found to be related to the amount and nature of the clay minerals, 'oxides' and organic matter present. Sorption behaviour of the Canterbury Plains soils was predominantly determined by the type of clay minerals present. The Craigieburn and Taupo soils showed adsorption characteristics similar to soil 'oxides', and the peat to that of humic acid. Desorption behaviour of the soils at pH 5.0 was completely reversible or partially reversible and influenced by the content and nature of the clay minerals, oxides and organic matter. The ease and extent of boron desorption by the soils were in the order of Selwyn > Craigieburn > Temuka > Taupo > peat. A pot trial to assess the boron availability of six Canterbury Plains soils following four extraction methods using radish (Raphanus sativa L.) was conducted. The extraction media evaluated were hot deionised water, hot 0.02M CaCI₂, CaCl₂-mannitol and a boron specific resin. The hot extraction procedures gave higher extractable boron values than the lower temperature methods. All the extraction procedures showed significant correlations with measured soil and plant parameters. The modified hot CaCl₂ method with a small addition of charcoal was the preferred extractant to determine boron availability. Nutrient interaction studies involving different nitrogen and phosphorus rates with varying rates of lime and boron were conducted using lucerne (Medicago sativa L.) and turnip (Brassica rapa L.) grown on a Selwyn sandy loam soil. Highly significant interactions between nitrogen, lime and boron and phosphorus, lime and boron on dry matter yield, boron concentration and boron, uptake were found. The addition of boron generally increased dry matter yield, boron concentration and uptake. With added boron, increasing rates of nitrogen, phosphorus and lime tended to behave in both an antagonistic and synergistic manner. Uptake of added boron in whole lucerne plants (tops and roots) varied from 25% to 59% and 35% to 61% for the nitrogen- and phosphorus-treated plants respectively. Similarly, the corresponding boron uptake values for whole turnip plants varied from 31.8% to 97.8% and 32.6% to 52.6%. Total boron uptake in lucerne was higher with the phosphorus treatments, and in turnip it was higher with the nitrogen treatments. The studies showed that soil pH, clay content, organic matter and oxides are the most likely factors controlling the solution concentration of boron and hence its availability for plant uptake and that the behavior of boron in soils can be interpreted in terms of their known colloidal components. The nutrient interaction studies showed interactions between boron and other elements but more work needs to be done in this area before firm conclusions can be drawn. It was shown that soils with a low adsorption and high desorption capacity, such as the Selwyn sandy loam, have a readily available pool of added boron for plant uptake, but would be likely to show rapid boron depletion under intensive cropping.