| Summary: | 博士 === 國立臺灣大學 === 農業化學研究所 === 97 === Boron is an essential micronutrient for plant growth. The uptake of B by plants depends mainly on B concentration in soil solutions. It is important to maintain B levels in soil solutions that are sufficient for plant uptake but nontoxic. Adsorption-desorption process governs B distribution between adsorbed and liquid phase. Boron availability to plants depends on soil properties as well as management practice like liming, fertilization and use of organic matters. The increase in soil pH associated with liming could also result in an increase in B retention by the soils and consequently a lower B availability. Therefore, the objectives of the study are on: (1) the factors affecting B adsorption-desorption in five red soils; (2) the effect of pH on the reversibility of adsorbed B; (3) the effect of compost application on B adsorption and the reversibility of adsorbed B.
In this study, boron adsorption and desorption as a function of B concentration and pH were measured in five fine-textured red soils. Immediately after adsorption of B, four consecutive desorption steps were carried out by successive dilution. The sorption results were described well by the Freundlich equation (P<0.01). Boron adsorption capacity increased as soil contains higher amounts of clay, free Fe2O3, free Al2O3, and amorphous Al2O3。Boron adsorption increased markedly as pH increased. This increase was more pronounced when Ca(OH)2, as opposed to NaOH, was used for pH adjustment.
The presentation of desorption data was based on the traditional isotherm approach and on the time-dependent isotherms. Apparent hysteresis was observed that both derivational families of desorption isotherms deviated from the adsorption isotherm for the five soils at their original pH. Moreover, hysteresis coefficients, ndes/nads and λtrad for the traditional approach, had a positive correlation with organic carbon content, but they had a negative correlation with free Fe2O3 and free Al2O3 contents. As pH increased, the hysteresis diminished, and adsorbed B became more reversible. This finding may be due to the fact that B adsorption and raising pH increase the negative charge on the surface, the outer-sphere boric acid complexes formed based on the Lewis acidity of the B metal center. Unlike the strong binding by ligand exchange reaction, the physically-bound boric acid may more readily return to the soil solution when solution concentrations decrease.
Adsorption-desorption of B was investigated in Lunkang, Laopi, Potu, and Tansui soils receiving varying doses of sugarcane compost (SC) or chicken manure compost (CM) (0, 2.5, 5, 10%). Application of sugarcane compost increased B adsorption in Lungkang, Potu, and Tansui soils except for the decreasing B adsorption in Laopi soil with 5% SC amendment. Application of 2.5% chicken manure compost considerably increased pH and B adsorption in the four soils. Because B adsorption on humus or clay minerals is pH-dependent, higher pH might be an important factor improving B adsorption. The effects of higher rate of chicken manure compost on B adsorption are different for the soils. Boron adsorption by Lungkang soil slightly increased as the rate of chicken manure compost increased. Boron adsorption by Potu and Tansui soils decreased with 10% CM amendment. This decrease might be due to the adsorption sites were occluded by organic matter. The degree of hysteresis slightly increased with 2.5% SC amendment. Hysteresis coefficients, ndes/nads and λtrad, decreased with a higher application rate of sugarcane compost, indicated adsorbed B became more reversible. The degree of hysteresis apparently decreased with the amendment with chicken manure compost.
The amount of B adsorbed was considerably greater after the organic matter had been removed from Lungkang soil by NaOCl. It suggested that a portion of adsorption sites are generally coated or occluded by organic matter and become available for B adsorption after removal of the organic matter. Boron desorption showed a hysteretic trend in the treated soil. But little effect of the presence of organic matter in hysteretic desorption was observed compared to the soil without organic matter removal at similar pH.
In conclusion, pH is the most important factor in determining B availability in soil solution. The changes of pH must be taken into consideration before liming or application of organic matter, and consequently the B status in soil solution must be satisfied by B fertilization in adequate concentration for plant growth.
|
|---|
