Graduate Thesis Or Dissertation
 

The relationship of exchange acidity and exchangeable aluminum in limed and unlimed profile samples of soil from western Oregon to the yield and chemical composition of alfalfa

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  • Acidity distribution within any given soil profile is dependent on mineralogical make-up of the parent material as well as environmental conditions prevalent during the weathering history of the soil. An experiment was designed to study the distribution pattern of exchange acidity, exchangeable aluminum, exchangeable manganese and pH of six acid soils of Western Oregon. 1N KCl extracts of four depths of each soil were obtained and the levels of the parameters mentioned above were measured. It was found that various soil series had different acidity distribution patterns within their profiles. Also, a highly significant linear correlation was observed between exchange acidity and exchangeable aluminum of each soil. Liming acid soils raises pH of the soil extract and improves its fertility status. The rise in pH is due to neutralization of acid groups or acid-producing species present in various forms and on various sites in the soil system. One group of these acid producing species is the exchangeable form of various hydrolyzable cations, especially Al⁺³, H₃O⁺, Mn⁺², also perhaps Fe⁺³, and others. These cations are extracted with solutions of 1N neutral salts such as KC1 and are neutralized by hydroxyls of the applied lime. A second experiment was designed to study the effect of liming, using an incubation procedure, on exchange acidity, exchangeable aluminum, exchangeable manganese and pH of these soils. It was observed that additions of increments of Ca(OH)₂ to soil resulted in progressive increase in pH of the soil extract and the relationship between pH and amounts of lime applied was curvilinear with the titration curves approximating linear patterns. As lime rates increase, there was a drop in both Al⁺³ and exchange acidity content of the soil extract as well as the amount of Mn⁺² extracted. For all these three parameters the initial drop was much sharper than the subsequent reductions occurring due to further additions of lime. Point of zero acidity or zero exchangeable aluminum did not necessarily coincide with pH of neutrality. Applications of lime to acid soils improves their crop raising ability, especially with regard to legumes. A third experiment was set up to study the response pattern of alfalfa (Medicago sativa L. var. Du Puits) to lime and phosphorous applications and the effect of such applications on aluminum, manganese, phosphorous, and calcium concentrations of plant tops. Significant yield responses were obtained in some cases but not in others. Phosphorous concentration of the tops was not significantly affected, but aluminum, manganese, and calcium concentrations were affected to varying degrees.
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