Adsorption, denitrification, and movement of applied ammonium and nitrate in Hawaiian soils
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1974
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Abstract
The factors influencing adsorption, denitrification. and movement of applied ammonium and nitrate in tropical Hawaiian soils were investigated. Suggestions are incorporated to maximize crop utilization of field-applied fertilizer nitrogen and to minimize loss of mineral nitrogen through leaching and denitrification. Some oxidic tropical soils, such as the Hydrandepts and Gibbsihumox, in Hawaii were found to adsorb nitrate significantly in pH ranges below 6. Non-specific anion adsorption is believed to be the major mechanism by which nitrate was adsorbed. The zero point of change (ZPC), which has a close relation with non-specific anion adsorption, is defined as the pH where the net sum of charges is zero. The high ZPC of the subsoil of the Hydrandepts was attributed to the extensive hydration of their iron and aluminum oxides. Hydrolysis and polymerization of the hydrated oxides were suggested as major mechanisms for the decrease of ZPC and pH on drying the Hydrandepts. Any change in the ZPC was shown to vary the nitrate adsorption. The significant decrease of nitrate adsorption due to dehydration of the Hydrandepts was explained by the change in the ZPC, pH, crystallinity, and surface area on drying. Since this dehydration process is irreversible, it was concluded that these soils should not be allowed to dry excessively by exposure to direct sun and wind so as to preserve their high exchange capacities, both for anions and cations. The surface of the Hydrandepts and Gibbsihumox became less positive or more negative on liming, and this was reflected in the (increased) ammonium and (decreased) nitrate adsorption by the limed soils. The finding that raising of soil pH beyond 5.5 with liming produced N03 repulsion in these soils should be taken into consideration in any liming program. Denitrification loss was found to be important only in soils with large amounts of water-soluble organic matter and nutrients. Available energy source appeared to play a dominant role in denitrification. N2 and N2O gases were the prime denitrification products in all the soils studied. Denitrification potential was very low in Oxisols with poor organic carbon (both water-soluble and total) content. In an infiltration study, it was found that the practically irreversible adsorption of ammonium was responsible for its retention in the Molokai soil. It was further shown that the higher the amount of water infiltrated, the deeper was the position of nitrate peak. An explanation is given on how to take advantage of the lag of nitrate peak with respect to the wetting front in the initially moist soil during transient water flow. For soils with the same initial moisture content, the depth of nitrate peak was in direct proportion to its wetting front. Thus by controlling the wetting front, one can control the depth of maximum solute concentration, irrespective of the rate of water application. In short, modified management practices based on the knowledge of nitrogen transformation and transport in soils as well as nitrogen uptake by crops will ensure efficient (nitrogen) fertilizer use in crop production with a minimum chance for the pollution of ground water by nitrate.
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Typescript.
Thesis (Ph. D.)--University of Hawaii at Manoa, 1974.
Bibliography: leaves 160-167.
xiv, 167 leaves ill
Thesis (Ph. D.)--University of Hawaii at Manoa, 1974.
Bibliography: leaves 160-167.
xiv, 167 leaves ill
Keywords
Soils -- Hawaii, Soils -- Nitrogen content, Soils -- Ammonium content
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Agronomy and Soil Science; no. 615
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