Finite difference method was applied to analyze one-dimensional unsaturated-saturated water movement in soil. Because it is difficult to obtain the exact solution analytically on usual physical phenomena in soil-water system, it follows that numerical analysis can be checked on only two ideal cases, namely, vertical infiltration into uniform soil column from its surface, and water table recharge above impermeable boundary. Then both stability and accuracy of numerical scheme were evaluated by comparing numerical solutions with exact ones, for various sets of grid sizes (A t , Az). Two iterative methods, the linear method and the Newton method were adopted to solve the system of non-linear algebraic equations derived from partial differential equation. In case of vertical infiltration,both methods were found to give sufficient stability, when satisfying von Neumann* s criteria. Their numerical errors could be minimized below 5%, by letting both grids sizes approach zero. However, too small sizes of finite differences caused the serious accumulation of round-off errors, when used to simulate water table recharge. It proved that the Newton iterative method was more accurate than the linear one, and was applicable to a wide variety of problems in soil-water system.
An introductory investigation on soil conditions in relation to K status and release characteristics of twelve soils reveal that the physical and chemical properties of the studied soils varied considerably with the variation of soil parent material, hydrology and climatic conditions. The texture of the soils varied from sandy to clay,organic matter content was rather low(6.9 —19.8g kg-1) .Except for Sara soil,the pH(l:1,water)of the soils ranged between 4.7 and 5.3,indicating that the soils are acidic. Sara soil contained about 60 g kg -1 CaCO3 and had an alkaline pH(8.2) .Among the twelve soils, seven soils had exchangeable K+ of more than 0.20 cmol(+)kg-1,which can be considered as the critical level of K for these soils.The exchangeable K+/exchangeable Mg2+ ratio of the soils varied from 0.05 to 0.14.The structural K was about 93 to 99% of the tota 1 K,while exchangeable(0.26—1.02%)and non—exchangeable (1.07 —8.33%)K comprised a small percentage of the total K. Non — exchangeable K in the soils determined by 0.7M H2SO4 and IM HNO3 methods varied from 0.11 to 3.26 and 0.44 to 3.94 cmol( + )kg-1, respectively. Rate of release of K from the soils was very high in the first extraction by both the IM CH3COONH4 (1.02—3.07mmol kg-1)and 0.3M NaCl(0.90—2.47 mmol kg-1)solutions.With IM CH3COONH4,the release of K was drastically reduced on the second day and virtually stopped on the third day of extraction. With 0.3M NaCl extraction,the rate of release of K was reduced gradually and even on the 11th day of extraction the release of K was considerable. Only K released with 0.3M NaCl showed a significant positive(r=0.70**)correlation with the non—exchangeable K.
The studied soil was in ripe condition(field n — value‹0.6)and consisted of more than 3 horizons including sulfuric horizon within the 1.2 m of the soil surface.The soil pH was low(‹4 : field)and ECe values(0.72—1.62 S m-1),soluble cations and anions,and exchangeable bases were relatively high throughout the profile.Water soluble SO/- —S content was more than 1 g kg-1 and the total S content at ground water saturated portion (C horizon) of the soil was about 12 g kg-1. The exchangeable hydrogen content(l. 1—4.4 cmol( + )kg-,)was very high,and it was attributed to the oxidation of pyritic sulfur.The high amounts of exchangeable and active Fe and Al contents of the soil can be used to categorize it as an actual acid sulfate soil. The soil had high amounts of kaolinite and illite minerals in the clay fraction(‹0.002 mm)throughout the profile. The soil had trace amounts of smectite,chlorite, vermiculite, quartz, feldspars and interstratified 0.71 nm/1.01 nm and 1.01 nm/1.42 nm minerals.The x—ray diffraction patterns showed that jarosite was present at the deeper parts of the profile, which was attributed to the oxidation of pyrite to jarosite.The amount of 0.71 nm mineral was high in the A horizon compared to in C horizon which might be due to pedogenesis.The soil is categorized into Inceptisol order,Aquepts suborder and Sulfic Haplaquepts great group.