Just twenty years have passed since the first papers were published on the application of chemical thermodynamics to the soil-water system (11, 14). Since then, soil physicists have used thermodynamics in an attempt to characterize and l earn more about this intricate system.

The products of pyrite oxidation, including solution phase Fe2+, Fe3+, S2O32-, S4O62-, SO32- and SO42- and solid phase Fe(OH)3, were measured under controlled conditions in order to investigate the behavior of pyrite in calcareous and alkaline soils. The distribution of sulfur oxidation products is pH dependent and can be interpreted in terms of metastable equilibrium among thiosulfate, disulfane disulfonate and sulfite. Thisulfate and sulfite predominate in the pH range greater than about pH 7 or 8. Sulfane disulfonates are more predominant at more acid pH. Solution concentration data were consistent with the presence of Fe(OH)3. Concentrations of thiosulfate and sulfane disulfonate were consistent with a redox equilibrium among solution iron and sulfure species at pH 6 to 9. Linear or zero-order kinetics were found to be sufficient for description of pyrite oxidation in this study. Linear kinetics were observed as electrical conductivity, solution sulfur products and solution plus solid phase iron products. The measurement of solution iron plus solid-phase iron oxide is a more rigorous approach to the extent of reaction than the measurement of sulfate. The rate of pyrite oxidation is pH dependent, increasing from 10-20 pmol(Fe) m-2 s-1 to 40-60 pmol(Fe) m-2 s-1 between pH 5 and 9. This is consistent with an oxidation mechanism involving the reoxidation of solution Fe2+ via a reaction between an iron hydroxide complex and hydrated oxygen as the rate-determining step. The effect o background electrolytes on oxidation rates at low pH also supports this interpretation. Pyrite oxidation rates in the presence of calcium carbonate, sodium bicarbonate, sodium thiosulfate and calcium-saturated bentonite can be related to the pH effect. Sodium thiosulfate and DTPA appeared to have specific inhibitory effects. Column studies show that the disposal of pyritic mine spoils or tailings by mixing with calcareous material may produce thiosulfate, a good reducing agent for toxic metals. Burial of lime below pyritic materials may protect groundwater quality more effectively than application of lime to the surface.

The genetic relationship between a Vertisol and an associated Mollisol with an argillic horizon was studied. These soils are taxadjuncts of the Hawkins and Ostler series, respectively. They occur in the Wasatch Mountains of northern Utah and have similar parent materials, altitudes, and slope percentages. Ostler soils have a dense cover of Gambel oak (Quercus gambelii) and occur preferentially on north aspects. Mule's ear (Wyethia amplexicaulis) is the dominant vegetation on Hawkins soils, which are most frequent on south aspects. Erosion is prevalent on Hawkins soils because the mule's ear does not adequately cover the soil surface during the fall, winter, and early spring. The dense oak woods and the litter cover on the Ostler soil protect it from erosion. Both soils dried sufficiently for cracks to develop in their clayey portions. The Ostler soil was drier, probably because it has more and longer transpiring vegetation than the Hawkins soil. The Ostler subsoil and the Hawkins solum both had cracks, slickensides, high clay contents, high COLE values, and smectite as the dominant clay mineral. These characteristics of high shrink-swell activity were most strongly expressed in the Hawkins soil. The Ostler subsoil clay was overlain abruptly by loam surface soil. The textural difference was attributed to lessivage, eolian contributions mixed with preexisting surface soil by soil animals, and in situ clay formation in the subsoil. Eolian additions were inferred from the presence of easily weathered silt and clay-size minerals in the Ostler A horizons and within the zone of mixing in the Hawkins soil. These minerals were not present or were less abundant in the Ostler subsoil and in the Hawkins C horizon. The formation of Hawkins soils (Vertisols) probably resulted from erosional exposure of the clay subsoil of Ostler soils or their precursors.

A robust mathematical model of one-dimensional flow for sloping, blocked-end border irrigation was developed using the four-point implicit method to solve the Saint-Venant equations, the volume-balance solution method, and the implementation of new algorithms to avoid numerical instability and solution divergence. The model has the capability of successfully simulating all surface irrigation phases in blocked-end borders for a range of inflow rates (0.01 - 0.05 m3/s per m), longitudinal slopes (up to 1.00%), and border lengths (100 - 500 m). To achieve numerical stability over the specified parameter ranges, the model was divided into three parts: (1) advance-phase simulation which uses the four-point implicit solution method of the Saint-Venant equations, with an algorithm that changes the spatial and temporal weighting, in addition to an algorithm that handles the water depth profile at the blocked-end downstream boundary upon completion of the advance phase; (2) simultaneous advance-recession-phase calculations using a hybrid algorithm to solve the governing equations; and (3) recession-phase simulation using the four-point implicit method until (and if) divergence occurs, then the volume-method is applied to complete the simulation. The three parts also involve the use of computational grid management algorithms and a parabolic equation which defines the Chezy coefficient as a function of water depth. The model incorporates the downhill simplex optimization method to determine the recommended inflow rate and irrigation cutoff time, maximizing a composite irrigation efficiency (water requirement efficiency and application efficiency). Different optimum values of inflow rate and irrigation cutoff time for a range of longitudinal slopes, border lengths, and soil types were generated. Most of the optimum values are for relatively high inflow rate and rapid cutoff time. In addition, exponential relations were developed, based on the simulation results, to determine the best irrigation time for maximization of the composite irrigation efficiency for specified, non-optimal inflow rates. The exponential relations are particularly useful in practice when it is not feasible to use the optimum inflow rate due to constraints at the water source, or because of irrigation scheduling issues.

Nitrification, the process whereby ammonical-nitrogen (NH i+4 - N) is changed into nitrate- nitrogen (NO3-N), is one of the more important biochemical processes associated with agriculture. Nitrate appears to be the form of N that most crops can most readily utilize.

Changes in the energy status of water that occurred in Milville silt loam and in bean (Phaseolus vulgaris L.) plants growing on it , during two consecutive 6-day periods while being irrigated with saline solutions, were studied. Total soil water potential (Ψ soil) (measured with thermocouple psychrometers), matric-potential (Ψm) (measured with tensiometers) were considered. In leaf samples, the total water potential (leaf) and the osmotic potential (Ψπ)--after freezing-- were determined with thermocouple psychrometers; by difference, the pressure potential (Ψp) was obtained. There is no evidence of close relationship between changes taking place in the values of the leaf water potential (or its components) and changes that occur in the energy status of the soil solution. Also, there is a lack of close relationship between the concentration of the solutions added and the resulting water potentials in the soil solution. Results indicate that the soil water potential exerts increasing control of leaf water potential as it decreases. In addition, there is a smaller range of values of leaf water potentials when the saltier solutions (C and D) are used for irrigations. Also, the osmotic potential is higher in leaves of plants grown in pots watered with saltier water. High values of pressure potentials (Ψp)--low turgor-calculated for leaf samples, where no wilting symptoms were visible, suggest that there are differences among plant species with regard to the turgor level at which wilting occurs.

An estimate of potential evapotranspiration is acquired to implement an irrigation scheduling program. Four equations, which estimate potential evapotranspiration (evaporation) were evaluated for prediction of Class "A" Pan evaporation. The equations used were Penman's combination equation, the Jensen-Haise temperature and radiation equation, and two of the Christiansen-Hargreaves polynomial regression type equations. Measured and calculated radiation was used together with two values of albedo. Four years of daily climate data from Rhodesia was evaluated by general linear regression methods. Despite the significant variations between the four annual regression lines of each equation, the use of confidence intervals indicate that the Penman and Christiansen-Hargreaves equation adequately predict Class "A" pan evaporation for irrigation control purposes. Using class "A" pan evaporation as the measure of potential evapotranspiration for sugarcane, a computerized irrigation scheduling model was developed. Controlled plant moisture stress was incorporated in the program with an irrigation coefficient related to the limiting effects of low soil moisture on plant transpiration. Five soil moisture regimes and two levels of irrigation were studied. Within the limits of the defined soil moisture assumptions, the program exhibited considerable flexibility in computation and the control of desired plant moisture stress.

A consideration of the physical principles involved in evaporative drying of soil indicates that several physical processes might possibly limit the rate of evaporation. These include the rate of supply of energy to the site of evaporation, the rate of transfer of water vapor through the sample or through the atmosphere above the sample, and the rate of unsaturated flow of water to the soil surface or to the site of evaporation.

Relationship between soil moisture and the absorption of phosphorus and other nutrients.

For many years man has been searching for a cure for lime-induced chlorosis. This disease is prevalent throughout the entire western United States and is characterized by a yellowing of the leaves, destruction of the chlorophyll, and eventual death of the plant. It has brought devastation of fruit orchards in Utah, costing fruit producers millions of dollars. Hardly any county in Utah is free from this destructive nutritional disease.