This thesis describes a series of experiments both in solution systems and soil systems to study the precipitation of calcium carbonate in soils and the effects of the precipitation on soil pH after urea had been applied. (1) A gas bubbling system has been established which introduces ammonia at a steady rate to the reaction solution and keeps it equilibrated at 0.00484 atm partial pressure of carbon dioxide. (2) In a non-seeded system, the effects of calcium, urea, Mg (magnesium), P (phosphate), and DOC (water-dissolved organic matter) on the precipitation were examined individually and in various combinations. Calcite and vaterite were found in the 10 mM CaC1₂ solutions with and without the addition of urea. When the solutions contained Mg, P, and DOC, vaterite was not found. Aragonite was found in the reaction solution containing 5 mM Mg. In high initial concentration of P (5x10^-4 M) , the formation of calcium phosphate (amorphous by X-ray analysis) catalysed the formation of calcite. The effects of urea and Mg on the precipitation are negligible compared with the effects of P and DOC. (3) In a seeded system, 16 sets of experiments with four sizes of calcite-seeds were carried out to study the precipitation rate of calcium carbonate. This was described by the equation LR=-4.113±0.132 + 0.379±0.029 LWA + LSI where LR=log (precipitation rate, PR, in mole litre^-1 min^-1), LWA= log (newly formed calcium carbonate, g ml^-1), and LSI=log (degree of supersaturation of calcium carbonate, SI). (4) A wide range of concentrations of urea (0.05, 0.1, 0.3, 0.5, 0.7, and 1 M) were added to three soils (Beg., Uni., and VWH) with or without the addition of 5 per cent of calcite (10-15 μm) to establish a rate model for the precipitation of calcium carbonate in soils. The precipitation model (in logarithmic form) in soils is lnPR=-9.47±0.30 + lnK_SOIL + 0.379±0.029 InWA + InSI - 1686±703 P - 6.13±3.02 DOC + 3854±1775 (P DOC) where P and DOC are the concentrations in soil solutions, and lnK_SOIL is the effect of soils on the precipitation, which is - 1.98, 0.43, and -0.10 for Beg., Uni., and VWH soils respectively. The amount of newly formed calcium carbonate is about a third to a half of the amount of ammoniacal-N released by urea hydrolysis. It was able to reduce the increase of soil pH by more than 0.6 pH units in some circumstances.