Effect of soil salinity and sodicity on urea hydrolysis in three soil orders [Sudan]
1995
Awad El-Karim, A.H. | El-Mahi, Y.G.
Laboratory experiments were conducted to study the effect of salinity (3, 20, 40, 100, and 200 meq.L-1) and sodicity (SAR = 0, 5, 15, 25 and 50) on urea hydrolysis in three soil orders, namely, Shambat gerf soil, S1(Enthisols); Gedarif soil, S2 (Vertisols) and Droushab soil S3(Aridisols). The samples were initially equilibrated with mixed - salt solutions (CaCl2 + NaCl) to render different SAR values. The rate of urea-N application was equvalent to 160 1b. feddan-1(1 fed = 0.42 ha) and the soil samples were incubared for three weeks under constant temperature and moisture conditions. The results indicated that for Shambat gerf soil (S1), the rate of urea hydrolysis decreased with increasing salinity level from 40 to 200 meq.L-1, specially at high levels of SAR. This was attributed to high concentration of salt which adversely affected the microbial growth and, hence, the microbially induced urease activity which caused reduction in the rate of urea hydrolysis. The initial rate of urea hydrolysis in Gedarif soil (S2) and Droushab soil (S3) were lower than that of S1. High clay content in S2 and, therefore, high surface charge density may have caused strong adsorption of urease and a decrease in its activity, and that high pH values in S3 probably suppressed the production of microbially induced urease activity through negative effect on microbial proliferation. Higher values of SAR decreased the rate of urea hydrolysis in most samples. This may be due to the high pH associated with increasin SAR values which adversely affects the soil environment and the adsorption of micro-nutrients. Also, the high contents of sodium and chloride may become toxic to microbes.
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