JWSS - Journal of Water and Soil Science

---

Field capacity (FC) and permanent wilting point (PWP) are important factors affecting irrigation scheduling and field management. FC and PWP can be estimated from some of the soil physical and chemical properties. Pressure Plate apparatus is usually used for determination of FC and PWP, but this is a time-consuming and laborious procedure besides, the apparatus may not be available in many laboratories. Samples were taken from 23 locations in Isfahan and Chaharmahal Va Bakhtiary provinces in central Iran. Soil texture, organic matter and cation exchange capacity were determined. Soil moisture at FC and PWP of the soils were measured with a pressure plate. Simple and multiple regression analyses were used to study the relationships between FC and PWP with sand, silt, clay, soil organic matter and cation exchange capacity.

 Results indicated that FC significantly correlated with sand, organic matter and cation exchange capacity in a stepwise model (r=0.97**). The PWP of soil also correlated significantly with silt, organic matter, and cation exchange capacity in a stepwise model (r=0.95**). Available water capacity (FC-PWP) correlated with sand in a stepwise model (r=0.82**). On the whole, results showed that FC and PWP can be estimated from some soil physical and chemical properties.

---

The urease enzyme plays an important role in the efficient use of urea fertilizer and some environmental risk assessment. Urease activities in 20 different soil samples of arid to semi-arid regions of Isfahan Province were determined and their correlations with some soil physical, chemical and biological characteristics were studied. Urease activities range from 5.3 to 79.2 µg NH4⁺ g^-1 soil 2hr^-1.

Results indicated that soil organic carbon was significantly correlated with urease activity (r=0.899***). None of sand, silt and clay percentages were significantly correlated with urease activity. Total nitrogen was significantly correlated with urease activity (r=0.797***). Electrical conductivity of saturated paste extracts were also negatively correlated (r=-0.499*) but sodium adsorption ratio (SAR), pH, equivalent calcium carbonate and cation exchange capacity failed to be correlated significantly with urease activity. No significant correlations were found between urease activity and total bacteria (on nutrient agar) or total fungi (on potato dextrose agar), but the bacteria that could colonize urea-agar media were significantly correlated with urease activity (r=0.47*). Multiple stepwise regression analysis showed that organic carbon accounted for most of the variation in urease activity.