عنوان مقاله [English]
The objective of this study was to determine the allowed and desired limits for heavy soils of texture (clay loom), as it has operational capability (friability), These limits were 6.34 mm for planting and 10.62 mm for harvesting in each raining or irrigation episode. We determined some factors that affect soil wetness and water content as follow: the amount and severity of rain, the condition of soil’s drainage, the percent of air moisture, evapotranspiration, the amount of runoff, the amount and severity of sunlight, the ambient temperature, the condition of vegetation and, the soil texture. The time needed for dryness and removal of soil moisture as evapotranspiration in each over- desired and allowed capacity rains are 5-7 days. Also, because of higher evaporation in planting time (late April and early May) than in harvesting time (November), and also higher surface temperature in spring, both result in moving the vapor from deeper layers of soil to surface. Therefore, as our result showed, the time needed for readiness of soil for machinery operation at time of planting are 5 days for planting (late April and early May) and 7 days for harvesting time (November). According to our calculation, 19.5% of rains at planting time and 7.5% at harvest time changed to run- off; and with taking this amount of run- off in account at allowed- limit calculation, and also because of texture of the studied soil, the maximum allowed rain in which the soil remains in proper condition for machinery planting and harvesting activities exceeded 7.6 and 11.41 mm in each rainy day. As a result, according to limiting factor of well doing of operations and also factors affecting these restricting factors, the number of working days for mechanized planting operation was found to be about 19.69 days and also 21.98 days for harvesting operations in farms with heavy soil with 98% of probability.