مجله مدیریت خاک
پیاپی 1 (پاییز و زمستان 1391)

Utilizing land resources based on their capability is necessary for an optimal land use and to prevent land degradation. The aim of this study was to determine the optimal cropping pattern using linear programming model to reduce land degradation at a local scale in Roin area, North Khorasan province. The decline in soil organic carbon due to land use change was considered as an indicator of land degradation. The results indicated the non-suitability of slopes higher than 20% and southern slopes for cereal rain-fed cropping. Because of the effect on soil organic carbon, alfalfa is permitted in southern slopes. The optimal cropping pattern included only chickpea, alfalfa and rangeland which result in an eightfold increase in net benefit and preventing land degradation. Implementation of optimal cropping pattern needs promotion and advising the people in the area.

Land suitability evaluation allows the optimum use of land and keeps soil quality as well. Considering high water table level in the most part of the study area (Farsan- Jouneghan plain), the qualitative land suitability evaluation was determined for rice to compare its suitability with the current cultivations in the area. Soil pedons were excavated in 700 m intervals and described. Then, representative pedons were chosen and the soil map of the area was prepared with the scale of 1:25,000. Land suitability evaluation was calculated for irrigated wheat and potato as the most common crops in the area and also rice as an alternative crop. Results showed that rice cultivation is the best choice for the soils with high water table in this area. The soils with moderate water table showed the best suitability for wheat whereas the soils without permeability (drainage) limitations showed the maximum suitability class for potato. The WRB system seems to be more appropriate system for the classification of the soils with high water table in the area because they classified as Gleysols at Reference Soil Group (RSG) level. Contrary, the soils with moderate water table could be recognized well as Endoaqualfs greatgroup by Soil Taxonomy.

The effect of gravel and soil column diameter was evaluated on the flow of water in a silty loam soil. The experiment was carried out in seven columns with nominal diameters of 40, 63, 90, 110, 160, 200 and 250 mm. Two soils sampling approach were used including soil sieved by the sieve No.10 (SS) and unsieved soil (US) which has gravel. The results also compared with the field. The result show that effect of column diameter was significant (P<0.01) on cumulative infiltration, soil basic water infiltration rate and saturated hydraulic conductivity. Overlay the soil column of 110 mm had the least difference in comparison with the field data. The cumulative infiltration measured 43.33 and 44.51 cm for SS and US, respectively. The infiltration rate was 0.29, 0.32 cm/min for SS and US, respectively. It measure 0.28 and 0.31 cm/min for SS and US, respectively on saturated hydraulic conductivity. It concluded that the measured values in soil column of 110 mm in diameter for sieved soil and unsieved soil can be considered similar to field values for this soil and experiment conditions.

Economical profit is one of the important factors, persuading farmers to plant a specific crop. So, in addition to qualitative and quantitative land suitability evaluation, economical one need to be revised, based on net or gross profit per surface area unit. The present research was done to evaluate land suitability, economically, by different methods for several varieties of almond in the Savadjan region in Chaharmahal va Bakhtiary province. In this regards, economical land suitability evaluation for several varieties of almond was carried out, using "internal rate of return", "gross profit", "net present value" and "benefit/cost ratio" methods. These methods are included in the ALES program. For model making in this program, a data base, including land units, yield, discount rate, constant and variable costs and return of one weight unit of several varieties of almond was established and then economical land suitability classes were determined. The results showed that economical land suitability evaluation based on gross profit, net present value, benefit/cost ratio and internal rate of return for 3 varieties mamai, rabie and shahrood 13 for 1388 to 1390 years changed from not suitable "N" to high suitable "S1". So, shahrood 12 is highest economically variety in these region gardens.

Utilizing land resources based on their capability is necessary to optimal land use and prevent land degradation. The aim of this study was to determine the optimal cropping pattern using linear programming model to reduce land degradation in a local scale in Roin area، North Khorasan Province. The decline in soil organic carbon due to land use change was considered as the indicator of land degradation. The results indicated the non-suitability of slopes higher than 20% and southern slopes for cereal rain fed cropping. Because of the effect on soil organic carbon، alfalfa is permitted in southern slopes. The optimal cropping pattern included only chickpea، alfalfa and rangeland which result in increasing eightfold of net growth and also decreasing land degradation. Implementation of optimal cropping pattern needs promotion and advising the people in the area.

Time Domain Reflectometry (TDR) is One of the modern and accurate methods in determination of soil water content that based on dielectric constant measurement. On the other hand Given that dielectric constant is affected by the soil characteristics، Therefore، calibration and determination of the relationship between dielectric constant and volumetric soil moisture is required for the practical application of this method. The purpose of this research is the evaluation of the effects of soil texture and organic matter on the accuracy of volumetric water content measured by TDR. For this purpose The experiments were carried out on five types of soil texture (sandy، sandy loam، loam، sandy clay loam and clay loam)، and three organic matter levels (2، 3. 5 and 5 percent) which contain litter fertilizer in three repetitions under quite random conditions. In order to evaluate and calibrate the results of the TDR system، soil volumetric water content was also measured by weight method. Results show that the accuracy of the volumetric water content values obtained from TDR system improved by increasing the percentage of clay and soil organic matter، so that TDR system with the relative error percentage (RE) 3. 97 and 11. 79 has been the most and least accurate، for sand with 2 percent and clay loam textures with 5 percent organic matter، respectively.