**3. Results and discussion**

### **3.1. Wheat yield**

Wheat is the main crop in the study area, covering 68% of the study area. Although only 14% of the basin is suitable for cultivation, most of the pasture areas have been converted to agriculture for the last 60 years. The agricultural areas in the basin are mostly shallow, varying between 20 and 50 cm. The mean plant-available water content for the cultivated areas is about 100 mm. Therefore, water stored in soils often fails to meet crop water requirement. In the dryland farming areas receiving less than 400 mm annual precipitation such as the Çelikli basin in Turkey, a winter wheat-fallow system is used to reduce the risk of uneconomical yield [24]. Wheat yield was measured at 115 sites with three repetitions in the basin (**Table 2** and **Figure 3**). While the wheat yields ranged from 600 to 3780 kg ha−1, only 4.78% of the cultivated areas had yield greater than 2500 kg ha−1 as shown in **Table 3**.


**Table 2.** Statistical results of the wheat yield.

**Figure 3.** The wheat yield distribution in the basin.


**Table 3.** Areal distribution of the wheat yield in the basin.

#### **3.2. Evaluation of soil degradation in the basin**

#### *3.2.1. Soil penetration resistance*

**3. Results and discussion**

98 Land Degradation and Desertification - a Global Crisis

**Table 2.** Statistical results of the wheat yield.

**Figure 3.** The wheat yield distribution in the basin.

Wheat is the main crop in the study area, covering 68% of the study area. Although only 14% of the basin is suitable for cultivation, most of the pasture areas have been converted to agriculture for the last 60 years. The agricultural areas in the basin are mostly shallow, varying between 20 and 50 cm. The mean plant-available water content for the cultivated areas is about 100 mm. Therefore, water stored in soils often fails to meet crop water requirement. In the dryland farming areas receiving less than 400 mm annual precipitation such as the Çelikli basin in Turkey, a winter wheat-fallow system is used to reduce the risk of uneconomical yield [24]. Wheat yield was measured at 115 sites with three repetitions in the basin (**Table 2** and **Figure 3**). While the wheat yields ranged from 600 to 3780 kg ha−1, only 4.78% of the cultivated

**Yield (kg ha−1)**

areas had yield greater than 2500 kg ha−1 as shown in **Table 3**.

Sample number 115 Maximum 3780 Minimum 600 Average 1794 Standard deviation 744 Coefficient of variation 0.41

**3.1. Wheat yield**

The penetration resistance values and their statistical results for the basin are given in **Tables 4** and **5** and **Figure 4a** and **b**. While 34.92% of the topsoils in the basin had under 2.0 MPa, the penetration resistance of all subsoils had penetration resistance values over 2.0 MPa. High PR values were attributed to soil texture (fine) and low water content of the soils. Penetration resistance is sensitive to soil water content. In addition, in subsoils, high penetration resistance could be attributed to the existence of a dense plow layer, which is mainly the case in cultivated fine-textured soils subjected to conventional tillage. The mean penetration resistance values of the surface and subsoils were 1.671 and 2.579 MPa, respectively, which are below 3.0 MPa above which growth of many crops is inhibited [25]. The penetration resistance was measured in 0– 20 cm only due to that soil depth was too shallow in grasslands. In general, the coefficient of variation and standard deviation for penetration resistance in topsoil were greater than in the subsoil due to soil tillage effect.


**Table 4.** Areal distribution of the penetration resistance, crusting index, and plant-available water content of the basin soils.


**Table 5.** Statistical results for penetration resistance and tendency to crust of the basin soils.

**Figure 4.** The penetration resistance (a and b), crusting index (c), plant available water content (d), surface runoff (e) and soil loss (f) maps of the basin.

The penetration resistance affecting crop yield is not a constant value and varies according to other soil properties. Indeed, it was reported that there was no penetration problem under 2000 KPa, but crop yields were affected over 2.00 MPa [26]. However, some researchers claim that the crop yield is affected by PR of >3.00 MPa [27–29]. Soil penetration resistance is a valuable indicator of the soil physical quality. A value of 2 MPa has been widely used as a critical limit to determine PR in both no-tillage and conventional systems [25, 26]. PR varies spatially as well as temporarily and is related to clay type, clay content, and soil water content. It is used to evaluate soil quality and to identify layers with increased compaction [30].

Soil compaction is a deterioration process that weakens the plant growth, reducing the soil porosity, slowing the infiltration rate, and restricting the root growth. The most effective factors on soil compaction are accepted as vehicular traffic and wetting-drying circles. Soil compaction impacts pore-size distribution and reduces total soil volume, increases surface runoff and soil erosion in sloping areas, causing ponding in level areas [31].

Results from recent studies showed that plant growth could continue in soil with PR values as high as 3.5 MPa in no-tillage conditions due to the presence of continuous and biological pores, which allowed plant-root development in areas with low PR [32]. The PR and soil moisture showed a spatial relationship where lower values of PR concentrated on smaller values of soil moisture [33]. In another study, PR values varied with the density of the soil, regardless of moisture and penetration rate. The relationship between PR and moisture was not always linear, once it is influenced by soil-bulk density [34]. While PR was indicated as a good indicator of physical soil-crust formation in scalped soils over time, it was not of any effect on biological soil-crust development, erosion behavior of the soils [35]. PR in shallow ploughing in autumn at 10–25 cm was significantly higher than deep ploughing at 45–50 cm in a research [36]. The root length of soybean was obtained as the most susceptible to soil compaction, and the change in soil PR was poorly related with the change in the degree of compactness [37]. The mechanized cultivation system presents greater soil PR values to penetration down to 0.15 m depths and less humidity, when compared to the manual cultivation system [38].
