2.1. Statistical analysis
The statistics of soil samples are presented in Table 3. According to the results of statistics, significant changes in soil mineral particles in the range of 4 to74%, 9 to 66%, and 6 to 46% have occurred for sand, silt, and clay particles, respectively, which caused the creation of eight textural classes, includes clay, silty clay, silty clay loam, clay loam, silt loam, loam, sandy clay loam, and sandy loam. Such diversity of textural classes can provide a difference in soil properties and their relationship with crop plant growth (Vinhal-Freitas et al., 2017). As can be seen in Table 3, among soil mineral particles, the highest coefficient of variation (CV) is related to clay. The high CV for clay is probably due to flood irrigation and erosion caused in farming lands by this type of irrigation because this type of irrigation causes the transfer of clay from different farms (Olsson et al., 2019).
The mean, minimum, maximum, and CV values of EC were 0.54, 0.22, 1.59 (dS.m− 1), and 57.91%, respectively (Table 3). Therefore, the soils are in the non-saline group. The data relating to soil pH showed that the mean, minimum, maximum, and CV values of this index were 7.4, 8.21, 7.86, and 1.8%, respectively (Table 3). Among all the measured soil properties, the lowest value of the CV was related to pH, which indicates the low variability of this soil properties. The mean, minimum, maximum, and CV values of CaCo2 were 0.85, 32.37, 16.26, and 8.29%, respectively (Table 3). These values show that the soils of the study area are low to moderately alkaline (Hazelton & Murphy, 2016).
Table 3
Statistical properties of studied soil in all districts
Soil parameters
|
Minimum
|
Maximum
|
Mean
|
Standard deviation
|
Coefficient of variation (%)
|
EC (dS/m)
|
0.22
|
1.59
|
0.54
|
0.31
|
57.91
|
pH
|
7.40
|
8.21
|
7.86
|
0.14
|
1.80
|
CaCO3 (%)
|
0.85
|
32.37
|
16.26
|
8.29
|
51.00
|
OM (%)
|
0.07
|
3.46
|
1.05
|
0.56
|
52.86
|
Total N (%)
|
0.004
|
0.17
|
0.05
|
0.03
|
52.86
|
P (mg/kg)
|
0.53
|
920
|
13.61
|
56.71
|
416.70
|
K (mg/kg)
|
2.92
|
2280
|
408.70
|
252.24
|
61.72
|
Fe (mg/kg)
|
1.56
|
3.31
|
2.39
|
0.34
|
14.38
|
Mn (mg/kg)
|
3.33
|
9.29
|
4.89
|
0.99
|
20.23
|
Zn (mg/kg)
|
0.30
|
0.77
|
0.44
|
0.09
|
20.75
|
Cu (mg/kg)
|
0.00
|
0.61
|
0.23
|
0.15
|
64.64
|
Clay (%)
|
6.00
|
46.00
|
19.59
|
8.38
|
42.79
|
Sand (%)
|
4.00
|
74.00
|
44.79
|
12.74
|
28.44
|
Silt (%)
|
9.00
|
66.00
|
35.68
|
8.10
|
22.71
|
Table 3 :HERE
Soil organic matter (SOM) is one of the most important indicators for evaluating soil fertility, which is of global interest (Herrick and Wander, 2018). In this study, the mean, minimum, maximum, and CV values of the SOM were 0.07, 1.05, 3.46, and 52.86%, respectively (Table 3). Jat et al. (2018) classified values less than 1 (%) in the very low class and values greater than 3 (%) in the high SOC class. Therefore, according to the average values of SOM, the lands of this region are in the moderate class. The mean, minimum, maximum, and CV values for TN were obtained as 0.03, 0.17, 0.05, and 52.86%, respectively. These values for AP were 53, 920, 0.61, and 416.70%, respectively. Also, according to the average obtained for AK (408.70%) in these soils, the amount of AK is moderate to very high (Table 3). The mean of micronutrients, including Fe, Mn, Zn, and Cu, were 2.39, 4.89, 0.44, and 0.23 (mg.kg− 1), respectively. According to these data, the micronutrient of these soils is in the low to medium class (Otieno et al., 2021; Roy et al., 2006).
2.3. Classification of soil properties based on the NIV index
The classification and determination of the NIV index value for the soil properties are given in Table 4. According to the results, it can be seen that EC is classified in the low class using both Gomez and Common methods. The optimal level of pH in the Gomez method is 7.4-8, but in the Common method, it is 6.6–7.3; based on the results, it can be seen that in the Gomez method, most of the samples are in the optimal class (85.43), but in the Common method, all the samples (100) were in the high class (< 7.3). The NIV index for CaCO3 was in the optimal class and the values of it for both Gomez and Common methods were 85.14 and 67.71, respectively, but the border between low and optimal classes in Gomez and Common methods had a big difference. In this case, the calculated values for Gomez and Common methods were 2.86 and 26.29, respectively. The values of the NIV index for the SOM were entirely different for the Gomez and Common methods. According to the obtained results, the NIV index for 57.62% of soil samples of Gomez method was in the optimal class, and for 76.57% of soil samples of the Common method was in a low class (Table 4).
The NIV index for TN using the Gomez and Common methods was classified in low the class. Although the Gomez method evaluated the amount of AP in the optimal class and the Common method evaluated it in a low class. In the Gomez method, the border between the low class (42) and the optimal class (49.71) is very close to each other, but in the Common method, the border between low class (78.29) and optimal class (10.57) is different. Both of the Gomez and Common methods evaluated the amount of AK in the optimal class. In the Gomez method, the border between the optimal class (71.55) and the low class (37.14) is very close to each other, but in the Common method, most of the samples classified the amount of AK in the optimal class (83.71) (Table 4).
The evaluation of the micronutrient showed that the amount of Fe for both Gomez and Common methods is in a low class. The value of this NIV index for Fe by Gomez and Common methods are 93.71 and 100%, respectively. The evaluation of the amount of Mn using the Gomez and Common methods showed that the Mn is classified as optimal (87.14) and low (100). According to the Gomez and Common methods, Zn was placed in the optimal (96.86) and low (68.86) classes, respectively. Also, Cu was in the low (100) class, according to the Gomez and Common methods (Table 4).
Since there is no typical classification record for soil texture characteristics, the Gomez method was used to classify soil texture in this study. According to the results obtained for the Gomez method, it can be seen that clay mineral particles were evaluated in the low class (69.14), sand mineral particles in the high class (65.14), and silt mineral particles in the optimal class (78) (Table 4).
Table 4. Percent sample category of soil properties and NIV by two classification methods.
Table 4 : HERE
3.3. Evaluation of soil fertility index according to elevation
The results of the NIV index based on elevation changes for Gomez and Common methods are given in Table 5. The evaluation of the NIV index EC as an indicator of soil salinity shows that both Gomez and Common methods at all elevations of 1600–2000, 2400 − 2000, 2400–2800, and < 2800 m were a low class. The percentage of soil samples of the NIV index in different elevations for the Gomez method were 54.41, 72.87, 76.23, and 56, respectively, and for the Common method, were 62.16, 86.82, 96.72, and 92 percent, respectively. According to the obtained results, the percentage of soil samples with low EC class in elevations of 1600 − 200 m and < 2800 m is lower than in medium elevation (2000–2400 and 2800 − 2400 m). Examining the NIV index for pH in different elevations showed that the Gomez method evaluated pH at the optimal level at all elevations and the percentage of samples in the optimal class at elevations of 1600–2000, 2000–2400, 2400–2800, and < 2800 m were 91.89, 83.72, 86.89, and 68%, respectively. Also, the NIV index classified the soil pH by Common in elevations of 2000 − 1600, 2000–2400, and 2400–2800 m in the optimal class, and elevation < 2800 m was classified in the low class. Gomez method evaluated the amount of CaCO3 at all elevations in the optimal class. But in this method, the value of the NIV index for CaCO3 decreased with increasing elevation. According to this method, the highest value of the NIV index for CaCO3 at an elevation of 2400 − 2000 m was 2.13, and the lowest NIV index value for an elevation of < 2800 m was 1.92. Evaluation of the NIV index using the Common method classified CaCO3 at the elevation of 2000 − 1600, 2000–2400, and 2400–2800 m was in the optimal class, and at an elevation of < 2800 m was in a low class. According to the results of the Common method, the highest value of the NIV index was obtained for CaCO3 at an elevation of 1600–2000 m with a value of 1.92. The lowest value of the NIV index for CaCO3 at an elevation of < 2800 m was 1.60. (Table 5, Fig. 2, and Fig. 3).
Examining the NIV index of SOM showed that in the Gomez method, the NIV index value increases with the increase of the elevation. The values of this index at elevations of 1600–2000, 2000–2400, and 2400–2800 m were in the optimal class and at elevation < 2800 m was in the high class. But using the Common method, these values were placed in 1600–2000, 2400 − 2000, and 2400–2800 m in the low class, and in the elevation of < 2800 m was in the high class. This trend was the same for TN, and the response to elevation changes was similar to SOM (Table 5, Fig. 2, and Fig. 3).
The NIV index for studying AP and AK changes with elevation change showed that the changes of these two nutrients using the Common method were closer to reality. As can be seen in Table 5, the value of the NIV index using the Gomez method for AP and AK at an elevation of 200–1600 and < 2800 m is in a low class, and elevations of 2400 − 2000 and 2400–2800 m are in the optimal class. But using the Common method, at all elevations, the amount of NIV for AP is in a low class, and the amount of NIV for AK is in the high class (Table 5, Fig. 2, and Fig. 3).
The changes in the NIV index using the Common method for micronutrients were also closer to reality. Evaluation of the NIV index for Fe and Cu using the Gomez and Common methods was in low class at all elevations. The NIV index for Mn was in the optimal class using the Gomez method at all elevations and in the low class using the Common method. The NIV index was in optimal class using the Gomez method in all elevations, in optimal class using the Common method at 1600–2000 m, and in low class in other elevations (Table 5, Fig. 2, and Fig. 3).
Table 5 : HERE
Figure 2 :HERE
Figure 3:HERE