Geospatial assessment of soil and land properties for mango suitability in Chandanhalli sub watershed using analytical hierarchic process

Introduction

Soil and land resources are declining both in quantitative and qualitative terms at a rapid rate worldwide primarily due to anthropogenic activates and secondly because of natural reasons. The pressure of producing more and more food on these limited resources to satisfy the demands of increasing world population has further aggravated the situation at alarming speed. Impact of climate change has also imposed unpredictable conditions in soil and land for cultivation of crop. Interventions like suitable soil and water conservation methods, nutrient management practices and adoption of crops based on soil and land properties can bring major positive change in productive potential of land. Each parcel of land has got its own potentials and limitations with respect to their physical, chemical and biological properties to support crop growth. Therefore to strengthen the decision support system of sustainable enhancement of productivity, site specific crop suitability is an important pre-requisite. Intensive data on inherent properties of site specific nature, influence on each other's potential and capabilities (Greenland et al., 1998; Richardson et al., 2023) [7,11] which plays vital role in evaluating land suitability for different crops there by becomes critical inputs in making strategies for increasing productivity (Pramanik 2016) [10] and making agriculture sustainable (El Baroudy 2016; Falasca et al., 2012) [4,5] . Chandanhalli sub watershed, covering an area of 5328.00 ha, was identified as study area where, mango is one among the commonly cultivated crop. With an objective to bring out site specific crop suitability plan for mango, a detailed Land Resource Inventory (LRI) was carried out to characterise surface and subsurface properties of soil and land. The outcomes of LRI were processed through AHP to develop site specific mango suitability map using GIS.

Materials and Methods

Study Area

Chandanhalli Subwatershed is located between 17°52'32.72" to 17°53' 23.164" N latitude and 77°6'58.283" to 77°6'57.47" E longitude in Humnabad taluk of Bidar district, covering an area about 5328.00 ha. (Figure1). The sub watershed comprise of 8 micro-watersheds coming under north-eastern transition zone with average annual rainfall of 847 mm. Soils of SWS are lateritic with undulating physiography.

Criteria selection

Criteria considered for assessment of mango suitability in the study area was done by referring to manual soil site suitability criteria for major crops (Naidu et al., 2006) [8] . The site specific physical and chemical properties like soil texture, depth, slope, pH, EC, and OC were recorded and used for assessing the land suitable for mango cultivation. GIS raster datasets on each of these indicators were acquired and processed from several sources for the study area. Table 1 presents the data sources used in study.

Standardization of criteria layers

All the criteria considered for cropland suitability analysis were standardized based on the soil classification and characterization guide for agricultural suitability by FAO and other guidelines defining natural and artificial constraints for agriculture suitability (Naidu et al., 2006) [8] . The crop requirements were matched with the soil and land characteristics to arrive at the crop suitability. The land suitability classification for different crops, viz., two orders were recognized, order S-Suitable and order N-Not suitable. Further, the orders have classes, sub-classes and units. Order-S has three classes, Class S1-Highly Suitable, Class S2-Moderately Suitable and Class S3-Marginally Suitable, N-Not suitable. There are no sub-classes within the class S1 as they will have very minor or no limitations for crop growth. Geostatistical interpolation technique kriging was applied to obtain the spatial layers of soil chemical properties. All these layers were classified and thematic maps were prepared as per the standards given by National Bureau of Soil Survey and Land Use Planning. These criteria maps are as shown in figure 3 and4.

Analytical Hierarchy Process (AHP)

The relative importance values were determined following Storie index (O'geen, 2008) [9] , where a Saaty`s scale score of 1 to 9 of the respective importance were obtained. A pair wise comparison matrix is derived using Storie rating method of 9 point importance scale based on thematic and fertility layers used for soil and land suitability for mango. To obtain the weighted super matrix, each column of the super matrix is weighted by multiplying the corresponding weights and normalized. The AHP captures the idea of uncertainty in judgments through the principal Eigen value and the consistency index (CI) (Saaty, 2004) [12] . Consistency Index (CI) as deviation or degree of consistency was derived using the following below equation,

𝐶𝐼= (𝜆𝑚𝑎𝑥-𝑛)/ (𝑛-1)

Where λmax is the largest Eigen value of the pair-wise comparison matrix and n is the number of classes (Alonso et al., 2006) [3] . Consistency Ratio (CR) is a measure of consistency of pair wise comparison matrix and is given by equation, CR=CI/RI Where, RIis the ratio Index. The value of RI for different n values is given in. When CR value is less than 0.10, the matrix indicates a reasonable consistency. The calculated results of weight would be accepted when the consistency ratio was satisfactory.

Weighted Overlay Analysis

Thematic layers of different surface and sub-surface characters with their respective ratings were analysed through AHP (Agarwal et al., 2013) [1] method to get their weightages. The analytical hierarchy process is a multi-attribute land evaluation technique which allows to determine the weights of these multiple and heterogeneous criteria indicating relative importance. Different thematic layers identified in the study with their respective weightages were further integrated with each other in ArcGIS 10.6.1 software as per weighted overlay technique using the spatial analyst tool (Table 3).

Results and Discussion

Physical and chemical properties of soil of an area are the important attributes which enable to asses the land suitable for different crops. To find out the site specific suitability for mango, soil and land properties criteria like soil depth, texture, slope, organic carbon, electrical conductivity (EC) and pH was processed through AHP and Weighted Overlay Analysis (Table 2 and3). The results indicate that soil depth with a weightage value above 30% is the most important factor in making land suitable for mango out of the six criteria. On the status of the remaining criteria OC &pH contribute18%, soil texture with 15%, soil slope with 13% and EC with 6% weightage have proved their hierarchy in deciding mango suitability of the study area. As soil and land properties co-exist & highly interdependent in creating favorable or unfavorable conditions for crop growth, Status of soil and land characters have clearly proved their ability to become suitable or not suitable for mango crop. Based on the outcomes of the weighted overlay evaluation.

The study area of 5328.00 ha falling under four different mango suitability categories as mentioned in table 4. The reasons for land parcels belonging to S1category are found strongly associated with deep to very deep soil depth where it allows the mango crop to have deeper and wider tap root system which enables to extract nutrient and moisture from deeper horizons. As these soils are belonging to very gently sloping lands, they become less prone to soil, nutrient and moisture loss. Therefore higher status of organic carbon is matching to the others conditions like neutral pH, Sandy clay loam texture.

On the contrary lands identified as not suitable (N) category can be confirmed by existence of severe limitation like very shallow to shallow depth and existence of lithic contact layer which blocks the root penetration. As most of the soils belongs to 3-5% slope and making them susceptible for erosion, runoff losses, resulting in low organic carbon, moderately alkaline pH, and dominance of weathered parent material which leads to increase in severity of limiting factors of mango crop While an area of 29.73% belonging to moderately suitable for mango in Chandanahalli SWS is in accordance with slight to moderate limitations in the form of moderately shallow soil depth leading to restrictive root penetration, drainage, moderate moiture and retention because of sandy clay texture. Large area of SWS (57.83%) has been categorised as marginally suitable with limitations like shallow soil depth which creates unfavourable conditions like restricted root penetrations, low moisture retention and poor support of land for the crop growth with gently sloping lands susceptible for erosion and loss of nutrients. These facts also coincides with low organic carbon status, moderately alkaline pH, and sandy clay texture of soil.

Conclusion

By applying weighted overlay analysis, LRI data can be used to crop specific and site specific suitability assessment. Such information can be an important input for developing decision support system model for judging crop suitability in a subwatershed. Findings of such studies can be extrapolated and used in crop specific land-use planning for large area with similar land and soil characters.

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