Evolution of land suitability and capability for some crops growth using LSSM model at Al-Gabbanah valley , Ibb , Yemen

This study aimed at finding the productivity of the land and its suitability for the growth of some agricultural crops (field crops, vegetable crops and fruits), namely sorghum, potatoes and figs. The study area was evaluated according to its production capacity according to Sys et al. 1991 for the first and second grades (S1, S2) using a special digital model to illustrate this cartography. These lands were also classified according to their suitability to grow three types of crops, vegetables and fruits according to Sys et al. 1993, and using a special digital model to illustrate this cartography. The studied crops were suitable for cultivation in the Al-Gabbanah valley, as the results showed that sorghum in the ground sectors (1, 4) is very appropriate (S1), while in the ground sectors (2.3) it is moderate suitability (S2). The potato yield showed very appropriate result (S1) in sectors (2,4) and moderate suitability in sectors (1,3). As for the fig crop, the results showed moderate suitability (S2) in all sectors.

Yemen has 19550 km of forests and other wood lands, which constitute almost 4 % of the total land area, (Wikipedia,(13).
Limited information are available on the soils of Yemen. Few soil surveys which were conducted previously were not sufficient or adequately correlated in national or international system to serve development needs. They varied in giving details and required complementary studies to respond to an increasing demand for soil resource information.
Ibb city is the capital of Ibb Governorate ( the area under study). It is situated on mountain ridge, surrounded by fertile land and is known as "the green City". The region of Ibb has many notable mountains such as Ba'dan, which overlooks most of the city. Ibb Governorate has many famous valleys.
Dar AL-Handasah (2) described the four main stratigraphic units outcrop in the Ibb city. These are from younger to older as follows: Quaternary deposits are represented by valley alluvium and terraces which unconformable overly the bedrock at the base of main valleys or on the slope terraces of the mountain ranges, respectively . The alluvial deposits are principally composed of gravel, sand boulders, and large detritus of volcanic rocks, while the terraces deposits are composed of loess with calcareous concentrations, alluvial fans, gravel, silt, loamy sands as well as sandy loam texture.
Ibb has a cool continental climate, varied in the mountainous highlands and mild in the central plains, while it is warm in the southern and western regions .it rains over most parts of the province. A summer seasonal rainfall in most districts reaches 800-1200 mm. The soil moisture regime for Ibb Governorate, according to the SOIL SURVEY Staff (9), is Ustic and/or Udic. The soil temperature regime is classified as Iso-thermic (Bruggeman,(1), fig.(2).
The aim of the current study is to investigate the characteristics and classification of the soils representing the four profile main valley AL-Gabbanah located at directorate Ibb from Yemen. This research could serve as a base management of these soils for sustainable agriculture. Therefore, this study aimes at finding the productivity of the land and its suitability for the growth of some agricultural crops (field crops, vegetable crops and fruits), namely sorghum, potatoes and figs.

Materials and methods:
The studied area is located at Ibb Governorate, Al-Makhader directorate, Yemen Republic, which is about 15 km from Ibb . Al-Gabbanah valley was chosen for the current study around the Ibb city. The studied area is (about 15 km) bounded between latitude 14 04'40.00 N and longitude 44 10'05.00 E, as shown in Figs. (3) and. (4).

Geomorphology and soil mapping using GIS
Geomorphologic map was carried out using digital image processing of land Sat7.0 ETM+ image ( Path/row 166/50) dated 2012, executed using ENVI software 5.0 (ITT, 2012) .Image was stretched using linear 2 %, smoothly filtered, and their histograms were matched according to  LilleSand and Kiefer (7). Image was atmospherically corrected using FLAASH module (ITT (6). GIS works were performed to produce geomorphologic and soil map for the studied area using Arc GIS software 10.1 ( ESRI, (4).

Field Work
Four soil profiles were chosen representing the different valleys and geomorphologic units and morphologically described according to FAO (5). Soils were collected according to the vertical morphological variation and prepared for the different physical and chemical analysis.

Physio-chemical Analyses
Particle size distribution was carried out according to KLUT (3). Electrical conductivity ( EC). pH, organic matter (OM), calcium carbonate (CaCO 3 ), gypsum, cation exchange capacity (CEC) and exchangeable Na percentage (ESP), were determined according to Page et al. (8).   Evaluating and classifying the soil according to its agricultural productivity is essential to narrow the gap between food production and consumption.

Evaluation of land capability:
Quantitative estimation of soil characteristics, namely slope, soil profile depth, drainage, erosion, texture, CaCO 3 , gypsum, salinity and sodicity, were used for evaluating land capability index according to Sys et al., (13). The mapping units were placed into grades according to their calculated capability indexes Table. (3).   (4) shows the values that were were used as a guide in rating the studied soils according to Sys et al (11).The ratings of soil characteristics, capability indexes and soil grades calculated for the mapping unites are presented in Table. (5) and Fig. (6).   The calculated soil capability index in Table. (5) reveals that the investigated soils can be classified into the following grades, Fig. (6).
Conventional methods of land evaluation assume that soil characteristics are homogenous within the land unit and hence it gives a suitability map of discrete value which does not represent the real situation. Therefore, there is a need to develop a method that takes the spatial variability of soil properties into account Fig. (7).
The evaluation of agricultural sustainability status helps in identifying specific indicators that constrain the achievement of sustainable agriculture.

Land Sustainable Spatial Model (LSSM)
LSSM was built according to ESRI (4) as follows: File geo-database was created. Spatial model for environment settings was specified. A series of values for input criteria was calculated resulting in five datasets. Each derived dataset was reclassified to a common measurement scale, giving each range a discrete and integer value between 1 and 4. Higher values were given to attributes within each dataset that are more suitable for sustainability classes. Conditional expressions were used to get sustainability raster classes. Datasets were weighed through setting equal influence with different scale values. Sustainability raster classes were converted into sustainability polygons in the geo-database and Sustainability layers, then, were created. Four suitability classes were selected by attribute (values). Hence, the final layers that represent sustainability classes (I, II, III and N) were resulted Fig. (8).

Model verification
However, quantitative assessment was executed for SLMSM, it is very important to identify and measure the map errors derived from the model. In this assessments, map data were compared with ground truth data obtained from two sources: 1-from field measurements & observations on farming system level and 2-from laboratory analyses that assumed to be 100% correct. The overall accuracy assessment of thematic maps recorded 98.34%.  They have mainly a slight limitation intensity of, sometimes ,erosion, irrigation water and ESP, and both carbonate and gypsum for all soils where profiles belonging to grade III. Also, they have moderate limitation intensity of soil texture.

Evaluation of soil suitability for growing main crops:
Studied soil profiles were evaluated to determine their suitability for growing 15 field crops, vegetable and fruits according to Sys et al., (12). The soil parameters used for estimating suitability index (Si) for the different crops were defined through matching climate condition, slope, soil profile depth, drainage, gravels, texture, CaCo 3 , gypsum, salinity, alkalinity and soil fertility (PH, CEC and O.M) with crop requirements. Land suitability classes are defined according to the value of the suitability index (Si) Suitability indexes and classes were estimated in the mapping units for the major field crops, vegetable and fruits which are shown in Table. (7) and represented in Figs. (9)(10)(11). Suitability class of Al-Gabbanah Soils: Data in Table (7) that illustrated in a Figs. (9)(10)(11) shows that the soils of this valley represented by profiles No's 1,2,3and 4 are very suitable S 1 and/or moderate suitable (S2) for growing sorghum, potato and figs from field crops. Also, the soils are disparately suitable from S 1 to S for growing vegetable and fruit crops. The suitability classes differ from site to another according to their parameters variations. From vegetables and from fruits at all sites of this valley are unsuitable. Also, it is considered as unsuitable at some sites.