Soil Characterization and Land Suitability Evaluation to Cereal Crops in Yigossa Watershed, Northwestern Ethiopia

A study was conducted in 2013 at Yigossa watershed, Northwestern Ethiopia to characterize the soils using FAO criteria and evaluate their suitability to cereal crop producton. Surface soil samples from different soil types were collected and analyzed in the soil laboratory of Amhara Design and Supervision Works Enterprise. The collected soils were analyzed for soil pH, organic matter, total N and available P. FAO guideline was used to evaluate the land suitability for selected cereal crops and geographical information system (GIS) was employed to analyze and map the soils within the watershed. Results indicated that the soils of the watershed were dominantly slightly acidic to slightly alkaline with pH ranging from 5.60 to 7.87. The organic matter content ranged from 1.88% in Nitisols to 3.56% in Gleysols and Regosols; while the total N content ranged from 0.09% in Nitisols to 0.18% in Gleysols and Regosols. Available P contents varied from 1.78 mg kg in Nitisols to 12.3 mg kg in Regosols. Analysis of land suitability analysis showed that 60.9% of Nitisols, 38.2% of Gleysols, 36.1% of Luvisols and 23.4% of Regosols were moderately suitable for maize (Zea mays L.); while 32.5% of the Nitisols, 50.6% of Gleysols, 73.0% of Luvisols and 73.9% of Regosols were moderately suitable for finger millet (Eleusine coracana L.). Moreover, 51.4% of Nitisols, 38.2% of Gleysols, 68.7% of Luvisols and 60.2% of Regosols were moderately suitable for teff (Eragrostis tef Zucc.); whereas 32.5% of Nitisols, 50.6% of Gleysols, 73.0% of Luvisols and 73.8% of Regosols were moderately suitable for rice (Oryza sativa L.). From the result of the study it was possible to conclude that different soils are variably suitable for different crops. In some cases, the same soil could be suitable for different crops bringing competing nature of crop land use types for the same parcel of land. However, yield per unit of land and return to investment dictates farmers’ decision on land allocation for a particular use.


Introduction
Ethiopia has a considerable land resource for agriculture.About 73.6 million ha (66%) of the country's area is potentially suitable for agriculture (Fasil, 2002) and the Ethiopian agricultural sector has a proven potential to increase food supplies faster than the growth of the population (Davidson, 1992).Crop production plays a vital role in generating surplus capital to speed up the overall socio-economic conditions of the farmers.However, the country is unable to feed its people due to various bio-physical and socio-economic constraints and policy disincentives.Land degradation manifested in the form of soil erosion is one of the major bio-physical constraints.
Soil degradation is becoming a major policy challenge in Ethiopia not only for increasing crop productivity but also for maintaining soil resource base for the future generation.Accordingly, the potential of the land for crop production to sustainably satisfy the ever increasing food demand of the increasing population is declining as a result of severe soil degradation (Lal, 1994).The net soil loss from cultivated fields due to erosion ranged from 20 to 100 t ha -1 per year -1 , with corresponding annual productivity loss of 0.1 to 2% of total production (Hurni, www.ccsen 1993).G. Y N (AN) a Ethiopia to (32.22

Soil Ch
The chemi from 5.6 slightly alk  (Yihenew, 2002;Getachew & Heluf, 2007;Abebe & Endalkachew, 2012).Slightly alkaline nature of Gleysols at soil surface could be attributed to the influx of the exchangeable bases particularly Ca 2+ and Mg 2+ by alluvial-colluvial depositions from adjacent upper slopes and less mineralization rate of organic matter.Moderate acidity of Nitisols in the study watershed could be caused by one or a combination of factors such as crop removal, continuous cultivation practices, leaching of basic cations, continuous use of only two acidifying fertilizers (urea and di-ammonium phosphate) coupled with the decomposition of organic residues (Landon, 1991;Ahmed, 2002).According to FAO (1998), moderately acidic soils may be deficient in phosphorus, calcium, magnesium and molybdenum.
The AP content was generally medium throughout the study watershed according to Landon (1991).This might be due to the influence of organic matter and added P containing fertilizers (Sharma et al., 2008).However, low AP in Gleysols might be due to severe leaching from the excessive moisture content observed in these soils.Nitisols in Ethiopia are reported to have low available P due to their inherently low P content, high P fixation by Al and Fe and complete removal of the biomass from the fields (Birru & Heluf, 2003;Abebe & Endalkachew, 2012).Studies made in Liberia also showed that acidic soils are generally deficient in AP (Getachew & Sommer, 2000).This soils would most likely respond to judicious phosphorus containing fertilizer application (FAO, 1998).
The OM content of surface soils ranged from 1.88 to 3.56% and TN varied from 0.09 to 0.18%.TN contents of soils generally ranged from less than 0.02% in sub-soils to greater than 2.5% in peat soils (Tisdale et al., 2002).Medium to high OM and TN contents at surface layer could be attributed to poor drainage condition of the soil that reduces the rate of organic matter decomposition; and the addition of farmyard manure and plant residues that are rich in carbon and nitrogen.Similarly, higher organic carbon and nitrogen in Nitisols and Luvisols were reported by Ahmed (2002); Yihenew (2002) and Getachew and Heluf (2007).Conversely, high organic matter and low nitrogen contents in Nitisols (Shimeles et al., 2006) and low TN in Luvisols (Belay, 1996) were reported in the highlands of Ethiopia.

Land Suitability Evaluation
The results of land suitability assessment showed that the four soil types available in the watershed varied in suitability for cereal crops.They were moderately suitable (S2), marginally suitable (S3), and currently not suitable (N) depending on the site prospected.Because of one or more limitation(s) of the land characteristics, none of the area in the study watershed fallen in to highly suitable (S1) class for the selected crops.As shown in Table 2, from the total area covered by Nitisols, 60.93% was moderately suitable for maize, 32.49% for finger millet, 51.39% for teff and 32.49% for rice cultivation.This is in line with the report of Henok (2010) and Kassa and Mulu (2012) in the Tigray Region.Woubet et al. (2013) also found that 40% of similar soils of Anjeni watershed were moderately suitable for maize and teff.A study in India on similar soils also showed that 50.28% of area was marginally suitable for maize cropping while 24.34% was highly suitable and 6.34% was moderately suitable (Mustafa et al., 2011).The major suitability limitations reported for these soils were low organic matter and nitrogen contents and acidic pH.Another study in Iran also indicated that soils with high pH, low organic matter, high salinity and alkalinity were non-suitable for maize production (Ali et al., 2008).
The current study also indicated that 36.13% of Livisols were moderately suitable for maize, 72.95% form finger millet, 68.73% for teff and 72.95% for rice production.The major limitation for teff cultivation was low available P and short length of growing period (LGP).This result partially disagrees with the conclusion of Rabia (2012) in the Kilte Awulaelo district (Tigray Region) where 3.6% of these soils were highly suitable for teff and slope had the major impact on its suitability.A study in India also showed that finger millet was extensively cultivated in nutrient poor soils (Mustafa et al., 2011).High pH was, however, the major limitation which excluded about 14.78% of the area from cultivation of finger millet.Similarly,38.22,50.59,38.22 and 50.59% of Gleysols were moderately suitable for maize, finger millet, teff and rice, respectively.Low available P in all soils and pH in Gleysols were the major amendable limitations.Similar study indicated that rice was highly suitable in wetland soils of Nigeria (Olaleye et al., 2002).However, the major limitations to rice cultivation could be low organic carbon and available P contents.Moreover,23.38,73.81,60.23 and 73.81% of Regosols were moderately suitable for maize, finger millet, teff and rice, respectively.However, large area of the land (44.59%) in these soils was found to be currently unsuitable for maize production.As per the observation made in the watershed, the main reason could be shallow soil depth and low water holding capacity of the soil.Currently not suitable 35.37 44.59 0.00 0.00 0.00 0.00 0.00 0.00 It was found that 30.98% of the study area was moderately and 57.55% marginally suitable for all crops (Table 3).In fact, Nitisols and Regosols were moderately suitable for all crop production, while Luvisols were marginally suitable.Gleysols were moderately suitable for maize, finger millet, teff and rice production.This indicates competing nature of crop land use types for the same land.Available P for all soils and pH for Nitisols, Luvisols and Gleysols were found to be limiting factors that should be optimized for sustainable and increased crop production and for effective use of the soils of the study watershed.

Conclusions
The study has produced map on the suitability of the different soil types of Yigossa watershed that will allow growing the right cereal crop at the right site for optimum yield and optimum return to investment for each of maize, finger millet, teff and rice.Proper agricultural practices such as liming and addition of organic matter would contribute to improving the availability of phosphorus and modifying soil pH.

Table 1 .
Soil types and their chemical characteristics in the study watershed

Table 3 .
Land suitability for major cereal crops