AS  Vol.12 No.11 , November 2021
Sustainability Assessment of Smallholder Farms in the Savannah Transition Agro-Ecological Zone of Ghana
Abstract: The ever-increasing human population has resulted in political pressure to increase crop production. Currently, smallholder farmers are expected to be environmentally considerate, adapt to extreme climatic conditions and deal with financial instability. Despite these recent developments, farmers and their farming activities in these rural environs of developing countries need to improve household food and income security on a sustainable basis. A minimum data set selected from extensive data was used to determine indicators for soil sustainability assessment. This method involved expert opinion and statistical data reduction techniques. The results indicated that SOC, MBC, BS, EC, Spo and sand were the most important variables selected as MDS using PC analysis. Forest and Grassland had a high sustainability index (SI) while Savannah woodland, Fallow and Cropland were rated not sustainable (NS). For Cumulative rating using the total dataset (CR-TDS), Forest had a high sustainability index (HS), Savannah woodland and Grassland were sustainable (S) compared to Fallow and Cropland, which were sustainable with high input (SWHI). Also, for cumulative rating using the minimum dataset (CR-MDS), Forest, Savannah woodland and Grassland had high sustainability (SH) indices compared to Fallow and Cropland, which were considered sustainable with high input (SWHI). Sustainability index (SI) and Cumulative rating (CR) using the total dataset (CR-TDS) had a strong correlation (R2 = 0.91, p < 0.05) compared to SI versus CR-MDS (R2 = 0.44, p < 0.05) and CR-TDS versus CR-MDS (R2 = 0.60, p < 0.05). These methods consider that these management goals dictate soil functions affecting the selection of indicators. Using PCA and variance analysis, silt, clay, EC, SOC, MBC and CEC relating to soil texture and fertility were identified as the most influential (sensitive) indicators for soil sustainability assessment. The selected soil attributes can serve as target indicators for soil fertility restoration, erosion control and management in the Nkoranza district. Therefore, a farm-level sustainability index for small-scale and commercial farms is proposed based on readily available data for the Savannah Transition Agro-ecozone of Ghana.
Cite this paper: Awoonor, J. , Yeboah, E. , Dogbey, B. and Adiyah, F. (2021) Sustainability Assessment of Smallholder Farms in the Savannah Transition Agro-Ecological Zone of Ghana. Agricultural Sciences, 12, 1185-1214. doi: 10.4236/as.2021.1211076.

[1]   Food and Agriculture Organization of the United Nations (2015) FAOSTAT Database. FAOSTAT Database Collections, Rome.

[2]   Godfray, H.C.J., Beddington, J.R., Crute, I.R., Haddad, L., Lawrence, D., Muir, J.F. and Toulmin, C. (2010) Food Security: The Challenge of Feeding 9 Billion People. Science, 327, 812-818.

[3]   Mungai, L.M., Snapp, S., Messina, J.P., Chikowo, R., Smith, A., Anders, E. and Li, G. (2016) Smallholder Farms and the Potential for Sustainable Intensification. Frontiers in Plant Science, 7, 1720.

[4]   Dirzo, R. and Raven, P.H. (2003) Global State of Biodiversity and Loss. Annual Review of Environment and Resources, 28, 137-167.

[5]   Legg, W. (2006) Policy Efforts to Achieve Sustainable Agriculture: An OECD Perspective. In: Brouwer, F. and McCarl, B.A., Eds., Agriculture and Climate beyond 2015, Environment & Policy, Vol. 46, Springer, Dordrecht, 265-277.

[6]   Gibbon, D. (2012) Save and Grow: A Policymaker’s Guide to the Sustainable Intensification of Smallholder Crop Production. Rome, Italy: Food and Agriculture Organization of the United Nations (2011), pp. 112, US$45.00. ISBN 978-92-5-106871-7. Experimental Agriculture, 48, 154.

[7]   Frater, P. and Franks, J. (2013) Measuring Agricultural Sustainability at the Farm-Level: A Pragmatic Approach. International Journal of Agricultural Management, 2, 207-225.

[8]   AGRA (2018) Africa Agriculture Status Report: Catalyzing Government Capacity to Drive Agricultural Transformation (Issue 6). Nairobi, Kenya.

[9]   Andrews, S.S. and Carroll, C.R. (2001) Designing a Soil Quality Assessment Tool for Sustainable Agroecosystem Management. Ecological Applications, 11, 1573-1585.[1573:DASQAT]2.0.CO;2

[10]   Edwards, C.A., Grove, T.L., Harwood, R.R. and Colfer, C.P. (1993) The Role of Agroecology and Integrated Farming Systems in Agricultural Sustainability. Agriculture, Ecosystems and Environment, 46, 99-121.

[11]   Pretty, J.N. (1995) Regenerating Agriculture: Policies and Practice for Sustainability and Self-Reliance. Earthscan Publications, London.

[12]   Pretty, J.N. (1995) Participatory Learning for Sustainable Agriculture. World Development, 23, 1247-1263.

[13]   Goswami, R., Saha, S. and Dasgupta, P. (2017) Sustainability Assessment of Smallholder Farms in Developing Countries. Agroecology and Sustainable Food Systems, 41, 546-569.

[14]   Bechini, L. and Castoldi, N. (2009) On-Farm Monitoring of Economic and Environmental Performances of Cropping Systems: Results of a 2-Year Study at the Field Scale in Northern Italy. Ecological Indicators, 9, 1096-1113.

[15]   Abraham, M. and Pingali, P. (2020) Transforming Smallholder Agriculture to Achieve the SDGs. In: Gomez y Paloma, S., Riesgo, L. and Louhichi, K., Eds., The Role of Smallholder Farms in Food and Nutrition Security, Springer, Cham, 173-209.

[16]   Pretty, J. (2008) Agricultural Sustainability: Concepts, Principles and Evidence. Philosophical Transactions of the Royal Society B: Biological Sciences, 363, 447-465.

[17]   Carter, M.R. (2002) Soil Quality for Sustainable Land Management: Organic Matter and Aggregation Interactions That Maintain Soil Functions. Agronomy Journal, 94, 38-47.

[18]   Warkentin, B.P. (1995) The Changing Concept of Soil Quality. Journal of Soil and Water Conservation, 50, 226-228.

[19]   Miller, F.P. and Wali, M.K. (1995) Soils, Land Use and Sustainable Agriculture: A Review. Canadian Journal of Soil Science, 75, 413-422.

[20]   Ghaemi, M., Astaraei, A.R., Emami, H., Nassiri Mahalati, M. and Sanaeinejad, S.H. (2014) Determining Soil Indicators for Soil Sustainability Assessment Using Principal Component Analysis of Astan Quds-East of Mashhad-Iran. Journal of Soil Science and Plant Nutrition, 14, 1005-1020.

[21]   Gomez, A.A., Kelly, D.E.S., Syers, J.K. and Coughlan, K.J. (1997) Measuring Sustainability of Agricultural Systems at the Farm Level. In: Doran, J.W. and Jones, A.J., Eds., Methods for Assessing Soil Quality, Vol. 49, The Soil Science Society of America, Inc., Madison, WI, 401-410.

[22]   Shukla, M.K., Lal, R. and Ebinger, M. (2004) Principal Component Analysis for Predicting Corn Biomass and Grain Yields. Soil Science, 169, 215-224.

[23]   Singh, M.J. and Khera, K.L. (2009) Physical Indicators of Soil Quality in Relation to Soil Erodibility under Different Land uses. Arid Land Research and Management, 23, 152-167.

[24]   Dhaliwal, S.S., Sharma, B.D. and Bijay, S. (2009) Micronutrient Status of Different Land Use Systems in Relation to Soil Quality and Sustainability under Different Watersheds in Submontaneous Tract of Punjab. Annals of Arid Zone, 48, 103-112.

[25]   Lal, R. (2004) Soil Carbon Sequestration Impacts on Global Climate Change and Food Security. Science, 304, 1623-1627.

[26]   Ghana Statistical Service (2014) 2010 Population and Housing Census. District Analytical Report-Nkoranza North District. Accra, Ghana.

[27]   Ghana Statistical Service (2014) 2010 Population and Housing Census. District Analytical Report-Nkoranza South Municipality. Accra, Ghana.

[28]   Ghana Statistical Service (2010) 2010 Population and Housing Census. Accra, Ghana.

[29]   Peel, M.C., Finlayson, B.L. and McMahon, T.A. (2007) Updated World Map of the Köppen-Geiger Climate Classification. Hydrology and Earth System Sciences, 11, 1633-1644.

[30]   Kasei, C.N. (1990) A Synopsis on the Climate of the North of Ghana. 2nd Workshop on Improving Farming Systems in the Savanna Zone of Ghana, 24 April 1990.

[31]   Dickson, K.B. and Benneh, G. (1995) A New Geography of Ghana. 3rd Edition, Longman Group Limited, Essex, England.

[32]   Christensen, E. and Awadzi, T.W. (2000) Water Balance in a Moist Semi-Deciduous Forest of Ghana. West African Journal of Applied Ecology, 1, 11-22.

[33]   Walker, H.O. (1962) Weather and Climate. In: Wills, J.B., Ed., Agriculture and Land use in Ghana, Ministry of Food and Agriculture, Oxford University Press, London, 3-50.

[34]   Smith, G.K. (1962) Report on Soil and Agricultural Survey of Sene-Obosum River Basins, East Brong-Ahafo and Ashanti Regions, Ghana. US Agency for International Development.

[35]   Bationo, B.A., Tabo, R., Waswa, B.S., Okeyo, J., Kihara, J., Fosu, M. and Kabore, S. (2008) Synthesis of Soil Water and Nutrient Management Research in the Volta Basin. Ecomedia Limited, Nairobi.

[36]   Junner, N.R. and Hirst, T. (1946) The Geology and Hydrogeology of the Volta Basin. Gold Coast Geological Survey, Memoir, 8, 837-854.

[37]   IUSS Working Group WRB (2015) International Soil Classification System for Naming Soils and Creating Legends for Soil Maps. World Reference Base for Soil Resources 2014, Update 2015, 106.

[38]   Adu, S.V. and Mensah-Ansah, J.A. (1995) Soils of the Afram Basin, Ashanti and Eastern Regions, Ghana. Memoir No. 12, Soil Research Institute, Ghana.

[39]   Asiamah, R.D. (2008) Soil Resources in Ghana. Synthesis of Soil, Water and Nutrient Management Research in the Volta Basin. Ecomedia Ltd., Nairobi, 25-41.

[40]   Brand, B. and Brammer, H. (1956) Provisional Grassland Associations of Interior and Coastal Savannah Zones of Gold Coast. Gold Coast Department of Soil and Land-Use Survey. Technical Report No. 21, 17.

[41]   Aynekulu, E., Carletto, C., Gourlay, S. and Shepherd, K. (2016) Soil Sampling in Household Surveys: Experience from Ethiopia. The World Bank, Washington, DC and World Agroforestry Centre, Nairobi, 26, 17-18.

[42]   Gourlay, S., Aynekulu, E., Shepherd, K., Carletto, C., Gourlay, S., Aynekulu, E. and Carletto, C. (2017) Collecting the Dirt on Soils: Advancements in Plot-Level Soil Testing and Implications for Agricultural Statistics (No. 8057). The World Bank.

[43]   Vagen, T.G., Winowiecki, L.A., Walsh, M.G., Tamene, L. and Tondoh, J.E. (2010) Land Degradation Surveillance Framework (LSDF): Field Guide. International Center for Tropical Agriculture, World Agroforestry Centre, and the Earth Institute at Columbia University.

[44]   Adjei-Gyapong, T. and Asiamah, R.D. (2002) The Interim Ghana Soil Classification System and Its Relation with the World Reference Base Soil Resources. Rapport sur les Ressources en Sols du Monde (FAO).

[45]   Gardner, W.H. (1986) Water Content. In: Klute, A., Ed., Methods of Soil Analysis: Part 1 Physical and Mineralogical Methods, 5.1, American Society of Agronomy, Inc. Soil Science Society of America, Inc., Madison, WI, 493-544.

[46]   Blake, G.R. and Hartge, K.H. (1986) Bulk Density. In: Arnold Klute, Ed., Methods of Soil Analysis: Part 1 Physical and Mineralogical Methods, 5.1, The American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, WI, 363-375.

[47]   Bouyoucos, G.J. (1962) Hydrometer Method Improved for Making Particle Size Analyses of Soils. Agronomy Journal, 54, 464-465.

[48]   Rowell, D.L. (2014) Soil Science: Methods and Applications. Routledge, London.

[49]   Jackson, M.L. (1973) Soil Chemical Analysis. Prentice Hall of India Pvt. Ltd., New Delhi, 498 p.

[50]   Bremner, J. and Mulvaney, C. (1982) Nitrogen—Total. In: Page, A.L., Ed., Methods of Soil Analysis. Part 2: Chemical and Microbiological Properties, American Society of Agronomy, Madison, WI, 595-624.

[51]   Olsen, S.R. (1954) Estimation of Available Phosphorus in Soils by Extraction with Sodium Bicarbonate (No. 939). US Department of Agriculture.

[52]   Sparks, D.L., Page, A.L., Helmke, P.A. and Loeppert, R.H., Eds. (2020) Methods of Soil Analysis, Part 3: Chemical Methods, Vol. 14. John Wiley & Sons, Hoboken.

[53]   Thomas, G.W. (1983) Exchangeable Cations. In: Page, A.L., Ed., Methods of Soil Analysis: Part 2 Chemical and Microbiological Properties, 9.2.2, American Society of Agronomy, Madison, WI, 159-165.

[54]   Sparling, G.P. and Ross, D.J. (1993) Biochemical Methods to Estimate Soil Microbial Biomass: Current Developments and Applications. In: Mulangoy, K. and Merckx, R., Eds., Soil Organic Matter Dynamics and Sustainability of Tropical Agriculture. Wiley, Chichester, 21-37.

[55]   Dunteman, G.H. (1989) Principal Components Analysis (No. 69). Sage, Thousand Oaks, CA.

[56]   Kaiser, H.F. (1960) The Application of Electronic Computers to Factor Analysis. Educational and Psychological Measurement, 20, 141-151.

[57]   Wander, M.M. and Bollero, G.A. (1999) Soil Quality Assessment of Tillage Impacts in Illinois. Soil Science Society of America Journal, 63, 961-971.

[58]   Andrews, S.S., Karlen, D.L. and Mitchell, J.P. (2002) A Comparison of Soil Quality Indexing Methods for Vegetable Production Systems in Northern California. Agriculture, Ecosystems & Environment, 90, 25-45.

[59]   Andrews, S.S., Mitchell, J.P., Mancinelli, R., Karlen, D.L., Hartz, T.K., Horwath, W.R. and Munk, D.S. (2002) On-Farm Assessment of Soil Quality in California’s Central Valley. Agronomy Journal, 94, 12-23.

[60]   Lal, R. (1994) Sustainable Land use Systems and Soil Resilience. In: Greenland, D.J. and Szabolcs, I., Eds., Soil Resilience and Sustainable Land Use, CAB International Publishers, Wallingford, 41-67.

[61]   Adeyolanu, O.D. and Ogunkunle, A.O. (2016) Comparison of Qualitative and Quantitative Approaches to Soil Quality Assessment for Agricultural Purposes in South-Western Nigeria. Cogent Food & Agriculture, 2, Article ID: 1149914.

[62]   Nielsen, D.R. and Bouma, J. (1985) Soil Spatial Variability: Proceedings of a Workshop of the ISSS and the SSSA, Las Vegas, USA/Pdc296. Center Agricultural Pub and Document, Pudoc, Wageningen, The Netherlands.

[63]   Kassa, H., Dondeyne, S., Poesen, J., Frankl, A. and Nyssen, J. (2017) Impact of Deforestation on Soil Fertility, Soil Carbon and Nitrogen Stocks: The Case of the Gacheb Catchment in the White Nile Basin, Ethiopia. Agriculture, Ecosystems & Environment, 247, 273-282.

[64]   Xu, M., Zhao, Y., Liu, G. and Wilson, G.V. (2006) Identification of Soil Quality Factors and Indicators for the Loess Plateau of China. Soil Science, 171, 400-413.

[65]   Tan, Z., Tieszen, L.L., Tachie-Obeng, E., Liu, S. and Dieye, A.M. (2009) Historical and Simulated Ecosystem Carbon Dynamics in Ghana: Land Use, Management, and Climate. Biogeosciences, 6, 45-58.

[66]   Ray, S.K., Bhattacharyya, T., Reddy, K.R., et al. (2014) Soil and Land Quality Indicators of the Indo-Gangetic Plains of India. Current Science, 107, 1470-1486.

[67]   Mueller, L., Schindler, U., Mirschel, W., et al. (2010) Assessing the Productivity Function of Soils. A Review. Agronomy for Sustainable Development, 30, 601-614.

[68]   Mueller, L., Shepherd, G., Schindler, U., et al. (2013) Evaluation of Soil Structure in the Framework of an Overall Soil Quality Rating. Soil and Tillage Research, 127, 74-84.

[69]   Moncada, M.P., Gabriels, D. and Cornelis, W.M. (2014) Data-Driven Analysis of Soil Quality Indicators Using Limited Data. Geoderma, 235, 271-278.

[70]   Vasu, D., Singh, S.K., Ray, S.K., et al. (2016) Soil Quality Index (SQI) as a Tool to Evaluate Crop Productivity in Semi-Arid Deccan Plateau, India. Geoderma, 282, 70-79.

[71]   Lal, R. (2016) Why Carbon Sequestration in Agricultural Soils. In: Kimble, J.M., Lal, R. and Follett, R.F., Eds., Agricultural Practices and Policies for Carbon Sequestration in Soil, CRC Press, Boca Raton, 45-54.

[72]   Bhattacharyya, T., Pal, D.K. and Easter, M. (2007) Modelled Soil Organic Carbon Stocks and Changes in the Indo-Gangetic Plains, India from 1980 to 2030. Agriculture, Ecosystem, Environment, 122, 84-94.

[73]   Bationo, A., Kihara, J., Vanlauwe, B., Waswa, B. and Kimetu, J. (2007) Soil Organic Carbon Dynamics, Functions and Management in West African Agro-Ecosystems. Agricultural Systems, 94, 13-25.

[74]   Hartemink, A.E. (2006) Soil Fertility Decline: Definitions and Assessment. In: Encyclopedia of Soil Science, Vol. 2, Taylor and Francis, New York, 1618-1621.

[75]   Hartemink, A.E. (2006) Assessing Soil Fertility Decline in the Tropics Using Soil Chemical Data. Advances in Agronomy, 89, 179-225.

[76]   Bekunda, M., Sanginga, N. and Woomer, P.L. (2010) Restoring Soil Fertility in Sub-Sahara Africa. Advances in Agronomy, 108, 183-236.

[77]   Brady, N.C. and Weil, R. (2008) Nature and Properties of Soils. 14th Edition, Pearson Education, Upper Saddle River, NJ.

[78]   Takoutsing, B., Weber, J., Aynekulu, E., Martín, J.A.R., Shepherd, K., Sila, A. and Diby, L. (2016) Assessment of Soil Health Indicators for Sustainable Production of Maize in Smallholder Farming Systems in the Highlands of Cameroon. Geoderma, 276, 64-73.

[79]   Triantafyllidis, V., Kosma, A.K.C. and Patakas, A. (2018) An Assessment of the Soil Quality Index in a Mediterranean Agro Ecosystem. Emirates Journal of Food and Agriculture, 30, 1042-1050.

[80]   Yao, R., Yang, J., Gao, P., Zhang, J. and Jin, W. (2013) Determining Minimum Data Set for Soil Quality Assessment of Typical Salt-Affected Farmland in the Coastal Reclamation Area. Soil and Tillage Research, 128, 137-148.

[81]   Karlen, D.L., Wienhold, B.J., Kang, S., Zobeck, T.M. and Andrews, S.S. (2011) Indices for Soil Management Decisions. In: Hatfield, J.L. and Sauer, T.J., Eds., Soil Management: Building a Stable Base for Agriculture, American Society of Agronomy and Soil Science Society of America, Madison, WI, 39-50.

[82]   Arévalo-Gardini, E., Canto, M., Alegre, J., Loli, O., Julca, A. and Baligar, V. (2015) Changes in Soil Physical and Chemical Properties in Long Term Improved Natural and Traditional Agroforestry Management Systems of Cacao Genotypes in Peruvian Amazon. PLoS ONE, 10, e0132147.

[83]   Takoutsing, B., Weber, J.C., Tchoundjeu, Z. and Shepherd, K. (2016) Soil Chemical Properties Dynamics as Affected by Land Use Change in the Humid Forest Zone of Cameroon. Agroforestry Systems, 90, 1089-1102.

[84]   Emami, H., Neyshabouri, M.R. and Shorafa, M. (2012) Relationships between Some Soil Quality Indicators in Different Agricultural Soils from Varamin, Iran. Journal of Agricultural Science and Technology, 14, 951-959.

[85]   Govaerts, B., Sayre, K.D. and Deckers, J. (2006) A Minimum Data Set for Soil Quality Assessment of Wheat and Maize Cropping in the Highlands of Mexico. Soil and Tillage Research, 87, 163-174.

[86]   Mandal, U.K., Warrington, D.N., Bhardwaj, A.K., Bar-Tal, A., Kautsky, L., Minz, D. and Levy, G.J. (2008) Evaluating Impact of Irrigation Water Quality on a Calcareous Clay Soil Using Principal Component Analysis. Geoderma, 144, 189-197.

[87]   Liska, A.J. and Perrin, R.K. (2011) Energy and Climate Implications for Agricultural Nutrient Use Efficiency. In: Clay, D.E. and Shanahan, J.F., Eds., GIS Applications in Agriculture, Vol. Two: Nutrient Management for Energy Efficiency, CRC Press, Boca Raton, 1-17.

[88]   Acton, D.F. and Gregorich, L.J. (1995) The Health of Our Soils—Towards Sustainable Agriculture in Canada. Centre for Land and Biological Resources Research, Research Branch, Agriculture and Agri-Food Canada, Ottawa, 138.

[89]   Karlen, D.L., Mausbach, M.J., Doran, J.W., Cline, R.G., Harris, R.F. and Schuman, G.E. (1997) Soil Quality: A Concept, Definition, and Framework for Evaluation (A Guest Editorial). Soil Science Society of America Journal, 61, 4-10.

[90]   Mosier, A.R. (1998) Soil Processes and Global Change. Biology and Fertility of Soils, 27, 221-229.

[91]   Vance, G.F., Pierzynski, G.M. and Sims, J.T. (1994) Soil and Environmental Quality. Lewis Publishers, Boca Raton, FL.

[92]   Bouma, J. (2014) Soil Science Contributions towards Sustainable Development Goals and Their Implementation: Linking Soil Functions with Ecosystem Services. Journal of Plant Nutrition and Soil Science, 177, 111-120.

[93]   Bouma, J., Keesstra, S. and Cerdà, A. (2017) The Importance of Soil Science to Understand and Remediate Land Degradation and Desertification Processes. 19th EGU General Assembly, EGU2017, Vienna, Austria, 23-28 April 2017, Article ID: 16112.

[94]   Bouma, J., Van Ittersum, M.K., Stoorvogel, J.J., Batjes, N.H., Droogers, P. and Pulleman, M.M. (2017) Soil Capability: Exploring the Functional Potentials of Soils. In: Field, D.J., Morgan, C.L.S. and McBratney, A.B., Eds., Global Soil Security, Springer, Cham, 27-44.

[95]   Fine, A.K., van Es, H.M. and Schindelbeck, R.R. (2017) Statistics, Scoring Functions, and Regional Analysis of a Comprehensive Soil Health Database. Soil Science Society of America Journal, 81, 589-601.

[96]   Magdoff, F. and Van Es, H. (2000) Building Soils for Better Crops. Sustainable Agriculture Network, Beltsville.

[97]   Frost, P.S. (2017) Soil Health Characterization of Agricultural Land in Jharkhand. Cornell University, India. Thesis, Cornell University, Ithaca, NY.

[98]   Granatstein, D. and Bezdicek, D.F. (1992) The Need for A Soil Quality Index: Local and Regional Perspectives. American Journal of Alternative Agriculture, 7, 12-16.

[99]   Romig, D.E., Garlynd, M.J., Harris, R.F. and Mcsweeney, K. (1995) How Farmers Assess Soil Health and Quality. Journal of Soil and Water Conservation, 50, 229-236.

[100]   Andrews, S.S., Karlen, D.L. and Cambardella, C.A. (2004) The Soil Management Assessment Framework: A Quantitative Soil Quality Evaluation Method. Soil Science Society of America Journal, 68, 1945-1962.

[101]   Doran, J.W. and Parkin, T.B. (1994) Defining and Assessing Soil Quality. In: Doran, J.W., Coleman, D.C., Bezdicek, D.F. and Stewart, B.A., Eds., Defining Soil Quality for a Sustainable Environment, Vol. 35, The Soil Science Society of America, Inc., Madison, WI, 1-21.