Merit Research Journal of Agricultural Science and Soil Sciences (ISSN: 2350-2274) Vol. 3(10) pp. 159-167, November, 2015 Available online http://meritresearchjournals.org/asss/index.htm Copyright © 2015 Merit Research Journals Original Research Article Evaluation of organic carbon, available phosphorus, and available potassium as a measure of soil fertility Roland Clement Abah 1, 3 * and Brilliant Mareme Petja 2, 3 Abstract 1 National Agency for the Control of AIDS, Central Area, Abuja, Nigeria 2 Water Research Commission (WRC), Rietfontein Pretoria, South Africa 3 University of South Africa, Pretoria, South Africa *Correspondence Author’s Email: rolann04@yahoo.com This study evaluated organic carbon, available phosphorus, and available potassium as a measure of soil fertility in the Lower River Benue Basin. Soil samples were collected from farms under cultivation and analysed for physical, chemical, and micro-nutrient properties. Soil fertility status was derived using the soil reaction index, and nutrient index with respect to organic carbon, available phosphorus and available potassium, based on the specific rating chart. The soil irrigation quality was assessed by calculating the Exchangeable Sodium Percentage (ESP) and Sodium Absorption Ratio (SAR). Based on the soil rating chart, most of the soils sampled had medium (30.6%) to high (55.6%) percentages of organic carbon. Only 13.9% of the samples had organic carbon percentages categorised as low. Most of the soils sampled had low amounts of phosphorus (50%) and low amounts of potassium (86.11%). A good percentage (36.11%) of the samples had high content of available phosphorus. The soil ESP and SAR results indicated fertile soils and good irrigation quality. The study has made useful recommendations for farmers and encourages agricultural extension workers to efficiently focus on educating farmers on appropriate crop varieties for cultivation; the best methods to enhance soil nutrients; and the importance of irrigation farming. Keywords: Available Phosphorus, Available Potassium, Organic Carbon, Soil Fertility, Soil Micro-Nutrients, Soil Nutrients INTRODUCTION Agriculture has become a large economic and livelihood venture globally. The current global focus is to make agriculture sustainable. The sustainability of agricultural production systems have become a major concern of agricultural researchers and policy makers in both developed and developing countries (Rossiter, 1995; Medugu, 2006; IITA, 2008; Alademerin and Adedeji, 2010; Rosenberg, 2012). In order to achieve an effective sustainable plan for the development of agriculture, it is important to determine the available potential that exists through land studies. Especially since agriculture is a significant income earner in many parts of the world. The agricultural sector gross domestic product (GDP) growth rate in Nigeria is the highest contributor to non-oil GDP growth rate. After an initial dip from 6.64% in 2003 to 6.50% in 2004, the growth rate appreciated per annum from 2004 (7.06%) to 7.43% in 2007 (Azih, 2011). With increases in crop yield, agriculture has become an important contributor to the Nigerian economy in the past decade despite the predominance of the oil sector (Oji- Okoro, 2011). This is reflected in the status of Nigeria in the world ranking of crop production by the Food and Agricultural Organisation (FAO). Nigeria ranks high in the production of several crops and is the highest producer of cassava and yam in the world (FAO, 2013; 2015). According to the Benue State Ministry of Agriculture