IMPACT: International Journal of Research in Applied, Natural and Social Sciences (IMPACT: IJRANSS) ISSN 2321-8851 Vol. 1, Issue 5, Oct 2013, 27-32 © Impact Journals EFFECTS OF PRECIPITATION EFFECTIVENESS ON THE YIELD OF IRISH POTATO (SOLANUM TUBEROSUM) IN JOS-PLATEAU, NIGERIA SOLOMON. Z. WUYEP 1 , AMBROSE A. ZEMBA 2 & C. J. JAHKNWA 3 1 Department of Geography, Plateau State University, Bokkos, Nigeria 2 Department of Geography, Federal University of Technology, Yola, Nigeria 3 Department of Geography Adamawa State Polytechnics, Yola, Nigeria ABSTRACT This study investigates the effect of precipitation effectiveness on the yield of Irish potato in Jos-Plateau, Nigeria. Rainfall data for 20 years (1989–2008) was collected from National Root Crop Research Institute (NRCRI), Vom, Plateau state. Precipitation effectiveness indices including dry spell or drought, onset dates of rains, and hydrologic ratio were derived from and used together with total annual rainfall data to assess their effects on yields of Irish potato. The indices were subjected to various descriptive and inferential statistical techniques, including correlation and regression analysis. While the correlation analysis was employed to determine the degree of relationship between yields and the indices, the regression technique was adopted to identify the factors that are critical in influencing yields. Results reveal that while rainfall total correlates positively with yields, dry spell, onset dates of rains and hydrologic ratio exhibit negative correlation with yields. The correlation between onset dates of rains and yields was found to be statistically significant at 5% while others are insignificant. Step-wise regression analysis identifies onset dates of rains as the most critical factor affecting Irish potato in the area. The coefficient of determination of r 2 =0.219 shows that the index contributes about 22% to variations in the yields of Irish potato on the Plateau. It was recommended that further studies should consider combine effects of climate and other factors like farming practices, soil factors and cultural practices of the farmers so as to generate comprehensive information for better agricultural planning and development in the area. KEYWORDS: Irish Potato, Precipitation Effectiveness, Regression Analysis, Jos-Plateau INTRODUCTION Precipitation effectiveness indices are the major control of crop yield in the West African savannah region. It is not only the total amount of rainfall that matters; but how effective the rain is in terms of its time of occurrence, spread, intensity, frequency, and availability as soil moisture (Adefolalu, 1993). Irish potato plants are particularly sensitive to drought. Jones and Johnson (2000) observed that drought during the time of tuber formation in Irish potato reduces tuber yield greatly. Studies have revealed that the plant requires more frequent supply of water than many other tuber crops. It is the moisture and nutrients that are useful to the plant at sprouting to emergence stage of Irish potato development (Burton, 1989). Sale (1973), Susnochi et al (1978), Zaag and Burton (1978), and Wolfe et al (1983) have conducted studies on water stress analysis on different varieties of Irish potato and reported that the rate of leaf expansion in the plant are slowed down or stopped and leaf variation is shortened as a result of water deficit, which adversely affect the development of the crop. In addition, physiological processes such as leaf diffusive resistance, photosynthetic efficiency and partitioning of assimilations are adversely affected as water deficit increases, thereby negatively influencing the development and yield of Irish potato.