RESPONSE OF GROUNDNUT (ARACHIS HYPOGAEA L.) TO RHIZOBIA INOCULATION, NITROGEN AND PHOSPHORUS FERTILIZERS ON AN ALFISOL IN THE NORTHERN GUINEA SAVANNA OF NIGERIA

CHAPTER ONE
1.0 INTRODUCTION
1.1 Background
Groundnuts (Arachis hypogaea L) also known as peanuts, is a leguminous crop, a member of the genus Arachis in the family Leguminosae. It is widely grown in the tropics and subtropics due to its nutritional and economic importance. Groundnut is the most widely grown major legume worldwide cultivated in 118 countries and occupies more than 22.6 million ha that produce about 36.4 million MT, with average yield of about 1600 kg ha-1(Abate et al., 2012).
Groundnut seeds (kernels) contain 40 -50 % fat, 20 – 50 % protein and 10 – 20 % carbohydrates (ICRISAT, 2003). Groundnut seeds are nutritional source of vitamin E, niacin, calcium, phosphorus, magnesium, zinc, iron, ribloflavin, thiamine and potassium (Kumar and Shankar, 2013). In most of the developing countries it provides high-quality cooking oil and is an important source of protein for both human and animal diet and also provides much needed foreign exchange by exporting the kernels and cake. The uses of groundnut plant therefore, make it an excellent cash crop for domestic markets as well as for foreign trade in several developing and developed countries (FAO, 2002).

In Africa, groundnut is grown mainly in Nigeria, Sudan, Senegal, Chad, Ghana, Congo, and Niger. Average productivity is 1720 kg ha-1 in Nigeria which is poor when compared to the USA and other developed countries where it is close to 3500 kg ha -1 (Vara-Prasad et al., 2009). The crop is grown mostly by smallholder farmers under rainfed conditions with limited inputs (Samson, 2012). Majority of the soils of the Guinea savanna of Nigeria are inherently low in fertility especially organic matter, phosphorus and nitrogen (Odunze and Kureh, 2009; Oluwasemire and Alabi, 2004). To address this challenge, farmers in the northern Guinea savanna use strategies such as application of organic and inorganic fertilizers (Samson, 2012). The use of inorganic fertilizers is effective but costly (Obisesan et al., 2013); this therefore, leads to application at sub- optimal level that is, below the recommended rate (Olawale et al., 2009). On the other hand, organic fertilizers, such as crop residues and animal manures are bulky. They contain relatively low concentration of nutrients and handling them is laborious. ICRISAT (1995) recommended an application of 10 -12 t ha-1of chicken manure or 20 t ha-1 of well decomposed farm yard- manure for groundnut production. Wamba et al. (2012) gave the nutrient content of poultry manure as 21.76 N g kg-1, 8.74 P g kg-1 and 11.22 K g kg-1. This however depends on the handling conditions. Much of the N in manure may be lost to the air if they are allowed to dry out or stored for a long time. Unlike cereals, legumes are able to supplement their nitrogen (N) demand and contribute to soil N through various processes. Legumes fix atmospheric nitrogen (N2) in the soil through its symbiotic relationship with N- fixing bacteria. Legumes also add nitrogen to the soil through falling leaf litter and, to a lesser extent by decaying roots and root nodules below- ground, and thus they have great potential for restoring degraded soils. The organic matter produced by legumes is generally rich in nitrogen and of good quality, meaning that it decomposes quickly and is a good source of nitrogen for other plants (Giller, 2010). The ability of legumes to fix N2 allows farmers to grow them with minimal inputs of N fertilizer (Lupwayi et al., 2011). Therefore, farmers usually intercrop non-legumes with leguminous crops including groundnut having considered legumes to stabilize crop yields and also serves as a source of income and protein for their families. In the northern savanna of Nigeria, the bulk of groundnut is produced by small scale farmers using traditional system of mixed
cropping with maize, millet and sorghum. (Samson, 2012).

Nitrogen is considered as the most limiting plant nutrient for crop production in West Africa (Sangakkara et al., 2003). Despite its abundance in the atmosphere, plants are unable to use it  directly because it is present in an inert form (N2) and the nitrogen in the soil is lost through microbial dinitrification, soil erosion, leaching, chemical volatilization, removal of nitrogen containing crop residues from the field. As earlier mentioned, groundnuts like other legumes play a primary role of fixing atmospheric N through their symbiotic relationship with Rhizobium spp., usually associated with the host’s root system. This contributes nitrogenous compounds to the soil, either directly, by nodule excretion, or indirectly, by decomposition of root nodules and tissues (Giller, 2003). Biological Nitrogen Fixation (BNF) is an inexpensive, renewable resource option for smallholder farmers, permitting them to redirect limited farm investment toward other pressing household needs. One of the ways to increase biological nitrogen fixation is by inoculating legume seeds with rhizobium inoculants. Research has also shown that legumes grow best if there is some mineral N available as nodules form and a small amount of starter nitrogen (10 to 30 kg ha-1) at planting may increase total BNF over the crop‟s lifetime (Woomer, 2010). This starter dose of nitrogen is necessary due to the lag period between rhizobium colonization and the onset
of nodule functioning. The starter dose however, increases yield only on soils that are extremely deficient in nitrogen and where crop yield potential is high ( Kucey, 1989;Woomer, 2010).