Sustainable Land Management for Mitigating Climate Change

C. Integrated Nutrient Management

62. Depletion of soil fertility and nutrient imbalance are major constraints in improving productivity in many developing countries, especially in SA and SSA. Most cropland soils in developing countries are affected by negative nutrient balances; in Africa, nitrogen-phosphorus-potassium (NPK) depletion occurs at 20 to 40 kg/ha/yr throughout the continent (Smaling, 1993; Smaling et al., 1993; Sanchez, 2002). African farmers traditionally left lands fallow to restore nutrients and regain fertility, but because of growth in population and food demand, crops now grow continuously with little or no nutrient input. In the Sahelian zone of Sudan, Ayoub (1999) observed that crop yields were severely reduced by decline in soil fertility. Thus, soil fertility management and fertilizer use could strongly increase crop yield in the Sahel. Fertilizer use in SSA is low (NPK at 8.8 kg/ha/yr) (Henao and Baanante, 2006). This situation is attributable to the inaccessibility and high cost of inorganic fertilizers. In contrast with Africa, fertilizer use in SA is generally high (NPK at 100/kg/ha/yr). Fertilizer consumption in SA increased by a factor of 42 from 1961 to 2003 and accounts for much of the yield gain in the region during the period (Lal, 2007). However, there has been widespread decrease in the responses of crops to agricultural inputs in SA since 2000.

63. One possible reason for the observed decline in agronomic yields in SA is the loss of soil organic matter (SOM). In SA and SSA, crop residues and weeds are used as fodder for animals or for cooking fuel (Eswaran et al., 1999). Without input of organic matter, degraded soils have low water and nutrient capacities, so they often do not respond to the addition of inorganic fertilizer. Numerous studies indicate that there can be strong synergism in the use of both organic and inorganic fertilizers. In SA, manure is used for cooking fuel; in many parts of SSA, the poorest farmers use some crop residues as building material and might not have animals as a source of manure, and they are reluctant to use their small plots to grow crops that yield only green manures. Low SOM leads to a decrease in the abundance of important soil organisms, such as bacteria, fungi, termites, earthworms, insects, and small animals that inhabit the rhizosphere. It is important to note that SOM content can easily be improved by manuring. In the Sudano-Sahelian conditions, Mando et al., (2005a; b) observed that manure application (10 t/ha) increased sorghum yields by 56 to 70%. In Burkina Faso, Ouedraogo et al. (2007) concluded that a combination of organic manures and chemical fertilizers is essential to increasing and sustaining high yield of sorghum and other grain crops. In the Pampas of Argentina, Quiroga et al. (2006) reported a strong positive correlation between barley grain yield and SOM concentration. Increasing SOM concentration by 1 g/kg led to increased barley grain yield by 130 kg/ha.

64. Soil infertility owing to deficiency of essential plant nutrients, is a major constraint affecting crop yields in developing countries. It is estimated that as much as 50% of the increase in crop yields worldwide during the twentieth century was due to adoption of chemical fertilizers (Borlaug and Dowswell, 1994; Loneragan, 1997). Fertilizers played a major role in increasing agronomic production in SA, where fertilizer input between 1969 and 1995 increased from 20 to 145 kg/ha/yr (Hossain and Singh, 2000). Among macronutrients, N is the most limiting factor to enhancing crop yield (Eickhout et al., 2006). In addition to N, productivity of the rice-wheat system in Asia operates at low yield because of inadequate supply of other nutrients and inappropriate water use. Replacing lowland flooded with aerobic rice is a new development (Bouman et al., 2007; Kreye et al., 2009), which can save water and address the severe concern about rapid depletion of ground water in the Indo-Gangetic Basin (Kerr, 2009; Rodell et al., 2009). Similarly, use of genetically-improved high rise rice can adapt to inundation under extreme conditions of flooding (Voesenek and Bailey-Serres, 2009). Low productivity in SSA is, to a large extent, attributable to soil infertility (Sanchez, 2002). High C and N pools are also related to clay content and type, and other horizon characteristics (Schaefer et al., 2008), along with availability of plant nutrients and cations. In Haiti, Clemont-Dauphin et al. (2005) reported that availability of P, K are essential components of SLM options.

65. Plant nutrients needed to replenish what is annually removed from the soil to meet the global demand for food and fibers are estimated at 230 Mt (Vlek et al., 1997). Thus, it is important to adopt a holistic approach based on SLM practices that enhance INM (Gruhn et al., 2000). The latter recognizes the importance of nutrient recycling using crop residues and other biosolids such as manure and compost, increasing biological N fixation (BNF) through leguminous cover crops, using mychorrhizal inoculation, and applying chemical fertilizers and organic amendments. In this regard, establishing links between livestock production and cropland management is very important (Naylor et al., 2005).

66. Because of the widespread problems of soil degradation and prevalence of extractive farming, cropping systems in developing countries need to be reinforced with microelements (Zn, Cu, I, Fe, B). These elements must be supplied through the soil, including application of S and N (Soliman et al., 1992) and Zn (Wijesundara et al., 1991). There are several strategies for improving availability of macrominerals and microelements in the soil. These include (Welch and Graham 2004/2005):