Studies have shown that soil fertility declines after cropping and this becomes greater with each successive cropping. The situation is likely to be more serious under mixture since mixtures take up more nutrient than sole crops. Thus, declined in soil fertility result in decline in production in the tropics. This may be due to the nature of the tropical soil and the mining of the soil by the crops. This gives a negative balance of most of the essential nutrients at the end of each season. The decline of these essential nutrients will therefore lead to lower yield of these crops. Results from...