Tham khảo tài liệu 'food production approaches, challenges and tasks part 2', khoa học tự nhiên, công nghệ sinh học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 8 Food Production - Approaches Challenges and Tasks heavy rain showers before the subsequent crops roots can assess the nitrogen and or there may be gaseous losses involved following denitrification. Approaches to quantify the amounts of atmospheric nitrogen fixed by grain legumes erroneously base their estimates on the nitrogen located in harvested biomass above-ground Herridge et al. 2008 Unkovich et al. 2010 . This approach ignores the depositions belowground associated with crop roots Gregory 2006 deposits can benefit subsequent crops Laberge et al. 2011 . The initial transfers of nitrogen from annual grain legumes to subsequent crops are relatively small H0gh-Jensen et al. 2005 Laberge et al. 2011 . However H0gh-Jensen et al. 2006 concluded that N-rich leaf litter together with root residues from pigeonpea grown as annual crops well into the dry season is able to enhance a sustainable maize yield that is approximately twice the yield of maize grown in sole stand. The potential of the crops when cultivated as semi-perennials are yet not well-understood but has be partly tested in agroforestry systems Daniel Ong 1990 Odeny 2007 . 4. Semi-perennial legumes in Africa The current section will investigate the use of semi-perennial legumes in Africa in particular as this continent to a very limited degree has taken up the use of Haber-Bosch nitrogen. Traditional African agriculture is often pictured as inefficient and unproductive. The prejudice in the statement of a Rhodesian administrator in 1926 that intercropping is nothing more than hit and miss planting in mixtures Juggens 1989 as cited in Barrett et al. 2002a derogates a view that persists until today. The challenges remain of inducing a sustainable intensification by improving productivity and natural resource management. But much progress thas been made - both in understanding the socio-biophysical complexity and decision making rationality of the farmer as well as in feasible and attractive options. .
