Effect of organic and conventional farming systems on nitrogen use efficiency of potato, maize and vegetables in the Central highlands of Kenya

Summary

Increased per capita food production in the tropics is closely tied to soil organic matter and water management, timely nitrogen (N) supply and crop N use efficiency (NUE) which are influenced by farmingsystems. However, there is lack of data on the effect of organic farming systems on NUE and how thiscompares to conventional farming systems under tropical conditions. Therefore, the objectives of thisstudy were to determine the effect of conventional and organic farming systems at low and high management intensities on N uptake and N use efficiency of potato (Solanum tuberosum L.), maize (Zea mays L.),cabbage (Brassica oleracea var. Capitata), kale (Brassica oleracea var. Acephala) and Swiss chard (Beta vul-garis sub sp. Cicla). The organic high input (Org-High) and conventional high input (Conv-High) farmingsystems are managed as recommended by research institutions while organic low input (Org-Low) andconventional low input (Conv-Low) farming systems are managed as practiced by small scale farmers inthe Central highlands of Kenya. The study was conducted during three cropping seasons between October2012 and March 2014 in an ongoing long-term trial established since 2007 at Chuka and at Thika, Kenya.Synthetic N-based fertilizer and cattle manure were applied at ∼225 kg N ha−1yr−1for Conv-High and at∼50 kg N ha−1yr−1for the Conv-Low. Composts and other organic inputs were applied at similar N ratesfor Org-High and Org-Low. Nitrogen uptake efficiency (NUpE) of potato was highest in Conv-Low andOrg-Low at Thika and lowest in Org-High and Org-Low at Chuka site where late blight disease affectedpotato performance. In contrast, the NUpE of maize was similar in all systems at Chuka site, but was sig-nificantly higher in Conv-High and Org-High compared to the low input systems at Thika site. The NUpEof cabbage was similar in Conv-High and Org-High while the NUpE of kale and Swiss chard were similarin the low input systems. Potato N utilization efficiencies (NUtE) and agronomic efficiencies of N use(AEN) in Conv-Low and Conv-High were 11–21 % and 1.4–3.4 times higher than those from Org-Low andOrg-High, respectively. The AENof maize was similar in all the systems at Chuka but was 3.2 times higherin the high input systems compared to the low input systems at the Thika site. The AENof vegetablesunder conventional systems were similar to those from organic systems. Nitrogen harvest index (NHI) ofpotato was similar between Conv-High and Org-High and between Conv-Low and Org-Low. N partitionedinto maize grain was similar in all the system at Chuka, but significantly lower (P < 0.001) in Conv-lowand Org-Low at Thika site. The NHI of cabbage in Org-High was 24 % higher than that of Conv-High. Thestudy concluded that for maize and vegetables, conventional and organic farming systems had similareffects on NUpE, AEN, NUtE and NHI, while for potato conventional systems improved NUE comparedto organic systems. The study recommends that management practices for potato production in organicsystems should be improved for a more efficient NUE.