Effect of spatial arrangement and nitrogen levels on yield of potatoes and beans grown as intercrops
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Two field experiments were conducted at the University of Nairobi's Faculty of Agriculture farm (Field Station) to investigate the effect of planting patterns (PP) and nitrogen (N)application on growth and yield of intercropped potatoes and beans. Two varieties (V)of potatoes, the early maturing variety Annette (VI) and the late maturing variety B53 (V2)were intercropped with the early maturing Rosecocco (GLP-2) variety of beans during the short rains of November - March 1993 and long rains of May - August, 1994. The treatments used were four planting patterns, four nitrogen levels and two potato varieties, tested in a factorial experiment laid out in a completely randornised block design with three replicates. The planting patterns consisted of sole crop of potatoes or beans (PPI), potatoes and beans in alternate rows (PP2), potatoes and beans in alternate hills within the row (PP3) and potatoes and beans in the same hill (PP4).The nitrogen levels were Okg/ha (Nl), 37.5kg/ha (N2), 75kg/ha (N3) and 112.5kg/ha (N4). During growth, plants were sampled for dry matter, nodule number and nodule weight determination every fortnight starting from four weeks after emergence. At maturity, yield and yield components were determined for both beans and potatoes. Results showed that potato dry matter yield in sole crop out yielded the other planting patterns early in the season upto 6 weeks after emergence and beyond this, those planted on the same hill with beans were significantly higher than alternate hill and alternate row patterns. The higher nitrogen levels of 75kg/ha and II2.5kg/ha gave sole crop dry matter yields that were significantly higher than the intercrop patterns. Bean dry matter yields of sole crop and alternate rows were statistically similar at the lower nitrogen levels of 0 and 37.5kg/ha nitrogen but as the nitrogen was increased to 75kg/ha and 112.5kg/ha the sole crop out yielded all the intercropped intercropped planting patterns. XXVlll At zero nitrogen level, intercropping in the same hill increased potato tuber yields by 3.8% and 15.2% in Annette and B53, respectively, compared to intercropping in alternate rows, in the first season. In the second season same hill yields were higher than alternate row yields by 10.1%and 12.3% for Annette and B53 respectively. Potato yield advantage arising from increased proximity between the intercrop species disappeared at higher nitrogen levels of 75kg/ha and 112.5 kg/ha. Sole beans yielded significantly more than beans in the intercropped treatments. Bean yield responded positively to nitrogen fertilizer but higher nitrogen levels depressed this parameter in some cases. Intercropping had yield advantages over monocropping. Land equivalent ratios increased with increasing intimacy between the intercrops especiallyunder low nitrogen. The land equivalent ratio values for same hill planting were significantly higher than for the other intercrop patterns of alternate row and alternate hills at the low nitrogen levels of 0 and 37.5Kg/ha. The land equivalent ratio parameter declined with increase in nitrogen fertilizer to 75 and 112.5kg/ha., with the same hill intercropping exhibiting the highest decline at the highest nitrogen level. In close analysis of the two potato varieties, the results indicated that same hill yields of B53 benefited more from the close intimacy than the Annette at low nitrogen levels. The results therefore, while underscoring the benefits of intimacy in exploiting the complimentary effects of the legumes, also indicates that the potato variety which stayed in the field longer even after harvest of the beans, gained relatively more. This is consistent with the idea that Nitrogen released from dead and decaying roots and root nodules may continue to benefit an accompanying intercrop.