Use of Legumes and Lime to Improve Soil Fertility and Control Sheep Sorrel (Rumex Acetosella L.) Weed In Potatoes (Solanum Tuberosum L.) in Timboroa, Kenya.

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Wycliffe W.K
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Egerton University
Low soil pH and infertility exacerbated by continuous cultivation without adequate replenishment of mined nutrients, coupled with total harvest of crop residues as livestock feeds, have led to the spread of weeds in the North Rift, Kenya. This has resulted in low potato (Solanum tuberosum L.) yield averaging 7t as opposed to the potential of 30 t ha-1 or more. A focused farm survey, four field experiments and a greenhouse study were undertaken at Timboroa from long rains 2002 to short rains 2004 to determine the influence of household and farm characteristics on soil fertility, weed types and their distribution, to screen and identify suitable legumes for growing in the region for use as green manures / smother crops in the control of weeds, with special emphasis of sheep sorrel (Rumex acetosella L.) weed. The effectiveness of lime applied at 0 or 6 t ha-1 alone or combined with CAN applied on potatoes at three rates (0, 60 or 120 kg N ha-I ) in controlling sheep sorrel weed was also investigated. Phosphorus was applied at potato planting stage as TSP at a uniform rate of 90 kg P ha-1 • The survey and soil analysis confirmed that soils in the region were acidic with pH (H20) of 4.3 and were deficient in P, Ca and Mg. The survey also showed that inadequate quantities of farmyard manure was used on crops because few livestock were kept. In the screening experiment, lupine (Lupin us albus L.) and purple vetch (Vicia benghalensis L.) gave significantly (p<0.05) higher ground cover and biomass than other legumes and were identified as the best bets for the region. Soil incorporation of lupine and garden pea (Pisum sativum L.) significantly (p<0.05) reduced sheep sorrel weed biomass by 18.0% and 9.0%, respectively while application of 120 kg N ha-1 together with incorporation of lupine reduced sheep sorrel weed density equivalent to hand weeding twice. Potato yields increased with N application only up to 60 kg N ha-1 but continued increasing in limed plots suggesting availability of more nutrients such as P and Ca with reduced soil acidity. The lack of significant potato yield increase beyond 60 kg N ha-1 when only N was applied was attributed to more potato vegetative growth at the expense of tuber production. Liming alone significantly (p<0.05) reduced sheep sorrel weed density and biomass resulting in increased potato yields. The study established that wild radish (Rhaphanus raphanistrum L.) weed density and biomass increased with liming, suggesting that the weed should not be allowed to form seed if lime is to be adopted a strategy for controlling sheep sorrel weed.