Oliver, O., Muthomi, J., Narla, R., Ojiem, J., Nderitu, J Effect of time after incorporation of lablab green manure on establishment of common beans Oliver, O., Muthomi, J., Narla, R., Ojiem, J., Nderitu, J University of Nairobi

Introduction Practice of ploughing or turning into the soil undecomposed green plant tissues for the purpose of improving soil fertility Green manure contributes to soil nutrient pool through mineralization of decaying biomass thereby promoting soil fertility and in turn improve crop production. However, undecomposed plants residues have been reported to reduce crop establishment and increase incidences of pests and diseases. Therefore, there is need to determine the appropriate time for green manure incorporation before planting.

Materials and methods The effect of time after incorporation of lablab green manure on bean root rot and crop establishment was determined by incorporating lablab green manure at 0, 7, 14 and 28 days before planting. The plots were planted with beans and the treatments were arranged randomized complete block design with a split plot arrangement Soil samples were collected before incorporation of lablab green manures, at planting, two, four and six weeks after planting. Data was collected on population of soil microorganisms, crop emergence, root rot incidence and severity, population of root rot pathogens and yield.

Results Effect of different incorporation time on plant stand plots incorporated at planting had much lower germination percentage (46%), Plots planted 28 days after incorporation had the highest germination percentage (79%). The germination followed a pattern of 28 days incorporation > 14 days incorporation >7 days incorporation 0 days incorporation Figure 1: Percentage stand count of common beans under different lablab green manure incorporation times

Effect of application times on severity and incidence of root rot High root rot incidence was recorded in plots incorporated at planting (fig 2) Severity was highest in plots that had been planted immediately after incorporation (Fig 3) Fig 2: Percentage root rot incidence under different green manure incorporation times Fig 3: Percentage root rot under different green manure incorporation times

Effect of different incorporation times on yield Incorporation of green manure 28 days before planting resulted in maximum grain (4.3 t/ha-1) Plots incorporated at planting recorded the lowest values for yield attributes of common beans (1.1t/ha-1) Fig 4: Common bean grain yield (t/ha-1) as affected by different green manure incorporation times

Population dynamics of root rot pathogens in the soil following different green manure application times F. solani population increased from 27000cfus g−1 to 45000cfus g−1 then reduced to 25000 cfu while F. oxysorum slightly increased to 25000 cfus g−1 then reduced to cfus g−1 soil (Fig 5) F. solani increased from 27500 to 33000 CFUs g soil and later reduced to 15000 cfus g soil (Fig. 6) In both incorporation times, Pythium population was low but slightly increased second week after planting Fig. 5: CFUs/ g soil of root rot pathogens at planting Fig. 6: CFUs/ g soil of root rot pathogens 7 weeks before planting

F. oxysporum slightly increased from 7500 to 2000cfus g soil (Fig 8) F. solani slightly increased second week after emergence from 23000 cfus g-1 to 25000 cfus g-1 then reduced cfus g-1. (Fig 7) F. oxysporum reduced from 20000 cfus g soil to 15000 cfus /g soil Pythium population reduced during the sampling times F. solani population reduced from 38000 to 18000 cfus g soil but later increased. F. oxysporum slightly increased from 7500 to 2000cfus g soil (Fig 8) Pythium slightly increased but remained low during sampling period Fig. 7: CFUs/ g soil of root rot pathogens 14 days before planting Fig. 8: CFUs/ g soil of root rot pathogens 28 days before planting

Population of antagonist in the soil Plots incorporated at planting had the lowest population of antagonists when compared with other incorporation times Table 1: Mean CFUs /g soil of antagonists isolated from soil at different incorporation times

CONCLUSION Incorporation of lablab green manure at planting reduced emergence, yield and stimulated the population of root rot pathogens while reducing population saprophytic fungi which could be antagonistic to root rot fungi The results of the study showed that increasing the time after incorporation of green manures before planting will allow proper decomposition and reduction in root rot incidence but increased grain yield.

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