ORGANIC LETTUCE GROWTH AND NUTRIENT ACCUMULATION IN RESPONSE TO LIME AND HORSE MANURE COMPOST

A field experiment with organic lettuce (Lactuca sativa) was arranged as a randomized block factorial design with increasing rates of composted horse manure (0, 20 and 40 t ha^-1) combined with lime (0 and 8 t ha^-1) to assess compost mineralization, lettuce growth and nutrient accumulation in lettuce shoots. Commercial yield was significantly increased from 25.6 t ha^-1 for the unfertilized control treatment to 35.8 t ha^-1 attained with 40 t ha^-1 of manure compost and 8 t ha^-1 of lime. Mean yield increase with lime for the overall compost treatments was 15% whereas with 20 and 40 t ha^-1 compost was 18 and 26%, respectively. Nitrogen content and N accumulation in lettuce significantly increased with the application of 40 t ha^-1 compost but differences between 20 and 40 t ha^-1 were not clear. Lettuce uptake of P, K, Ca, Mg and Fe was not significantly different between experimental treatments except for occasional differences found for K. Apparent organic N use efficiency was highest for 20 t ha^-1 of compost when lime was applied (18.3%) and increased also with lime application for treatments with 40 t ha^-1 of compost. This study shows the potential of horse manure compost combined with lime to increase organic lettuce yield and N uptake in low-fertility soils and for organic horticulture.