Horticultural crop yields and nitrogen uptake response to green manure, farmyard manure compost and organic commercial fertilizer

A 3-year field organic crop rotation was set up in a sandy loam soil with a cover crop of rye and vetch over the autumn/winter for green manure followed by potato and lettuce (1st year), Swiss chard and turnip (2nd year) and Portuguese cabbage and carrot (3rd year). A randomized block design was arranged with green manure (GM), GM with 20 and 40 t ha(-1) of farmyard compost manure (C20 and C40) and GM with 1 and 2 t ha-1 of commercial organic fertilizer (CF1 and CF2) to access nitrogen uptake and crop yield. Lettuce, turnip and carrot yields were significantly increased, by 90, 115 and 56%, respectively, for C40 compared with CF2, probably because of the rapid nitrogen mineralization of the commercial fertilizer during the previous crop, decreasing nitrogen availability for the 2nd crop of the season. There were no significant differences in potato yield between C40, CF1 and CF2, whereas Swiss chard yield increased for CF2 compared with C40, probably because of the long growth period of potato associated with the increased nitrogen availability from compost application. Portuguese cabbage yield, unlike Swiss chard, did not increase for CF2 compared with C20 and C40, even with the same short growth period, probably because of the effect of continuous compost and green manure application that increased nitrogen availability in the medium/long term. The application of 40 t ha-1 farmyard manure compost and rye with vetch as green manure has the potential to enhance crop yields during the organic horticultural rotation.