RESPONSE OF PROTECTED ORGANIC PEA CROP TO INCREASED APPLICATION RATE OF MANURE COMPOST

Legumes are important as soil fertility building crops in organic vegetable production but biological N fixation is not sufficient to achieve high yields of grain legumes. In this study the response of a grain pea crop (Pisum sativum ‘Karina’) to soil application of composted organic cow and horse manure (0, 30 and 60 t ha^-1) was evaluated throughout a randomized block design experiment, in a greenhouse located in NW Portugal, from October to April, and compared with that of a conventional pea crop fertilised with 150 kg ha^-1 of mineral N (MF).
The yield of organic pea crop grown with manure compost (4.2 t ha^-1) was not significantly different between different compost rates and was more than twice of that of unfertilized crop (1.6 t ha^-1), whereas MF treatment yield was 9.3 t ha^-1. Plant dry matter content was similar for organic crops grown with compost and for MF, as well as N, P and Ca plant nutrient contents. However, the MF crop had lower K and Mg plant contents and higher Fe content compared to organic crops. Nitrogen plant accumulation for the MF crop was 81.7 kg ha^-1, while for the unfertilized crops was 12.8 kg ha^-1 and for the organic crops grown with 30 and 60 t ha^-1 of compost was, respectively, 33.1 and 31.5 kg ha^-1. The N recovery rate for the mineral N in the MF crop was 45.9%. Compost N mineralization rate was 7.2 and 2.9% respectively for compost rates of 30 and 60 t ha^-1. This decrease in compost N mineralization was probably due to negative effects on soil characteristics induced by the higher compost rate. This study shows that soil application of compost at a rate of 30 t ha^-1 increased organic pea yield, and that the higher compost rate is not recommended under the prevailing local conditions.