Soil nitrogen in an organic apple orchard

Introduction The lack of vigour of an organic apple orchard (Smoothee cultivar) planted in 1994 pointed out the management of tree nitrogen supply in this orchard. Serious damages due to Dysaphis plantaginea, the rosy aphid, were also noted on trees. As tree vigour and therefore aphid population levels are related to nitrogen nutrition, the monitoring of soil nitrogen availability was necessary to analyse the causes of these problems. Besides, the aim was to optimise the orchard organic nitrogen fertilisation. Materials and methods From 1994 to 1998, the fertilisation management mainly consisted in compost supply (10 t.ha-1). From 1999 to 2001, autumn compost supply was reduced to 5 t.ha-1, complemented with organic quickly mineralised fertilisers in spring (2 x 20 kg.ha-1 N). Lysimeters have been installed (density of 32 lysimeters per ha) in the orchard at 80 cm from trees in the areas under irrigation emitters; at 35 cm depth in 1999, at 35 cm and 50 cm depth in 2000 and 2001. These lysimeters allowed to sample the soil solution and to measure its nitrate content each week along the vegetative period. Growth of the trees, yield, fruit quality and pests were monitored. Soil and leaf analyses were performed in order to evaluate their nitrogen status. Results During the first year of the experiment (1999), nitrate content in soil solution was low and slightly varied among the tested places. In 2000 and 2001, the orchard homogeneity and the very low risk for nitrate leaching were assessed, as nitrate content in soil solution was comprised between 5 and 15 ppm, and remained lower at 50 cm than at 35 cm depth. Analyses of soil samples before and after the vegetative period led to the same conclusions: soil nitrate content varied between 20 and 30 kg.ha-1 at the end of winter, 15 and 25 kg.ha-1 in autumn. These results suggested that soil nitrate content was low enough to limit nitrate leaching, but sufficient to ensure an appropriated nitrate availability for the apple trees (Bussi and Gojon, 1997). The soil C/N ratio increased (from 8.6 in 1998 to 9.8 in 2001), as a result of the soil organic matter increase due to compost supplies. In our soil conditions, a close to 10 C/N ratio, as measured in 2001, appears to be an adequate level (Delas and Molot, 1983). Leaf nitrogen content decreased from 23.7 g.kg-1 in 1999 to 20.4 g.kg-1 in 2001, and fruit nitrogen content from 2.45 g.kg-1 in 1999 to 1.69 g.kg-1 in 2001; nevertheless these N content remained quite suitable for apple production (Sharples, 1980). Yield varied from 15 to 23 t.ha-1 according to the year, and first choice fruit represented 51 to 75 % of the total harvest. These yield fluctuations were mostly a consequence of the serious damages caused by the rosy aphid. Damages by rosy aphid were serious in 1998 and 2000. In 1998, damages might have been favoured by high soil nitrate content, as a consequence of the 10 t.ha-1 compost yearly applied. In 2000, results showed that trees were not overfertilised, which excluded a stimulation effect of nitrogen. Weather in spring 2000 was wet and hot, which was likely to favour the increase of aphid populations, despite pest management before blooming. Conclusion Soil solution and soil nitrate content are useful tools to check the adequacy of organic fertilisation. They may allow to quickly detect excess or lack of nitrogen fertilisation, in order to correct it. Leaf and fruit nitrogen contents are diagnostic elements to evaluate the nitrogen fertilisation effectiveness in organic apple orchard. In our conditions, the optimisation of nitrogen fertilisation was simultaneous of an effective control of the rosy aphid in 1999 and 2001. However, rosy aphid damages in 2000 were serious, suggesting that this problem is most probably related to multi-factorial causes. References Bussi, C., Gojon, A. (1997). Nitrate reductase activity in leaves of peach trees. J. Hort. Sci., 72 (3), 347-353. Delas, J. and Molot, C. (1983). Effet de divers amendements organiques sur les rendements du maïs et de la pomme de terre cultivés en sol sableux. Agronomie, 3 (1), 19-26. Sharples, R.O. (1980). The influence of orchard nutrition on the storage quality of apples and pears grown in the United Kingdom. In : Mineral Nutrition of Fruit Trees, 17-28