NITROGEN REQUIREMENT OF BELLE DE BOSKOOP APPLE TREES AT DIFFERENT DENSITIES GROWN UNDER DRY CONDITIONS

Amongst the various factors considered in determining nitrogen dressings in apple orchards, the moisture condition of the soil is one of the major ones. In The Netherlands under average moisture conditions 50–100 kg N per ha is given annually, but on dry soils dressings range from 100–150 kg N. On extremely good soils quite a few growers do not fertilize at all, although some nitrogen is then supplied by calcium nitrate and urea sprays.

Of late, more and more evidence is being accumulated on the relative drought-susceptibility of trees growing at narrow spacings. In single-row systems where grass strips are applied, this susceptibility may be further worsened by enhanced competition as row distance and width of the herbicide strip decrease. Since nitrogen uptake and the efficiency of nitrogen dressings are much influenced by moisture supply and competition for nitrogen, and besides, an increased need of nitrogen can be expected on account of the greater number of trees and a higher production per ha, it seems obvious to assume a relationship between fertilizer response and plant density. Such a relationship was found in a field experiment with Belle de Boskoop apples on M.9 on a shallow-rooted, rather dry soil. There were four plant densities and five nitrogen rates. In the initial years, growth and production of the trees were strongly influenced by dry periods in most of the years whereas the whole growing season of 1976 was extremely dry. The following preliminary conclusions have been drawn.

1. Aiming at minimum nitrogen dressings or even omitting fertilization in grass strip orchards carries a far greater risk of biennial bearing in dense, than in wide plantings. Reduced flower initiation brought about by nitrogen deficiency is especially to be feared when high cropping coincides with drought.
2. In seeming contradiction with the foregoing is the observation that the highest nitrogen requirement was not found in the densest planting system. High dressings promoted tree growth in the widest planting but had no effect or even reduced growth in the densest systems, the latter possibly as a consequence of excessive salt concentrations in the root zone during dry periods.
3. Therefore, in the two densest plantings a rather low nitrogen dressing was required for maximum production in the first six rather dry years. Under more normal, moist conditions possibly a higher optimum dressing would have been found, compared with the widest planting.