Short-term effects and long-term after-effects of fertilizer application on the flowering population of green-winged orchid Orchis morio

The green-winged orchid Orchis morio has declined dramatically in England due to loss of habitat through the agricultural improvement of old hay meadows. A range of replicated fertilizer treatments was applied over a six year period to plots in an old hay meadows containing O. morio, to determine short-term and long-term after-effects. The numbers of flowering spikes per plot were counted annually for 23 years, commencing two years before experimental treatment. Inorganic fertilizers returning equivalent amounts of N, P and Mg and 80% of K to that removed in the annual hay crop significantly decreased flowering spike numbers during the six years of treatment. The fertilizer treatments, overall, significantly decreased flowering spike numbers, although the rates of N applied were low by agricultural standards (e.g. 22–88 kg ha−1 N). Two applications of P at a rate of 40 kg ha−1 during the six year period greatly reduced flowering spike numbers, apparently permanently. Except in the case of the 40 kg ha−1 P treatment, the decrease was closely correlated with the increase in hay yield following fertilization, suggesting that competition between O. morio and the rest of the vegetation was responsible. The effect of P on O. morio was out of all proportion to its effect on hay yield, suggesting that it may have been toxic to the orchid. In retrospect we conclude that fertilizer effects would have been easier to detect if all plants of O. morio had been counted and not just flowering plants whose numbers fluctuated greatly. Even so, we have detected detrimental effects of low levels of fertilizer, and hence any application of inorganic or organic based fertilizers to old meadows should be avoided where conservation of O. morio is an objective