Forest fertilization — present state and history with special reference to South German conditions

Abstract

Until well into the eighteenth century, forestry was largely confined to the poorer soils and sites too difficult for farming properly and ravaged by destructive lumbering, burning and grazing. Up to the mid-20th century there was much extraction of litter. All this resulted in nutrient impoverishment which persists to the present day. There has been some improvement through corrective fertilization. Such treatment over the past 30 years brought about yield improvement of around 3 m³/ha/year compared to untreated areas. This, no doubt, was largely because of the former widespread deficiencies of nitrogen and phosphate and weathering under high rainfall during the course of the experiments. Returns from fertilizer were very variable and such large improvements were not always the rule. Whether such results are still obtainable today when growth performance in many areas is so much improved or under different climatic conditions may be doubtful.

Today there is a need more for ameliorative treatment to revitalize the forest and to stabilize or improve soil fertility. Many examples have shown that fertilizer treatment designed to improve growth can also improve toleration of natural and man-made adverse factors. Where this did not apply, the reason was found to be inappropriate nutrient combinations. For instance, it is only quite recently that the importance of magnesium deficiency has been recognized in many areas. Today it is possible to identify specific nutrient deficiencies before deciding on a fertilizer programme. The full potential of the forest can only be realised when all the major and trace elements are available in sufficient quantity from the soil and when nutrient contents in the plant are in equilibrium. The use of fertilizers in the complex environment of the forest ecosystem calls for discretion, the more so if the bad effects of man-made adverse factors are to be countered. Very soluble fertilizers used at a high rate can have undesirable side-effects, notably too rapid mineralisation of organic matter. So there is much to be said for using slower releasing fertilizers, but, where the need for improvement is urgent, soluble fertilizers are to be preferred. This is the case in nurseries and during the establishment phase where adverse factors (competition from the natural plant cover, wild animal damage and frost) must be outgrown. Quick releasing fertilizers are also recommended to ensure the right nutrient interrelationships in the tree, the earlier the necessary correction is made the better.

In the future, fertilizer policies must be reviewed in the light of change in the level of pollution and, possibly also in the light of change in climate. Knowledge acquired up to now will be a useful foundation for future work.