Principles of forest fertilisation — illustrated by New Zealand experience

Abstract

While part of the world is still being deforested, in another part agricultural land is being relinquished and forests are being replanted. The common factor that links these divergent trends is that foresters have had very little choice over which land changes use and tended to get land agriculturalists do not want. Some has been poor, infertile land. Thus forestry today is practised on land some of which is inherently susceptible to nutrient deficiency. Although natural forest is rarely nutrient deficient there are fewer and fewer places where truly natural forest exists. Past nutrient removals in timber, branches, leaves and even forest floor litter have interrupted the nutrient cycle. The land coming back into forestry from agriculture can suffer deficiencies caused by past agricultural history. Thus nutrient deficiency can be expected, and fertiliser use will be an important component of managing these forests. There are certain factors relating to forest fertilisation that distinguish it from agricultural fertiliser practice.

• 1. Foresters generally are reluctant to use fertiliser. This is partly because of the uncertainty of obtaining an effective and economic response and partly because of the environmental overlay associated with forests.

• 2. Foresters are more ready to use fertilizers to protect their crop from gross damage (because that is more easily defined) than they are just to increase yield (because they have been unable to justify this in economic terms).

• 3. Forest land can be rough or steep and the crop itself is dense. Land-based applications of fertiliser are difficult. Aerial applications predominate. Compared to agricultural use, the cost of application is high relative to the cost of the fertiliser, however the total cost over the life of the crop is likely to be less than in a succession of annual crops.

• 4. There is only limited sideways nutrient transfer within the crop, hence accurate application is essential.

• 5. There are considerable advantages to using slow release fertilisers to lengthen the duration of response.

• 6. The nutritional management of plantations on poor soils can be very complex because the fertiliser need varies both in quantity and quality over time.