Forest fertilisation is the fastest way to increase the growth of trees, improve the profitability of commercial forestry and grow forest carbon sinks. However, forest fertilisation should be targeted at those sites where the effects of fertilisation that increase growth and carbon sequestration are the strongest and where the environmental impact of fertilisation can be minimised.
When planning fertilisation, many factors must be considered, such as water protection, the fertiliser used and the quantity applied. Because of this, it is advisable to seek expert assistance in the planning of fertilisation.
Forest fertilisation includes growth fertilisation and remedial fertilisation.
Growth fertilisation
The growth fertilisation of mineral soils mainly targets managed pine and spruce forests on relatively dry and fresh heaths, which are mature cultivated forests or forests due for regeneration felling.
Nitrogen is the main nutrient in the fertiliser. A lack of nitrogen limits tree growth on mineral soils. A fertiliser containing phosphorus and/or boron is used where necessary.
In suitable locations in Southern Finland, a single application of nitrogen fertiliser produces an additional growth of about 12–20 m3/hectare in less than 10 years. In Northern Finland, the increase is 60–80 % of the growth increase in Southern Finland.
The fertiliser is usually applied by helicopter or on the ground by a forwarder. Depending on the fertiliser type and the site, 500–800 kilograms of fertiliser are applied per hectare. Forest owners can also apply fertiliser themselves using, for example, all-terrain vehicles and seedling trays.
Remedial fertilisation of forests prevents and corrects growth issues
Remedial fertilisation prevents or corrects growth issues caused by a nutrient imbalance in the soil.
Growth issues typically occur in the tree's crown or branches.
In practice, the remedial fertilisation of forests means fertilising a peatland forest with ash or boron at a site where the soil is deficient in boron.
Financial support is available for remedial fertilisation: Financial support for remedial fertilisation (in Finnish)
Ash fertilisation of a peatland forest
Ash fertilisation is particularly beneficial to moderately nutrient rich, fertile and drained peatlands with thick peat layers. These sites usually have enough nitrogen but are deficient in phosphorus and potassium.
Nutrient deficiencies cause growth issues that typically occur in the tree’s crown or branches. If conditions at a site are too harsh, ash fertilisation has very limited benefits, and the investment is usually not economically viable.
When used at the correct site, ash fertilisation is a long-lasting and effective way to increase tree growth. Improving the growth of trees increases groundwater evaporation, which in turn reduces the need for ditch network maintenance in forested areas.
Wood ash will begin to have a nutritional effect as early as a year after fertilisation. Depending on the site and the ash, the effect can last for over 20 years.
On average, ash fertilisation produces a growth increase of 2–4 cubic metres per hectare annually. The effects will become apparent within two to eight years.
Depending on the quality of the ash and the site, between 3,000 and 5,000 kilograms of ash are applied per hectare. The ash is applied with a forestry tractor in winter when the ground is frozen, or by helicopter when the ground is thawed. Mapping the terrain of a site will provide an accurate picture of what should be considered during fertilisation and where fertiliser should be applied.
See the map service for potential ash fertilisation sites (in Finnish).
Boron fertilisation of forests
Boron fertilisation is used to prevent growth issues or to correct existing growth issues caused by a boron deficiency. Boron deficiency is common in former agricultural land, particularly in spruce forests growing on land previously used for slash-and-burn cultivation in Eastern Finland.
A boron deficiency can cause significant losses in quality and growth. The profitability of boron fertilisation is due to the fact that future logging will produce more valuable timber from the forest. Boron fertilisation restores the trees’ capacity to grow and increases carbon sequestration.
Boron fertiliser is applied to the forest mainly in liquid form, either by foresters or from the air by drones. Boron can also be applied during nitrogen or ash fertilisation. In these instances, boron has already been added to the fertiliser used.