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Nutrient requirements in forestry

The maintenance of satisfactory nutrient levels in growing trees is an important part of plantation management. One or more applications of fertiliser will sometimes be required to temporarily or permanently stimulate growth. Achieving satisfactory nutrient levels will help promote:

  • A vigorous and healthy crop
  • Earlier establishment of the trees
  • A decrease in the period of risk from weed growth, frost and vermin
  • Earlier canopy closure after which nutrient recycling can increase 

The most important nutrients in forestry are phosphorus, nitrogen and potassium. Phosphorus is of fundamental importance from an early stage for good root development. Both nitrogen and potassium are important for photosynthesis.

Fertiliser application at establishment

All land being considered for planting should undergo a preliminary soil assessment by a registered forester. This investigation should determine if the soil is suitable and if so what level of nutrient inputs are required, if any.

For example, deposits of marl and calcareous materials can be found at varying depths beneath midland peats. Where marl occurs within 70cm of the soil surface, sites are classed as unplantable. In the event of a soil reaction with 10% hydrochloric acid, a detailed sampling and analysis of the soil is required. A competent forest soils laboratory should always be used to assess soil samples.

The nutrient requirement for forests at establishment should match the quality and character of any given site. In general broadleaf trees are more site demanding and have higher nutrient requirements. Ideal broadleaved sites will rarely require fertiliser inputs.

Compound fertiliser (e.g. 10:10:20 or 18:6:12) may be required on enriched peats and other site categories but nitrogen deficient soils will very rarely be suited to broadleaf planting.

Phosphate inputs are more widely required for coniferous plantations.

The Forestry and Water Quality Guidelines published by the Forest Service, DAFM contain specific measures regarding fertiliser application and storage. Implicit within these measures is the protection of all permanent or seasonal streams, rivers and lakes (aquatic zones) from fertiliser discharge.

Correct setback distances from all aquatic zones must be observed during the storage or application of fertilizers and may be advised by the relevant Fisheries Board. Application of fertilizers should only take place during the months of April to August inclusive, avoiding all drains, silt traps and application during periods of prolonged rainfall.

Remedial fertiliser applications

The nutrient requirements of trees have to be satisfied throughout the rotation in order to achieve good growth rates high yields and healthy trees. These requirements depend on the soil type and tree species planted.

Trees growing on infertile peaty or mineral soils may begin to lose vigour and display symptoms of nutrient deficiency in the years after planting. These deficiencies can occur despite correct fertiliser application at planting time. It is important to walk your forest and monitor trees regularly for possible nutrient deficiencies. It is also important to control competing weeds prior to application of fertiliser to young trees. This will ensure that the trees rather than the weeds benefit from the nutrient application.

Phosphorus deficiency

On very poor unenclosed sites, an initial application of phosphate may not be sufficient. A supplementary application may be required to sustain tree growth after a number of years. Typical phosphate deficiency symptoms include:

  • Poor height / leader growth
  • Dull foliage colour
  • Possible necrosis of older needles in severe cases
  • Needles growing parallel to branches

Application of all phosphate fertiliser should be broadcast and evenly applied, taking care to avoid drains and silt traps. Granulated rock phosphate is ineffective on soils with a pH of 6 or greater. In these situations, an application of superphosphate may be appropriate.

Nitrogen deficiency

Trees growing on peat or mineral soils of low fertility may sometime lose vigour a few years after planting due to limited soil nitrogen. For example, competition from heather on infertile peats may contribute to “heather check” of Sitka spruce resulting in the following symptoms:

  • Decrease in growth
  • Light-green or yellowing of foliage
  • Uniform yellowing over the whole tree
  • Smaller needles/leaves

One or more applications of nitrogen in the form of urea may be required where deficiencies occur. It is important to ensure phosphate levels are also adequate at this stage and supplement where necessary. Nitrogen fertilizers are more soluble in the soil than phosphates, and are leached out relatively quickly.

Urea should therefore be applied in April/May so that trees can benefit immediately. Application should be made around the effective rooting area. If trees are smaller, nitrogen should be applied to each plant over an area of about 1 square metre around each tree. If canopy closure is expected within 2 or 3 years, application may be broadcast.

Potassium deficiency

Trees growing on midland fen peats may require an application of muriate of potash to prevent or correct a potassium deficiency. Typical symptoms of deficiency may include:

  • Partial yellowing of needles (base of needle remains green and top is yellow)
  • Complete yellowing of needles at the ends of the shoots (Douglas fir, spruces and larches only, not pines)
  • Pronounced symptoms towards end of winter

Foliar analysis

Where symptoms occur, foliar analysis is recommended to determine the nature and the extent of a possible nutrient problem.

August is the appropriate time for foliar analysis of broadleaves and larches.

All other conifers should be sampled in November or December.

Approved forestry laboratories provide written reports that identify deficiencies and recommend corrective fertiliser applications.

How to take foliar samples is explained here.

Other factors such as poor drainage, lack of adequate vegetation control, exposure, frost and disease may sometimes produce deficiency-like symptoms on trees. Therefore it is vital to find the source of any possible growth problem before attempting to rectify it.

Rectifying problems

Nutrient problems can sometimes be hidden and not immediately visible in established forests. It is advisable to have foliage tests carried out on trees showing only a growth increment decline but not necessarily showing a discolouration of foliage. Potential nutrient problems should be tackled in good time.

Correcting situations where trees have gone into check or are beginning to stagnate can be both difficult and costly and may affect their long term economic viability.

Relevant publication