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Forestry Fact Sheet Number 12

Introduction

This fact sheet aims to assist growers to determine whether fertilising their Tasmanian blue gum plantations will be economically beneficial.

Why fertilise?

Fertilising aims to improve growth by ensuring that an adequate level of nutrients is available to the tree, or to relieve nutrient deficiencies or imbalances.

Responses to fertiliser

ForestrySA has found little or no response to a range of trial fertiliser applications in Tasmanian blue gum (Eucalyptus globulus); however some sites are expected to respond to fertiliser.

The decision to fertilise should be determined by the likely benefits in terms of improved volume production and/or potential improvement in plantation health.

Growth responses to fertiliser applications can vary widely with:

• Fertiliser type,
• Rate of application,
• Method of application (air, ground, machine or manual),
• Timing, positioning, and
• Site factors including:
                §      rainfall,
                §      soil type,
                §      previous land use and fertiliser history,
                §      level of weed control,
                §      cultivation, and
                §      age of plantation. 

Rainfall

Where water availability is not limiting (mainly on sites that receive more than 750 mm), studies have found that the growth of E. globulus is likely to increase with the addition of both nitrogen (N) and phosphorus (P) (Beadle et. al 1991, Gerrand 1991, and Weston 1991). If the availability of soil water is a major factor limiting growth on the site, the magnitude of the response to a given application of fertiliser will be proportional to the water available during the rotation (Beadle et al 1991).

Soil Type

Soil depth largely determines the water holding capacity of the site, while texture and colour indicate the soil’s inherent fertility. In general; freely drained pale coloured sands more than two metres are relatively infertile and are more likely to respond to fertilising.

Previous Land Use

Where the previous land use was grazing on good improved pasture it is likely that there has been adequate nutrition for Tasmanian blue gum growth requirements. Where soils have been recently cropped, or pastures are run down, nutrition levels are difficult to predict. Previously applied fertiliser that has accumulated lower in the profile below the rooting zone of pasture plants may be available to tree crops by the second or third year.

Weed Control

Weeds compete for nutrients and water, depleting soil moisture earlier in the growing season and preventing further uptake of nutrients. Unless the site around each tree is completely bare of weeds through spring and summer, fertiliser may stimulate weed development and potentially hinder Tasmanian blue gum growth. Weed control alone can improve seedling growth in the first year by up to 70% as well as ensure good survival. Response by Tasmanian blue gums will be small if weed control is not effective. 

Cultivation

Greatest growth responses are generally recorded when fertiliser applications are combined with good site preparation and weed control. Site preparation, especially mounding, favourably redistributes humus, which is essential for nutrition in sandy soils. Ripping allows deeper penetration of tree roots through harder soil layers and generally increases access to soil moisture and nutrients.

Age of plantation

Tasmanian blue gums should not require fertiliser in the year of planting except on very infertile sands. Additional nutrition is likely to be most important in the first two to four years. As the trees age and the leaves intercept most of the available light, the rate of nutrient uptake changes and internal cycling becomes increasingly important.

Deficiencies

There are many resources available to assist with the identification of nutrient deficiencies. Correct identification of the disorders becomes difficult where multiple deficiencies are present, and professional help should be sought.

In general, deficiencies of nitrogen (N), phosphorus (P) and potassium (K) usually manifest themselves in the older leaves, while deficiencies of iron (Fe), copper (Cu), zinc (Zn), sulphur (S), boron (B) and calcium (Ca) are first expressed in young leaves. Magnesium (Mg) deficiency appears simultaneously in both older and younger leaves. Manganese (Mn) deficiency appears on expanding leaves, extending to both older and younger leaves, and is associated with alkaline soils (high pH), peaty topsoils in depressions, and shallow and light textured soils on limestone.

Fertilising

Formulations

The chemical analysis of fertiliser is shown as a percentage of the total fertiliser for major nutrients N, P, K and S. For example Pivot 900 (16-8-9-11) indicates that this formulation contains 16% nitrogen, 8% phosphorous, 9% potassium and 11% sulphur.

Fertilising plantations

If the rainfall at a site is greater than 750 mm per annum, and the site does not have a history of improved pasture, then fertilising to increase growth is likely to be necessary. If the cost of fertilising is about $300/ha, a growth response of at least 11m³/ha is required by the time of clearfelling assuming a return of $29/tonne to justify this additional expense. Predicting the extent of a growth response is currently difficult, given the lack of research in the Green Triangle Region.

Calculating application rates

Generally fertiliser rates for pines in the Green Triangle Region are 20-40 kg of nitrogen per hectare for one to two year old trees and 50-80 kg/ha for trees two to four years old. Tasmanian blue gum fertiliser needs are likely to be similar.

To calculate the amount of fertiliser required per hectare to apply 20 kg/ha of nitrogen using a fertiliser with the NKPS analysis of (16-8-9-11) divide the amount of nutrient required by the percentage of it in the fertiliser as follows:

20 kg/ha of nitrogen / 16%= 125kg/ha of fertiliser.

Application Methods

Nitrogen-based fertiliser can kill trees in the in the first year. A 15 cm space between the tree and the closest point of application is essential, especially on light, sandy soils to avoid direct contact with tree roots. In the first year fertiliser should be applied as individual tree dosages (either tablets or loose). This can be on the ground, either as single band semicircle around the tree or in the soil in a single slot, placed beside the tree not closer than 15 cm and not uphill of the tree. Alternatively, fertiliser can be deposited as a band along the line of cultivation or under the mound, provided there is a distinct separation between the tree and the fertiliser.

Aerial or ground based broadcast applications are not recommended until the trees are 14 months old. There is little information available on foliar application of copper and zinc in Tasmanian blue gums, although this is routinely carried out in pines.

Timing

Regeneration of the root system of transplanted trees takes about five weeks in cool soils and the increase of soil temperature in early spring determines when to start fertilising. Generally September is the preferred time to fertilise as soil temperatures are high enough for root uptake of nutrients. Fertilising earlier risks loss of nutrient by leaching.

Conclusion

Few fertilising studies have been conducted in South Australia to date. Preliminary work done in the Green Triangle would tend to support the findings from studies in other states (Beadle et al. 1991, Gerrand 1991, Weston 1991, Sands and Mulligan 1990).

Limitation of water availability where annual rainfall is less than 750 mm is likely to override any gains from the addition of fertiliser on previously fertilised sites. Unfertilised sites and cases of specific deficiencies should respond to fertiliser.

Foliar analysis at twelve months can be used to indicate the need for remedial fertiliser to address deficiencies.

Addition of fertiliser will only be effective with adequate moisture, good site preparation and good weed control.

References

Beadle, C. L. et al 1991 ‘Comparative growth rates of eucalyptus in native forest and in plantation monoculture’, in Growth and water use of forest plantations: proceedings of the International Symposium held at the Hotel Ashok Raddison, Bangalore, India. I. R. Calder, R. L. Hall, and P. G. Adlard (eds), John Wiley and Sons Ltd., Chichester; UK, pp 318-331.

Dell B., Malajczuk, N., Xu, D. and Grove, T. S. 2001. Nutrient disorders in plantation eucalypts. Australian Centre for International Agricultural Research (ACIAR) Monographs MN74, Canberra, ACT.

Gerrand, A 1991 ‘Predicting eucalypt plantation in Tasmania from site and soil parameters’, in:  Productivity in Perspective: Third Australian forest Soils and Nutrition Conference: Melbourne, 7-11 October, 1991. P. J. Ryan (ed.) Forestry Commission of NSW, Sydney, NSW, pp.61.

Sands, R. and Mulligan, D. R. 1990 Water and Nutrient Dynamics and Tree Growth Forest Ecology and Management, vol. 30, pp. 91-111.

Weston, C. J. 1991 ‘Factors limiting the growth of eucalypts across a range of sites in Gippsland, Victoria’, in: Productivity in Perspective: Third Australian forest Soils and Nutrition Conference: Melbourne, 7-11 October, 1991. P. J. Ryan (ed), Forestry Commission of NSW, Sydney, NSW, pp. 158-159.

For further information contact PIRSA Forestry

Disclaimer: While this publication may be of assistance to you, the government of South Australia and its officers do not guarantee that it is without flaw of any kind or is wholly appropriate for your particular purpose. The Government therefore disclaims all liability for any error, loss or other consequence that may arise from you relying on any information in this publication.

Last Revised February 2008