Forest Fertilization Guidebook Table of Contents]

Appendix 3.

Guidelines for screening trials


First-year increases in needle weight and foliar nutrient concentration can provide a useful quick index of long term stemwood growth response to fertilization. As such, fertilizer screening trials can be established in candidate stands to predict which stands are likely to respond to fertilization. If established in the spring, the first year foliage response can be obtained in the fall of the same year. The needles of fertilized and unfertilized trees are weighed to find if there has been an increase in needle weight as a result of fertilization. Foliar analysis will show any change in the nutrient status of the trees associated with the fertilizer application. The results can then be used to rank candidate stands on the basis of fertilization response potential and to finalize plans for the next year’s fall operational fertilization program.

Screening trials should only be established after a candidate stand meets all other biological, operational, and wood supply tests. Screening trials are most appropriate for species with little fertilization research data or inconsistent growth response.


The predictive reliability of screening trials is dependent on how well the changes in needle weight and foliar nutrient concentration reflect subsequent stemwood response. As such, four important criteria must be met:

  1. The species examined must be determinate (i.e., leaf primordia are initiated in the previous year).

  2. Only the current year’s foliage is used in the analysis.

  3. The methods used for collecting, handling, and drying foliage samples are consistent with those outlined in Appendix 2 and in Land Management Report No. 20.

  4. The trees selected for sampling are representative of the stand being considered for fertilization.

If these criteria are met, the following guidelines can be used to establish screening trials within candidate stands.

Mini-plot establishment

Within the candidate stand, two adjacent dominant or codominant trees with room for crown expansion should be selected for each of 15 mini-plots. The paired trees should all have similar diameters, heights, and live crown lengths, and should be growing in areas of the stand that are relatively homogeneous in terms of understorey vegetation, microsite position, soil, and stand structure. Selected trees should have no obvious symptoms of insect or pathological activity and should not have heavy cone crops.

A 0.01 ha mini-plot should be centred around each pair of sample trees and plot boundaries identified with flagging. A minimum distance of 5 m should separate the outer boundaries of adjacent mini-plots. Both of the sample trees within each mini-plot should be clearly marked with paint and flagging. Prepare a map showing the location of the 15 mini-plots within the candidate stand.

Fertilizer measurement and application

Fertilizer screening trials normally include three treatments:
  1. control (i.e., unfertilized)
  2. N alone
  3. complete nutrient blend (e.g., NPKSMgB).

This makes it possible to evaluate the foliar response to N alone as well as any additional response associated with the application of other growth-limiting nutrients. In most cases, it is possible to identify which of the additional nutrients included in the blend is responsible for the incremental response. In the event of a larger growth response with the complete blend, the predicted extra response will have to be weighed against the additional costs of adding other nutrients to the operational mix.

The following fertilizers and rates are recommended for use in screening trials:

N – urea (46-0-0) @ 200 kg N/ha
P – triple superphosphate (0-45-0) @ 100 kg P/ha
K – sulphate potash magnesia (0-0-22-11Mg-22S) or muriate of potash (0-0-60) @ 100 kg K/ha
Mg – sulphate potash magnesia or magnesium sulphate (0-0-0-10.5Mg-14S) @ 50 kg Mg/ha
S – sulphate potash magnesia or magnesium sulphate @ 100 kg S/ha

Micronutrients (e.g., Cu, Zn,) can also be added to the complete blend where deficiencies are suspected.

The specific nutrients to be included in the complete blend and the amount of each actual nutrient to be added per hectare should be discussed with the fertilizer supplier prior to ordering. The calculated amounts of fertilizer to be applied per mini-plot should be carefully checked prior to application.

The amount of fertilizer to be applied to each 0.01 ha mini-plot should be pre-weighed and bagged. Each of the two fertilizer treatments (N alone and complete blend) will be applied to five of the mini-plots. To ensure even fertilizer distribution, the fertilizer for each mini-plot should be divided into two equal portions with each portion being spread evenly over one-half of the plot. The five control mini-plots are left untreated.

Foliage sampling and analysis

Foliage samples will be collected from both trees in each mini-plot in the fall after fertilization. The collection, handling, and drying of foliage samples must be consistent with methods outlined in Appendix 2 and in Land Management Report No. 20.

The analysis of the foliage samples can be done by a number of commercial laboratories in the Pacific Northwest. The analysis must include determination of the weight of 100 needles (or fascicles for lodgepole pine) for each of the 30 sample trees (two trees per mini-plot). An equal weight of foliage from each of the two trees per mini-plot should then be combined to form a single composite sample for chemical analysis. After grinding, the 15 composite samples will be analyzed for all macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Cu, Zn, Mn, B). Because it is particularly useful in diagnosing S deficiencies, foliage should also be analyzed for sulphate-S.

Interpretation of results

The interpretation of the needle weight results is quite straightforward. The average increases in first year needle weight of fertilized plots relative to the control will be used to assess the fertilization response potential of the candidate stand. Those stands with the largest increases in needle weight probably have the highest stemwood growth response potential. Differences in needle weight response between the N only and complete blend treatments will give an indication of the likelihood of other nutrient deficiencies.

The interpretation of the foliar data is considerably more complicated. The direction and magnitude of shifts in first-year foliar nutrient concentration, nutrient content, and needle weight are used to make diagnostic interpretations about the nutritional status of foliage with respect to both added and non-added nutrients. In cases where the complete blend gives a larger needle weight response than N alone, the nutrients(s) responsible for the extra response must be identified. A forest nutrition specialist should be consulted to help interpret foliar data.

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