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