Forest Fertilization Guidebook Table of Contents]

Soil moisture and nutrient regimes

Moisture and nutrient regimes of forest soils affect tree responses to fertilizing and must therefore be carefully considered.

The most reliable information on soil nutrient availability is obtained from foliar analysis. While evidence of chlorotic tree foliage can indicate soil infertility, caution should be used in making interpretations based on these indications (see Crown condition).

Lastly, the plant community often reflects soil moisture and nutrient regimes. Use regional guides to relate plant associations to soil moisture and nutrient conditions.

Soil moisture regime

For coastal Douglas-fir, the largest and most consistent responses are found on slightly dry to fresh sites (e.g., submesic and mesic).

In the B.C. interior, sites that are slightly dry to fresh should receive the highest priority (e.g., submesic and mesic), with lesser priority assigned to drier (subxeric) and wetter (subhygric and hygric) sites. Because soil moisture, either by deficiency or excess, may exert the primary limitation on tree growth, fertilization of stands with very dry (xeric and very xeric) or very wet (subhydric) conditions should be avoided.

Nutrient regime

Although benefits from fertilizing occur most consistently on infertile sites, very infertile sites (site quality is low) are unsuitable for treatment because natural growth rates are too slow. Avoid very rich sites also, since the soil probably contains adequate nutrients. Stands on poor and medium regimes should respond best, and therefore will be given the highest priority. Moisture and nutrient regimes are two of the main factors that determine site quality as discussed in the following section.


Site quality

Site quality considerations vary between coastal and interior sites.


Research data in coastal Douglas-fir stands indicate that the magnitude of growth response to fertilizer application is greater on poorer sites than on good sites. Based on the current Inventory Branch (Ministry of Forests) classification, preference should therefore be given to fertilizing poor and medium sites. Response on good sites will be small relative to the size of the trees. However, the absolute response on good sites (periodic increment) can be large compared to the response on medium or poor sites. Fertilization on good sites should be considered when a large absolute response is predicted (e.g., on sites having a moist soil moisture regime and medium soil nutrient regime). Fertilizer should not be applied on low sites, since they are unlikely to ever become economically productive.


Preference should be given to fertilizing medium sites (e.g., submesic to mesic soil moisture regimes and poor to medium soil nutrient regimes) in the B.C. interior. A lower priority should be assigned to good sites until such time that additional research information is available. Fertilization on poor sites will have low priority since site productivity is probably influenced strongly by inadequate soil moisture or extreme climate. Even where relative growth responses are favorable, the absolute volume gains on these lower productivity sites may be too small to make fertilization profitable. Fertilizer should not be applied on low sites.

Crown condition

The size and condition of live crowns provide an indication of the nutrient status and productive potential of stands. Considerations related to crown size and foliar characteristics are described below.


In unspaced and recently spaced stands, evidence of many trees with short, narrow crowns suggests competitive stress is, or has been, strong. Application of fertilizer to stands in this condition will enhance crown expansion by stimulating growth of branches and foliage. Assign a priority to the treatment of these stands provided they meet the criteria described in the guideline in the section on stand density.

Foliage color

Small, yellowish, and sparse foliage throughout the stand may indicate that one or more soil nutrients are deficient. In this situation fertilization may achieve a substantial growth response. However a chlorotic appearance may also be caused by drought or pathological conditions. In the absence of chemical analysis of soils or foliage, the interpretation of visual symptoms requires expertise and local knowledge. Look for other symptoms that may identify insect, disease or animal damage. Also consider soil conditions, ground vegetation, and rainfall patterns to infer drought.

If foliar appearance in a stand can be reliably interpreted to indicate nutrient deficiency and sufficient response is anticipated to make the treatment economical, the site can be assigned a high priority for fertilizing. However, an absence of visual symptoms does not preclude the possibility of growth limiting nutrient deficiencies.

Nutrient diagnosis

Foliar analysis

Foliar analysis is a useful method of indicating possible nutrient deficiencies. Foliar analysis should only be undertaken after a candidate stand meets all other biological, operational, and wood supply criteria. Because of the cost of sampling, foliar analysis should only be done on sites which have the potential for a return on investment. It is also important to assess soil and site conditions in conjunction with foliar analysis in order to facilitate the interpretation of analytical results.

Tables 1 and 2 in Appendix 2, show interpretations of macro and micronutrient foliar concentrations. Consult a tree nutrition specialist when interpreting results of foliar analysis.

Appendix 2 outlines the foliar sampling procedure.

Adequate time must be scheduled for foliar analysis. Labs routinely take 46 weeks to analyze samples.

On the coast, sampling of candidate Douglas-fir stands is not normally required. However, foliar sampling should be carried out if local experience suggests that multiple nutritional problems are likely. Candidate hemlock and spruce should be analyzed to ensure that they are N deficient and that other nutrients (e.g., phosphorus) will not limit response.

Based on the results of the analysis, stands that are nitrogen (N) deficient only, with no potential N-induced deficiencies, should receive the highest priority for treatment. Second priority should be stands that are N deficient but that also likely have one other deficiency. For example, lodgepole pine stands with pre-fertilization foliar sulphate S levels <60 ppm are likely sulphur (S) deficient and may not respond well to N fertilization. Fertilizers applied to S deficient stands should contain both growth limiting nutrients (e.g., 35-0-0-10S). Nitrogen fertilization of lodgepole pine stands with low pre-fertilization foliar B levels (i.e., <10 ppm) may result in severe top die-back deficiency symptoms. It is recommended that stands with pre-fertilization foliar boron (B) levels <10 ppm not be fertilized or that a small amount of B (i.e., 1.53 kg/ha) be added to the fertilizer mix. Stands with complex nutritional problems, deficient in many nutrients, should not be fertilized.

Screening trials

Screening trials utilize foliar analysis and small scale fertilizer trials to rapidly estimate fertilization response potential. As with foliar analysis, they should only be done when a candidate stand meets all other biological, operational, and wood supply criteria and the potential response from the treatment is not known. If a year and a half lead time is available after stand ranking is complete, consider installing some screening trials. A screening trial is used to diagnose specific nutrient deficiencies and to predict which stands will likely respond to fertilization. Screening trials may be appropriate for species with little research data or inconsistent response. Screening trials are not normally necessary for coastal Douglas-fir as sufficient predictive capabilities are now available. Western red cedar or western hemlock are not appropriate for operational screening trials because of their indeterminate buds. Research trials are necessary to obtain response data from indeterminate species. A screening trial can be initiated in the spring with response determined from foliar samples taken in the fall of the same year. The results can be used in planning the next years fall fertilization program. Screening trial methodology is described in Appendix 3.

Insects, disease, and small wildlife

The susceptibility of a stand to certain damaging agents may, or may not, be increased by fertilization. The degree of damage that can be accepted will vary by forest health factor and severity of impact. A forest health specialist should be consulted in situations where insect, disease, or animal factors may affect the priority rating of candidate stands.

Laminated root rot (Phellinus weirii) is the most significant fungal pathogen affecting growth and survival of Douglas-fir. On coastal sites, fertilization appears to have little effect on the incidence or spread of the disease. Minimally infected stands (<6% incidence of trees infected) close to rotation age may be treated. Fertilization of these stands may help them achieve harvestable size before they become severely infected. Evaluate immature Douglas-fir stands infected with laminated root rot and concentrate fertilizer operations on non-infected strata. Little is known about the effects of fertilization on the spread of other root diseases. However, N-fertilized Douglas-fir in the Intermountain region (Northern Idaho, Montana, and eastern Washington) is thought to be more susceptible to armillaria root rot (Armillaria ostoyae). Plans to treat stands that are infected with any type of root disease should be reviewed by forest health specialists.

The spruce weevil (Pissodes strobi) is the principal insect that may affect priorities for fertilizing in coastal immature Sitka spruce stands. Refer to forest health specialists for advice since increased levels of weevil attack following fertilization have been observed.

The western spruce budworm is found in parts of the Vancouver, Kamloops, and Cariboo Forest Regions. The implications of budworm infestations in relation to fertilizer programs cannot be generalized. Problems regarding specific candidate stands should be referred to forest health specialists.

In the interior, sharp increases of red squirrel feeding damage on lodgepole pine have been observed after fertilization. If any pre-fertilization damage is noted in the general area of the stand, consult regional forest health specialists and stand tending foresters for advice on whether to fertilize.


Consult with local forest health specialists and stand tending foresters when dealing with laminated root rot, tomentosus, armillaria, white pine weevil, western spruce budworm, and red squirrel in areas being considered for fertilization treatments. Treatment may increase stand risk to damaging agents.

Operational factors

The following four operational factors should be considered during the evaluation of candidate stands.

Location: Distance to haul the fertilizer affects transportation costs. Also, costs of future commercial thinning and final harvest are partly determined by hauling distances to manufacturing plants and markets.

Access: Conditions of access also affect costs of transporting material and personnel in fertilizer operations, in addition to later expenses of hauling timber to manufacturing plants.

Slope: Costs of future management (e.g., commercial thinning) and harvesting usually increase as terrain becomes steeper. Furthermore, flying over steep or irregular, contoured land may not be conducive to efficient and uniform aerial distribution of fertilizer.

Project and Block Size: Project and block sizes affect efficiency and cost of operation. Large-scale programs (e.g., >300 ha) every 25 years are generally more cost effective than small-scale programs every year.

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