7. FERTILIZATION IN INTERMEDIATE-AGED STANDS

By age 5 or earlier, a plantation’s potential to use N and P typically outstrips the available soil supply resulting in restricted leaf area development and growth (Figure 3, Allen et al. 1990). By time of canopy closure, stands are generally very responsive to additions of N or N+P rather than P alone, as long as gross P deficiencies were ameliorated at or soon after planting. Volume growth responses vary depending on stand/site attributes and the rates of N and/or P applied. Results from an extensive series of intermediate-aged fertilizer trials in loblolly pine stands established by the Forest Nutrition Cooperative indicate that over 85 percent of the stands fertilized were responsive to additions of N+P. Growth gains averaging 30% (50 ft³/acre/year) over a six-year period following a one-time application of 200 lbs/acre N and 25 lbs/acre P are typical. Responses of over 100 ft³/acre/year are possible on some sites. For the majority of stands, both N and P are deficient with responses to additions of N+P much greater than the additive effects of the two elements applied alone (i.e. N and P additions are synergistic).

In intermediate-aged stands, little response is typically observed to additions of P alone except on very P-deficient sites as indicated by low foliar P concentrations and very low leaf areas. Applications of 200 lbs/acre of DAP (labeled as P established in Figure 4) are typically used as the remedial treatment in these stands. Additions of N alone can actually be detrimental to growth on these severely P-deficient sites.

The large increase in area fertilized has been due primarily to the dramatic increase in fertilization of intermediate-aged stands (Figure 4). On most sites, a prescription of between 150 to 200 lbs/acre of N plus 25 lbs/acre of P is used for loblolly or slash pine. Lower doses of N are recommended for longleaf pine (Blevins et al. 1998). Typically, this treatment is applied as an application of DAP and urea to provide the additional N required to reach the desired N rate. Although most intermediate-aged stands will be biologically responsive to N+P fertilization, financial returns are strongly dependent on fertilizer cost, the product mix, and stumpage price that can be realized for the additional wood produced. Application of 200 lbs/acre N plus 25 lbs/acre P presently costs from $65 to $90 per acre. At this price, fertilization is a very attractive investment with internal rates of return exceeding 15% or more. Until recently, fertilization has been delayed until after the first or second thinning to maximize returns on investment. This is now changing as the need for nutrients earlier in a plantation’s life is recognized and increasing wood values have made even early fertilization a very attractive financial investment.

The diagnostic techniques for identifying intermediate-aged stands that will be biologically responsive to fertilization have recently undergone substantial revision. In the past, stand attributes (e.g. basal area and site index), soil/landscape type, foliar concentrations (Colbert and Allen 1996), and experience were used together to identify responsive stands and prescribe the appropriate elements and rates to apply (Allen 1994). Unfortunately, the ability to predict the responsiveness of an individual stand was limited. This situation has now changed with the application of recent Forest Nutrition Cooperative research that quantifies the linkages among stand productivity, leaf area, and nutrient availability (e.g. Albaugh et al. 1998). Differences between a stand's current leaf area and its potential leaf area are now being used to estimate responsiveness to nutrient additions and the appropriate timing, elements, and rates of application. Quite simply, leaf area of a fully stocked stand (basal area >100 ft²/acre) should be 3.5 or greater; otherwise, the stand is probably in need of N+P. The probability and magnitude of response will be greater at lower leaf areas.

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