Forest fertilization, is it for you?

Spring 2000


Fertilization is a forestland management tool that is getting new attention. Increased tree growth, reduced incidence of disease, and lower levels of tree mortality are potential benefits from a forest fertilization program.

Several factors should be considered to determine whether fertilization is a viable management tool on a given parcel. These include landowner objectives, forest condition, and economic feasibility. Currently, the prevailing opinion among forestry professionals is that when used appropriately, forest fertilization is a worthwhile activity.

The two major goals of fertilization are to increase the production of wood, and to reduce the incidence of disease. Studies have shown that on the appropriate site, both of these objectives can be accomplished with fertilization.

The ability to increase wood production through nitrogen fertilization has been well documented. Recently, pioneering research has indicated that fertilizing with potassium, and possibly other trace elements (primarily sulfur and boron), may also lead to a reduction in tree mortality caused by root disease and other pests. The importance of potassium in forest health makes sense since the lack of this nutrient has long been associated with agricultural crop disease problems.

Two tree species that are highly susceptible to root disease, Douglas-fir and grand fir, have much greater nutrient demands then other more root disease resistant species. Consequently, it appears that Douglas-fir and grand fir sites containing soils of low fertility are particularly prone to having root disease problems.

New research has also discovered a possible link between the presence of root disease and the parent material of forest soils. For example, trees growing in soils formed from basalt parent materials are less likely to be affected by root disease than those growing in soils derived from granite. The primary reason for this appears to be the greater availability of potassium in basalt-based soils than in granite-based soils.

The numbers: Although results vary widely, fertilization has the potential to increase wood production by 1,500 board feet per acre over ten years and to generate an average 23% net return on the investment in that same time. A rough estimate of fertilization costs, including fertilizer and using a helicopter for application, is $120.00 per acre for nitrogen and potassium fertilizer, and $160.00 per acre for nitrogen and potassium with trace element supplements. These costs are greatly influenced by the size of the parcel to be fertilized. On very small land areas, hand application of fertilizer is a viable option.

Of course, natural sources of fertility are also present on forestland. Approximately 60% of the potassium and 75% of the nitrogen in a tree is located in the needles and smaller branches. With this in mind, it is important to leave as much of this material in the woods as possible after a logging operation. In some instances, aesthetic needs or high wildfire risks make this management strategy prohibitive.

What does this all mean? The following information highlights a few points to consider:

  • Forests respond best to fertilization if they have been thinned and exhibit good individual tree vigor.
  • In most instances, potassium should be included in the fertilization mix to increase forest health.
  • Fertilization is somewhat expensive, but usually provides an attractive financial return.
  • Harvest operations should strive to leave as much slash in the woods as is acceptable from a visual and fire risk standpoint.

While forest fertilization is a relatively new management strategy, research on the subject is encouraging. The increased growth potential and disease resistance can dramatically change the productivity and character of your forest. If you are interested in pursuing a fertilization program, please contact our office.

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