Richard E. Miller

Family forest owners and gardeners share a common goal: To protect and maintain their productive soils and to improve their less productive or degraded ones. Most know that productive soil supports plant growth through a vigorous community of soil organisms, physical structure and favorable moisture, nutrition, temperature and aeration.

A wide variety of life forms reside and interact in forest soils. Mycorrhizae are important for water uptake, storage and transport; they release compounds that enhance absorption of certain nutrients. Some species of bacteria in nodulated plant roots and in decaying wood fix (convert) nitrogen (N) from the air into N-forms that are available for plants and other organisms. Certain fungi and bacteria protect particular tree species from pathogens. Various microbes secrete organic glues that bind soil particles into water-stable aggregates that ensure movement and storage of air and water in the soil.

Above the forest soil are leaf litter and woody debris that help protect structure of the mineral soil from the direct impact of rain, heavy equipment or logs. This forest floor (duff) also contributes to soil humus and nutrient cycling, when fallen plant and woody tissues are decomposed by the resident host of invertebrates, fungi and other microorganisms that are themselves food for small mammals and birds. Forest floor depth and composition result from past natural environment and human-caused disturbance such as timber harvesting and site preparation. Forest floor depth indicates the balance between the rate at which organic debris is deposited and decomposed. Some forest floor is always desirable. However, deep accumulation of forest floor (exceeding about five inches) usually indicates slow rates of organic matter decomposition and nutrient cycling, which restrict growth or diversity of soil organisms and plants.

Protecting Soil Productivity

Natural and human-caused disturbances affect soil structure, nutrient status, moisture, temperature, pH and organic matter; litterfall, forest-floor composition and plant-species composition can also be changed. For example, fire frequency, and especially intensity, influence losses of organic matter and nutrients. Conservation and addition of nitrogen is particularly important because this nutrient element usually limits forest productivity. Soil compaction and disturbance alter soil structure and pore space, disrupting flows of gases and water essential to soil organisms and plants. Only when bulk densities are low can roots readily exploit the soil for water and nutrients. Plants, especially large trees, also require a well-distributed root system to provide physical support and avoid windthrow.

To keep their soils healthy, family forest owners should:

o Minimize severity of human-caused disturbance. Activities that extensively displace or compact mineral soil are undesirable, especially on steep topography (which is more likely to erode) or shallow soils (which inherently have limited volume for rooting).

o Ensure adequate sources of small and large woody debris to replenish what is lost through both natural events and human activities. Large debris protects soil and forest floor, especially on slopes, by forming natural barriers to erosion. Small debris helps buffer the soil from compaction; decomposing debris renews organic matter in the mineral soil and keeps vital nutrients cycling.

o Control vegetation if necessary to ensure adequate survival of seedlings, but retain some plant cover. Intensive silviculture characterizes many shrubs and hardwoods as "brush" and slates them for removal as vegetative competitors. Yet these plants, often first to colonize sites after disturbance, stabilize sites and lay the foundation for later successional species.

o Promote conditions (for example, plant cover and woody debris) that moderate climate near the soil surface. Harsh sites, such as those with extremely dry or cold soils, are most susceptible to reduced productivity and ecosystem resiliency.

o Learn more about your soils and their response to heavy equipment and natural disturbance. Does rain enter your soil rapidly? Can you easily break through your soils with your shovel? Soil characteristics and current condition should be evaluated carefully on a site-specific basis wherever activities with heavy equipment are proposed. Published soil survey reports by the Natural Resources Conservation Service can provide useful information about soil series on your land and their characteristics.

Improving Soil Productivity

Current productivity of some forest soils can be improved. Their poor productive capacity could result from past human activities or from natural limitations in their soil-forming factors (parent material, climate, organisms, topography, time and their interactions). Quality soil develops when each of these factors has been favorable. For example, in the Northwest, many forest soils are developing on parent material deposited by glaciers or volcanic flows or ejecta less than 15,000 years ago. Much more time and favorable climate are required for these geologic deposits to weather to clays and to accumulate organic matter. Both clay and soil humus are important for moisture and nutrient retention and supply.

Soil fertility can be improved by fertilization to correct native or induced nutrient deficiencies. These shortages limit soil processes, hence growth of soil organisms and aboveground vegetation, including trees. The practicality of using fertilizers to replace nutrients lost in harvesting (exported) and in site preparation is worth considering for naturally infertile soils. Such losses of nitrogen range from 400 to over 1,000 pounds N per acre. For full replacement, however, more would have to be added because not all applied N is captured by the trees, vegetation and soil. Thus, conservation of existing nutrients is a preferable alternative. If less wood volume, especially tree crowns, were harvested or destroyed in site preparation, then lesser amounts of nitrogen would need replacement.

When sufficient (but not excessive) amounts of growth-limiting nutrients are applied, the soil ecosystem and dependent larger ecosystems usually develop faster. Applying nitrogen fertilizer is a proven means of accelerating production of large trees in some forest types in the Pacific Northwest. About 70 percent of coast Douglas-fir stands grow faster after application of about 200 pounds N per acre; response by stands of western hemlock and other species is less predictable, though sometimes successful. Other benefits of fertilization include darker green foliage and higher quality forage for animals. Fertilizing with elements other than N has received limited research. West of the Cascade Range, elements other than N have seldom provided adequate stimulation of tree growth to justify the additional costs. East of the Cascades, their benefits to tree performance are better recognized.

Intensive silviculture prescribes fertilizers to increase growth; refertilization is conventionally scheduled at about 10-year intervals. Multiple-use forestry (creation of wildlife habitat or restoration forestry) shares this goal to accelerate growth and development of large trees. Current urgings for longer rotations and less intensive harvesting and site preparation could reduce the need for frequent fertilization because: (1) fewer nutrients would be exported in harvested material; (2) more N could be added by the natural process of N-fixation; and (3) nutrient needs of older trees could be satisfied by internal recycling of nutrients.

Other remedial treatments can also benefit degraded soils. For example, tilling or ripping compacted soil can improve soil structure or density; and removing trees can increase the decomposition rate of the forest floor by physically disturbing it, changing the microclimate and providing conditions for understory vegetation.

Glossary of Soil Science Terms-(From the Soil Science Society of America)

Soil productivity-The capacity of a soil to produce a certain yield of crops or other plants with a specified system of management.

Soil structure-The combination or arrangement of primary soil particles (sand, silt, clay) into secondary units or peds. The secondary units are characterized on the basis of size, shape and grade (degree of distinctness).

Bulk density-The mass (weight) of dry soil per unit bulk volume.

Soil series-The lowest category of U.S. system of soil taxonomy. Soil series are commonly used to name dominant or codominant polypedons represented on detailed soil maps. The soil series serve as a major vehicle to transfer soil information and research knowledge from one soil area to another.

Further Reading and Other Sources of Information about Soils

Perry, D. A.; Meurisse, R., et al., eds. 1989. Maintaining the long-term productivity of Pacific Northwest forest ecosystems. Timber Press, Portland, OR. 256 pp. Chappell, H. N.; Weetman, G. F.; Miller, R. E., eds. 1992. Forest fertilization: Sustaining and improving nutrition and growth of western forests. College of Forest Resources, University of Washington, Seattle. Forest Institute of Resources Contribution No. 73. 302 pp.

USDA Natural Resources Conservation Service, Soil Biology Primer (PA-1637), 101 SW Main, Suite 1600, Portland, OR 97204-3221; 503-414-3012; www.or.ncrs.usda.gov/soil/mlra.html.

USDA Natural Resources Conservation Service Soil Survey Division: www.statlab.iastate.edu/soils/homepage.html/main.html Soil Quality Institute: www.statlab.iastate.edu/survey/SQI/

Soil Science Society of America, Division S7-Forest and Range Soils http://soilslab.cfr.washington.edu/S-7/index.htm

Richard E. Miller is a member of the Washington Farm Forestry Association and former research soil scientist with the Pacific Northwest Research Station, USDA, Forest Service.

This article appeared in the Northwest Woodlands Magazine, Summer 2001- Published quarterly by the World Forestry Center as a benefit of membership in the Oregon Small Woodlands Association, Washington Farm Forestry Association, Idaho Forest Owners Association and Montana Forest Owners Association.