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By Steve Webster
Although most forest soils in the Pacific Northwest provide enough nutrients to sustain adequate tree growth, adding nutrients by fertilizing will increase growth in many instances. This increased growth is called response. The amount of response and the best fertilizer prescription depends on many things such as soil type, tree species, age, stand condition and climate.
Coastal forests
Douglas-fir stands, which predominate in coastal forests, are quite responsive to nitrogen (N) fertilization. Responses range from more than 25 percent on site index class IV (low) lands to less than 10 percent on site index class I (very high) lands.
After summarizing a long-term research study, the University of Washington reported an average response of about 1,400 board feet for unthinned Douglas-fir when fertilized with 200 pounds of urea-N per acre. Thinned stands responded better, producing about 1,600 more board feet per acre when fertilized (Figure 1). The explanation given for this difference is that the individual trees in thinned stands have more moisture, light and other resources and can consequently better utilize the extra nitrogen.
Assuming a stumpage value of $500 per thousand board feet for second-growth Douglas-fir, this extra wood in unthinned stands would have a value of about $700 at the time the stand is harvested. The value of the extra wood in thinned stands would be more because of the greater response and because the stumpage price should be higher.
Obviously, thinned stands should be given first priority for fertilizing. Additionally, low site stands should be given priority over high site stands, because, as noted above, the gain is greater on low site stands than on high site stands. Readers might note in a companion article that the reverse appears to be the case with seedling fertilization, where higher site lands with greater moisture availability appear to be more responsive than drier low site lands.
Fertilizing with 400 pounds of N per acre produced nearly 40 percent more response than did 200 pounds of N per acre. Nevertheless, 200 pounds is probably more economically profitable than 400 pounds.
Helicopter application of nitrogen is the most common method and the cost ranges from $65 to $90 per acre depending on the size of the unit, accessibility and the helicopter company. Ground applications using manual procedures or tractors should be less costly and will vary depending on the method.
Achieving increased growth worth $700 or more per acre at a cost of $65 to $90 per acre makes fertilizing appear to be an attractive silvicultural investment. Remember, however, that when calculating financial returns from fertilizing, one should use compounded fertilizer costs rather than current costs. For example, if fertilizer were applied today to a 30-year-old stand at a total cost of $75 per acre, but that investment was not returned until the stand is harvested 30 years later at age 60, the fertilizer cost compounded for 30 years at a rate of six percent is $431 per acre. Fertilizing stands within 10 to 20 years of harvest is often the most profitable, because the response is the same as for younger stands, but the investment period is shorter.
Western hemlock response to nitrogen is erratic. Sometimes growth increases, sometimes not. The reason remains unknown. Noble fir appears to be responsive, but more research is needed to quantify volume response. An interesting side note is that noble fir stands are often fertilized with nitrogen to improve the color of the foliage and make the boughs more valuable as a greenery product. Little is known about the response of western redcedar and Sitka spruce.
Experimental fertilization with nutrients other than nitrogen, such as phosphorus, potassium, sulfur, lime and trace elements, has not in most circumstances resulted in greater growth.
For more information, contact a your local WSU County Extension Office for a copy of EB 1800, Fertilizing Coastal Douglas-fir Forests or contact the Bulletins Department at WSU(www.cahe.wsu.edu). at http://caheinfo.wsu.edu.
Inland forests
For inland forests, response to fertilizer is more complex than for coastal forests. From research conducted by the University of Idaho, habitat type and soil parent material have emerged as important variables influencing response to fertilizer.
In general, inland forests of Washington, Oregon and Idaho are responsive to nitrogen fertilization, but the response to a single application of 200 pounds urea-N per acre ranges from a high of about 1,600 board feet per acre on highly productive grand fir types to a low of about 400 board feet per acre or less on ponderosa pine types with low productivity. The percentage increase in growth may be similar across habitat type, but the volume increase varies considerably depending on the unfertilized growth rate.
In eastern Washington and northern Idaho, stands growing on light-colored granitic, meta-sedimentary and mixed glacial soil parent materials are responsive to potassium as well as nitrogen. The recommended fertilizer prescription for these stands is 200 pounds urea-N urea N per acre along with 100 to 200 pounds of potassium K per acre as potassium chloride. Stands growing on basaltic, dark-colored granitics or sandstone parent materials are responsive only to nitrogen, with the recommended application being 200 pounds urea-N per acre. Refer to county soil survey reports prepared by the Natural Resources Conservation Service to determine soil parent materials for your property. These reports are available at county conservation district offices.
Adding to the complexity of fertilizing inland forests are research findings of response to sulfur, boron and phosphorus on certain soil or parent material types found in inland forests. The need for nutrients other than nitrogen in many inland forests is not yet fully understood. For more details, see WSU Extension Bulletin EB 1874 entitled Fertilizing Eastern Washington Coniferous Forests available from WSU County Extension Offices or the Bulletins Department at WSU at (www.cahe.wsu.edu). http://caheinfo.wsu.edu.
Duration of response
The effect of nitrogen fertilization lasts about eight years with the greatest increase in growth occurring in the first five years. Repeated fertilizations will produce responses similar to the first fertilization. If, for example, a thinned Douglas-fir stand was fertilized at the ages of 30, 40 and 50 years and not harvested until it was at least 58 years old, one could expect the stand to have about 4,800 more board feet per acre than if it had not been fertilized.
Wood quality
Concern has been raised by some that faster growing fertilized trees will produce wood with fewer annual growth rings per inch and will be of lower quality, strength and value. Research on this subject has, indeed, found that accelerating tree growth rates with fertilizer (or other treatments) does tend to lower the specific gravity of the wood, resulting in some minor loss of strength. However, for most applications of the wood products coming from second-growth Douglas-fir, this small loss in strength is of little consequence. For applications where fineness of wood grain is important, fertilization may not be desirable.
Steve Webster is a Washington State University Forestry Extension Agent based in Chehalis.
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