1992

Loblolly Pine Growth Response Favors TVA Experimental Fertilizer

AUBURN, Ala. - Traditionally, foresters have been reluctant to use fertilizers due to the cost, even though nitrogen (N) normally shows a growth response. However, forest fertilization has become more common during the past two decades as more reliable diagnostic techniques have been developed to determine nutrient deficiencies. Recent Alabama Agricultural Experiment Station research with an experimental fertilizer is helping to identify fertilizer sources which are more effective at promoting growth responses.

Urea has been the primary nitrogen source of choice in forestry due to its high nitrogen analysis. However, urea has two potential major disadvantages: (1) high ammonia losses under conditions of high soil temperature and (2) toxicity of urea to seed and seedlings due to potentially high concentrations of ammonia.

Urea nitric phosphate (UNP), a granular fertilizer of grade 27-9-0, is an experimental fertilizer that has been proposed by the Tennessee Valley Authority as an alternative to urea-phosphate blends. Studies have shown that UNP has the potential to reduce ammonia loss by as much as 50 percent over urea, due to a relatively low pH, a significant fraction of the nitrogen in the nitrate form, and free calcium that can react with carbonate from hydrolyzed urea. There also are indications that UNP has fewer adverse effects on seed germination than urea.

A 14-year-old loblolly pine stand, located on a Piedmont site near Auburn, was used to compare growth response (height, diameter and bole dry weight) to UNP and urea plus triple superphosphate (TSP). The growth response of these two fertilizers also was compared to an unfertilized control. The actual fertilizer treatments consisted of: (1) no fertilizer, (2) 200 pounds per acre N and 67 pounds per acre P205 from UNP and (3) 200 pounds per acre N as urea, plus 67 pounds per acre P2O5 from TSP.

Field plots (0.05-acre measurement plots within 0.25-acre treatment plots) were established in the winter of 1988 and inventoried for diameter, height and bole dry weight. Fertilizer treatments were applied the following spring and the plots were inventoried again in the winter of 1989 to assess one-year response.

Although no differences in response were apparent for diameter after one growing season, the UNP treatment was greater than the unfertilized control for both height and bole dry weight. The urea plus TSP application resulted in intermediate height and bole weights, showing no difference from either the UNP or the control treatment. Although large treatment responses are not yet apparent, these first-year results indicate that, in this situation, UNP did produce a growth response and was the preferable fertilizer treatment.

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Auburn, AL 36849
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Contact Jamie Creamer, 334-844-2783 or jcreamer@auburn.edu
by D.T. Crawford and B.G. Lockaby