Tree Fertilization Trials in Illinois

Materials and Methods

Four tree species, each in a block of 100 trees, were selected for study. The pin oak, Quercus palustris, had been planted in 1969; the tulip tree, Liriodendron tulipifera; Norway maple, Acer platanoides; and honey locust, Gleditsia triacanthos f. inermis, had been planted in 1970 at 12-foot intervals.

Seventy-five trees in each block were selected for the test. A point on the trunk approximately 3 feet above the soil was permanently marked with paint and measured with a diameter tape in April, 1976. Average diameters in the spring of 1976 were pin oak, 2.4 inches; tulip tree, 3.0 inches; maple, 1.7 inches; and honey locust, 2.0 inches. An array based on tree diameters was prepared and the trees were divided into 15 groups of 5 trees with each group containing small, medium, and large trees. The treatment each group received was randomly selected.

The nutrient formulations selected for this study were high in nitrogen and low in phosphorus and potash. Three complete fertilizers, 20-4-8, 30-3-10, and 34-3-7, were compared with ammonium nitrate, 33.5-0-0. One hundred square feet of soil surrounding each tree were treated. The quantity of fertilizer each tree received was individually packaged.

Three rates of the complete fertilizers were applied based on actual nitrogen content. Nitrogen was used at 2, 4, and 6 pounds per 1000 square feet of area. Ammonium nitrate was applied only at the 6-pound rate. The measured quantity of fertiizer was broadcast onto the soil surface as uniformly as possible by hand.

One complete fertilizer formulation (20-4-8) was placed in holes in the soil. Holes were either drilled with an electric drill or formed by pushing a punch bar into the soil. Twenty-four holes 1 inch in diameter and 15 inches deep were placed around each tree to be treated in a 2 foot by 2 foot grid. A measured quantity of fertilizer was placed in each hole.

The dates for treatments were April 28-30, 1976; March 30-31, 1977; and April 13-14, 1978. The same trees received the same treatment each year.

Precipitation during April-June every year from 1976 through 1979 was below normal. Precipitation data from a weather station about 2 miles from the test plots are given in Table 1.

Tree growth was determined from trunk diameter measurements in October of 1976, 1977, 1978, and 1979.

Results

Fertilized trees grew more than unfertilized trees (Table 2). Fertilizer formulation had no effect: 20-4-8, 30-3-10, and 34-3-7 gave essentially the same response when equal quantities of N were applied. The rate of 6 pounds per 1000 square feet gave significantly more growth than the 2- and 4-pound rates which in turn were not much better than no treatment. There was little or no advantage to placing the nutrients in soil holes. A slight advantage was gained from the use of complete fertilizers rather than nitrogen-only fertilizers in this study.

Differences in species responses to fertilizing were noted. The tulip trees growing at the rate of almost 1 inch in diameter per year increased only 5% following treatment at the 6-pound nitrogen rate. Honey locust and pin oak growing at % inch per year increased by 10% and 17%, respectively. The smallest trees in the test, the Norway maple, growing at less than 0.4 inch per year, increased diameter growth only 4% when fertilized.

Trees treated and trees not treated in 1976-1978 all grew at equal rates in 1979. There was no growth response from residual fertilizers.

Discussion and Summary

The trees included in this study were growing well with trunk diameter growth increases of ½ to 1 inch per year, and the trees appeared normal. It appears that nutrients were not a major limiting factor for plant growth.

Moisture was quite likely a growth-limiting factor. Nutrients applied to the soil surface are dependent upon rainfall to move them into the soil occupied by tree roots. Rainfall amount and number of days with 0.5 inch of precipitation were deficient in April, May, and June of every year in which this test was conducted (Table 1). Since the trees were growing in a bluegrass sod, the grass may have used most of the supplemental nutrients applied to the soil surface. It would be interesting to know what the tree response would have been in these dry years if the grass competition had been eliminated.

Soil in east-central Illinois, when not disturbed by construction activities, is normally an excellent medium for tree growth. Established trees perform well.

In an earlier tree nutrition study in Urbana (2), tree growth increased 15 to 55% with fertiization. in the earlier tests the trees were growing slowly and were more closely spaced and nutrients were probably the limiting factor.

Results from the present study indicate that supplementing the nutrients naturally available in these soil plots was not a suitable cultural practice. Arborists should not expect results from tree feeding when there are factors other than soil fertility that are primarily responsible for limiting tree growth.