Preemergent Herbicide Trials with Direct-Seeded Black Locust Grown in Different Soils

Materials and Methods

Seed were scarified with concentrated sulfuric acid for 60 minutes. Twenty-five seeds were planted in individual 3.8 liter plastic nursery containers at 0.6 cm depth in the following soil types:

• * Eudora, silty-clay loam with a pH of 7.5, 2.0% organic matter, 146 kg available phosphorus (P), and 560 kg exchangeable potassium (K) per hectare.

• * Shellabarger sandy loam with a pH of 6.1, 1.6% organic matter, 37 kg P, and 575 kg K per hectare.

• * Cass loam/sandy loam with pH of 7.7, 1.2, organic matter, 22 kg P, and 336 kg K per hectare.

The following day, three replications of each of eight herbicide treatments were applied to the soil surface of randomly selected containers. Eight treatments (Table 1) using the best herbicide rates from our previous trials (6, 7) were prepared. Treatments were: alachlor, chloroxuron, DCPA, EPTC, napropamide, oxadiazon, pro-fluralin, and a control. A 1 ml aliquot stock solution was mixed with 232 ml of water to simulate a 1.3 cm irrigation per pot. Water and fertilizer were provided as needed throughout the experimental period. Seedling counts were made at 6-day intervals. Sixty days after seeding, plant height and final survival counts were recorded. Plants were cut at the soil surface and oven-dried at 65°0 C for 48 hours for dry weight measurements. Separate tests were conducted for each soil type, thus only ranking between herbicides should be compared.

Results and Discussion

Some herbicide treatments resulted in a significant survival reduction of black locust in some soil types (Table 1). Although grown at a different time, survival was less in soil than in peat/sand mixture as previously reported (6, 7), but usually herbicide comparisons within the different growing media were similar. Compared to the control, all plant measurements were significantly lower for oxadiazon in all three soil types (Tables 1, 2), which is contrary to previously reported results in both growth chamber (7) and peat/sand mixture studies (6). Significant survival differences (compared to control were found with chloroxuron, EPTC, profluralin, adn DCPA only in the loam/sandy loam soil.

Black locust seedlings were significantly shorter than the controls only in soil treated with oxadiazon (Table 2), while no difference was found in the earlier study with peat/sand. Generally, treatments did not cause individual dry weights to differ from the control (Table 2), except with oxadiazon, where survival was low. Survival with chloroxuron was low, but the few surviving seedlings, with little competition, grew to a large size.

Summary and Conclusions

Growth effects between herbicides within any given soil type did not generally differ from those found in the peat/sand mixtures. The one exception was oxadiazon, which severely reduced survival in soil, but not in peat/sand, indicating that high organic matter may be necessary to allow safe use of oxadiazon. Because most of the herbbicides used in this study did not decrease seedling survival or growth in soil, they could provide feasible alternatives to hand-weeding in tree nurseries and establishment of direct-seeded tree plantings.