. Auxins and plant growth. Auxin; Plant growth promoting substances. 290 Auxins in Agriculture Soil herbicides applied before the crop plant emerges depend to a considerable extent upon positional selectivity. The herbicide is applied to the surface of the soil where the bulk of the weed seeds germinate, while the crop seed is at a lower depth. For this reason herbicides which are readily leached are somewhat less dependable than less mobile materials. If the chemical remains strictly on the surface of the soil, large amounts may be lost by volatilization in the heat of the sun, so some small
. Auxins and plant growth. Auxin; Plant growth promoting substances. 290 Auxins in Agriculture Soil herbicides applied before the crop plant emerges depend to a considerable extent upon positional selectivity. The herbicide is applied to the surface of the soil where the bulk of the weed seeds germinate, while the crop seed is at a lower depth. For this reason herbicides which are readily leached are somewhat less dependable than less mobile materials. If the chemical remains strictly on the surface of the soil, large amounts may be lost by volatilization in the heat of the sun, so some small movement into the soil is desirable. Soil herbicides may also be applied after the crop plant has emerged. Most generally this is done by utilizing some device to keep the herbicide from excessive contact with the crop plant. In some cases the herbicide is sprayed onto the soil with nozzles held below the foliage level of the crop plants, or pelleted formulations are some- times used to avoid retention of the herbicide on the foliage. The sodium 2,4-dichlorophenoxyethyl sulfate is advantageous for this pur- pose, as there is no contact injury sustained by the foliage. FACTORS IN SUCCESS The use of auxin herbicides to remove weeds selectively often represents something of a compromise between reduced competition by weeds, and injurious effects of the sprays on the crop itself. This has been pointed out particularly clearly by Woodford (1950). Various types of injuries to crop plants have been observed subse- quent to removal of weeds by auxin herbicides. An interesting set of observations by Andersen and Hermansen (1950) indicates that the UJ | C/) ^ y .751- < .50 O .25 Z § 0 Opposite Spikelets April. o« ^Supernumerary Spikelets. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Leopold, A. Carl
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