In the field of weed control a great deal of attention is being given to the control of weeds after they germinate; in other words, pre- and post-emergence weed control. Control of dormant weed seeds in the soil has received little attention except in seedbed fumigation and in turf renovation. Many of the fumigant type materials were selected and screened for the control of nematodes and soil-inhabiting insects. It was discovered that some of them will control dormant seeds as observed by reduced weed stands as compared to non-treated areas.

Wood (13) has estimated that 61,000,000 acres, or 60 per cent of the cropped land of the Great Plains region of North America, are infested with wild oats, Avena fatua L. The wide-spread seriousness of this weed and the fact that there is no cropping or tillage practice known for its ready control, point out the importance of finding a feasible chemical method.

In recent years weed research workers have begun to investigate the possibility of using herbicides for the selective control of wild oats (Avena fatua L.) in field crops. One of the first herbicides to show promise for the control of wild oats was IPC (isopropyl N–phenylcarbamate). Although this herbicide was effective on wild oats, only a few broad-leafed crops proved tolerant to it. More recently, another herbicide, CDAA (2–chloro–N,N–diallylacetamide), has been found to be effective in controlling wild oats without damaging corn and soybeans, and several field tests indicate that it may be used successfully in flax. To be effective both herbicides must be applied as pre-emergence or pre-planting treatments. They kill wild oats as the wild oats germinate, and consequently must be placed in the vicinity of the germinating seed if a good job of killing is to be done.

The present extensive use of herbicides in soil application has resulted in a great deal of interest in the breakdown of these chemicals in soil (1, 4, 6, 8). An increasing demand exists for information that will permit fairly accurate predictions of the residual life of these chemicals (4). The breakdown of chemicals in soil has been repeatedly shown to be the result of microbiological activity in part but other factors are also known to contribute to the disappearance (1, 4, 5, 7, 8, 9).

On rich alluvial soils where corn is the principal crop, Johnsongrass is the most serious perennial weed in Ohio at least as far north as the 40th parallel, and it has spread rapidly north of that area, even into southeastern Michigan. Many people still credit the statements of Piper (3), Vinall (6), and Bennett (1) that Johnsongrass behaves as an annual north of the 38th parallel. Pammel and King (2), to be sure, reported Johnsongrass as a perennial weed in southwestern Iowa in 1919 and the junior author's experience with it as a weed in Ohio, from the 40th parellel south, began in 1918. At Columbus these infestations have lived through winters in which the temperature was below 0° F for prolonged periods and the ground was frozen to a depth of 6 to 8 inches. There seems to be no hope for the eradication of Johnsongrass by mere freezing.

The pre-emergence use of 3–(3,4–dichlorophenyl)–1,1–dimethylurea (diuron) offers a means of controlling weeds in cotton during the extremely critical period of stand establishment (1, 2, 6, 8, 9, 10, 11). The amount of diuron required to give effective weed control without injury to cotton has been observed to vary between locations. Factors such as temperature, moisture, light conditions, and physical and chemical properties of the soil may be involved in causing variation in the activity of this herbicide.

Since the aerial brush control spraying operations in northern Minnesota have little research basis, the Lake States Forest Experiment Station has been carrying on some fundamental.studies aimed at improving the results of this increasingly important aid to forest management. These consist of analyzing two factors basic to all foliage spraying jobs whether they are done from the air or from the ground. These are:

1. The minimum amount of herbicide (in terms of acid equivalent) required to control a unit-area of brush of a given species and density, and

2. The minimum amount of solution needed to distribute the herbicide uniformly over the sprayed area.

Along the lower Coastal Plain, from the western panhandle of Florida to the Carolinas, lie approximately ten million acres of deep sands. In northwest Florida the sandhills support chiefly scrub oaks and wiregrass (Aristida stricta). Oaks are mainly turkey oak (Quercus laevis) and bluejack oak (Q. incana). Once an understory to longleaf pine (Pinus palustris), the scrub oaks gained dominance as heavy, indiscriminate logging removed the pines. Today only scattered pines remain, and the oaks and wiregrass prevent seed from these pines from restocking the area.

In recent years the relationships of 3–(p–chlorophenyl)–1,1–dimethylurea (monuron) phytotoxicity and various factors in the soil have received considerable attention (1, 3, 5, 6). Similar information for 2–chloro–4,6–bis–(ethylamino)–s–triazine (simazin) is lacking because of its later development as a herbicide. The experiments reported here were initiated to compare (a) the toxicity of monuron and simazin in solution culture and in soil, (b) the inactivation of the two compounds in soil and, (c) their downward movement through soil columns.

Systemic herbicides applied to leaves must move from the leaf surface into uninjured phloem in sufficient quantity to be translocated throughout the plant. Contact herbicides must at least enter the leaf. Leaves of terrestrial plants are adapted to exchange of gases —water vapor, CO2, O2—but are not adapted to absorb foreign solutes in applied solutions and to translocate them systemically; insectivorous species and some water plants may be exceptions. Failure of herbicidal effectiveness often may be due to lack of penetration. However, selectivity towards herbicides is not clearly related to differential absorption (5, 82, 172, 176).

Foliar application of herbicides, plant regulators, and nutrients is a well established practice. Yet, the many contradictory reports in the literature (7) indicate that the routes of entry into and movement within leaves are not clearly known. This situation has probably arisen because presently available techniques do not precisely establish the location of the penetrant on the cellular level.

One of the factors to be considered in the establishment and maintenance of an orchard is the competition from weeds for the available supply of moisture and nutrients. Mechanical elimination of competitive plants is time consuming, expensive, and may not be done as early or as frequently as it is needed each year. The use of herbicides as a supplement to such a program now seems worthy of serious consideration. In some instances, as will be seen below, the use of mechanical methods may be eliminated altogether.