The presence of 2,4–D in soil can seriously retard the germination and growth of crop plants. Cotton and tomatoes, for example, are extremely sensitive to soil in which there is 2,4–D. As little as one part in four and a half million parts of soil has reduced the germination of mustard seeds. So care should be taken in applying 2,4–D to crop areas.

On the other hand, some scientists believe 2,4–D may be useful in killing some kinds of weeds in soil. On this point, though, we need more information. In experiments we found that as little as 9 millionths of a pound of 2,4–D ( about the amount that can cover an eighth of an inch of the flat end of a toothpick) in a pound of soil reduced the germination of mustard seeds by 90 percent. The chemical soon becomes inactive in moist soil. After moist soil containing that amount of 2,4–D had lain fallow for a period of from 2 to 6 weeks, seeds of crop plants germinated as soon, and grew as vigorously, as they did in uncontaminated soil. The productivity of moist soil that is contaminated with 2,4–D may be reduced for a few weeks, but in dry soil the chemical may seriously reduce productivity for a year or longer.

P. C. Marth and I looked into the possibility of fertilizing grassy areas as they were being treated with 2,4–D to kill weeds. We used both liquid spray and solid mixtures of 2,4–D and fertilizer—enough urea to make a 3-percent water mixture of urea in a 0.1 percent 2,4–D spray. The spray killed plantain, dandelions, and other weeds, and at the same time the nitrogen in it stimulated the grass. Five gallons of the spray were applied to 1,000 square feet of lawn.

Fungi sometimes attack grass grown in a moist place. Fungicides, such as Fermate, can also be added to the 2,4–D spray mixture without lessening its weed-killing properties, but with a saving of time and labor.

Marth and I performed some experiments in which we applied mixtures of 2,4–D and a mineral fertilizer of the kind commonly used on lawns and pastures. We found that the mixtures enriched the soil so that lawn grass grew vigorously and at the time time killed common weeds such as plantain and dandelion.

It was learned that under some conditions 2,4–D can apparently be used to advantage in the production of Kentucky bluegrass seed. The acid was mixed with a commercial fertilizer (a commonly used one containing 10 percent nitrogen, 6 percent phosphorous and 4 percent potassium) and applied in a dry state evenly over areas of lawn that were infested with narrow-leaved plantain, sheep sorrel, and other common lawn weeds. The amount of seed produced by bluegrass on these treated areas greatly exceeded the amount produced by bluegrass on adjacent unfertilized areas and it also exceeded that produced on nearby plots that received fertilizer but no 2,4–D. The effect of the 2,4–D was twofold. It eliminated the weeds, and thus allowed the grass to obtain more of the fertilizer that was added to the soil; it was possible to harvest from the 2,4–D-treated areas a crop of bluegrass seed that was practically free of weed seeds, since practically all weeds in the area were killed before they produced seeds.

We started other experiments to study the usefulness of 2,4–D fertilizer mixtures in the production of other kinds of grass seeds.

2,4–D will not kill grasses that occur as weeds, such as crabgrass, quackgrass, and others, without injuring also the desirable grasses (bluegrass, fescue, or red top) present in the turf. It is simple enough to rid a lawn of common broad-leaved weeds, like dandelion and plantain, by spraying the area with 2,4–D, but after those weeds have been killed the task is only partly done, because as the broad-leaved weeds die and rot away, bare areas are exposed. In most localities the bare spots will soon become infested with weeds of the grass type that cannot be controlled with 2,4-D. We recommend therefore, that the control of lawn weeds be undertaken in the season (usually autumn) that is most favorable to the growth of lawn grass and that bare areas that result from killing the weeds in the turf be replanted to grass within 2 weeks or a month after the area is treated.

Research regarding 2,4-D has been under way only a short time; already it is considered to be one of the most successful weed killers we know. After further research, it will no doubt take its place as a safe and useful method of controlling many kinds of noxious weeds—but it is not a cure-all for all weeds.

British scientists have reported that two chemicals ( ethyl and phenyl carbamate and isopropyl phenyl carbamate) have been successfully used on an experimental basis to kill grass-types of weeds without injury to certain garden crops. Those and related substances are being studied to determine their usefulness in controlling weeds of the grass type.