Fumigants to be successful must be properly applied. The requirements are not unduly difficult to fulfill and the reasons for them are easy to understand, but more failures have resulted from faulty application than from any other cause.

The land should be thoroughly and properly prepared. The soil should be reasonably free from lumps and clods and should be moderately, not excessively, loose. Very light, sandy soils should be compact. When seedbeds are prepared with a rotary tiller, a few days should be allowed for the soil to settle, except if methyl bromide is to be evaporated under a cover.

The soil should be moist but not wet. When the soil is even moderately dry, the efficacy of fumigants in killing most organisms is somewhat reduced, and ample moisture is especially important for effective control of weeds and certain fungi. The nearer a weed seed is to germination, the easier it is to kill, and fumigants are more likely to kill the sclerotic produced by some fungi if those structures have been wet for a few days.

The soil should be warm. Some fumigants are more effective than others at low temperatures, but generally the temperature of the soil should be at least 50 F. and preferably 60 or above.

The fumigant should be injected at a uniform depth. If the ground is uneven, the chisels of a power applicator will inject the fumigant too near the surface or even on the surface when they pass over low spots. The proper depth varies with conditions and the pests to be controlled. When chemicals are injected into the soil, organisms located near the surface are often not killed. This is an inherent weakness of soil fumigation regardless of the pests involved, and it is especially serious when attempting to control weeds or damping-off in seedbeds.

In order to overcome this difficulty by increasing the concentration of gas near the surface, shallow application is recommended, with injection 3 or 4 inches deep. For field application under most conditions, 6 inches is usually recommended. In the sandy soils of Florida, roots that grow in the upper 2 or 3 inches escape injury by nematodes to a marked degree. Celery plants, growing on land that is heavily infested with the stubby root nematode, may produce a thick mat of more or less healthy roots in the upper 3 inches of soil, although all the roots deeper than that are completely destroyed. The same is true, though to a somewhat lesser degree, for plants injured by the root knot nematodes. Hence, for field applications in Florida, fumigants should be deeply injected, never less than 6 inches deep and preferably 7 or 8 inches.

Holes or furrows made by the applicator should be promptly and firmly filled. If loosely filled, the gas, instead of diffusing into the surrounding soil, will escape upward into the air.

Fumigants are toxic to plants and should have diffused out of the soil before a crop is planted. Otherwise the plants may be stunted. The time required for adequate aeration of the soil varies greatly and is influenced by many factors, including the kind of soil, the condition of the soil (especially temperature and water content), the fumigant used, the rate of application, weather conditions following application, and the crop planted. Chloropicrin, being extremely toxic, must be followed by thorough aeration. Some kinds of plants will tolerate low concentrations of ethylene dibromide without being seriously injured. The toxicity of dichloropropene mixtures is intermediate between those two extremes, but, to avoid all danger of injury, aeration must be fairly thorough. Methyl bromide is highly toxic, but the gas leaves the soil so quickly that aeration for 2 or 3 days is usually adequate.

The usual recommendation for dichloropropene fumigants is to allow 1 week for every 10 gallons per acre. For 41 percent ethylene dibromide, one should allow similarly varied but slightly shorter intervals. For most conditions, this rule allows a reasonable margin of safety, but there are exceptions. If the soil is wet or cool and if the content of organic matter is high, those intervals may not be long enough. On the other hand, if the soil is light and the conditions are hot and dry, shorter intervals may be enough.

The question has been raised as to the possibility of injury to the soil by use of soil fumigants and whether their continued application will result in chemical, physical, or biological changes that may have a deleterious effect on the growth of plants.

Three possibilities have been suggested: That the continued application of fumigants may eventually result in the accumulation of toxic residues; that the chemicals will kill beneficial organisms and thereby seriously interfere with the normal biochemical changes that occur in the soil; and that after land has been once fumigated the treatment must be repeated each year, otherwise injurious organisms may become more serious than would have been the case had the land never been treated.

Experiments have not been numerous enough or of sufficient duration to provide a final answer to the question of toxic residues, but results indicate that, with the fumigants now in use, we have little to fear on this score. Very definitely, fumigation kills beneficial organisms, but most of them seem able to reestablish themselves rather quickly. The effect of fumigation on the nonparasitic organisms of the soil generally is more transient than is its effect on the parasites. Obligate parasites, such as many of the nematodes, cannot reproduce except on a suitable host plant, and, although fumigation rarely exterminates them, at least one crop can be grown before their numbers increase sufficiently to cause serious damage.

The conversion of nitrogen from an ammoniacal to a nitrate form is a biological process. Destruction of nitrifying bacteria by fumigation may retard the process and result in an accumulation of nitrogen in the form of ammonia. Different plants differ in their ability to utilize nitrogen in this form. Tomatoes are reported as able to absorb ammoniacal nitrogen readily if the soil is neutral but not if it is acid. Hence it appears that the extent to which fumigation may possibly have an adverse effect on crops by reducing the nitrifying bacteria of the soil depends on at least two factors the kind of crop and the condition of the soil. I know of no instance in which fumigation has caused an accumulation of ammoniacal nitrogen sufficient to interfere seriously with the successful growing of any crop.

Fear that soil fumigation, if once begun but not continued, will be followed by an abnormal build-up of noxious organisms seems to have had its origin in results of experiments with the sugar beet nematode. Where land is heavily infested with that pest, the crop of sugar beets on fumigated areas may be very good, but the crop on unfumigated areas may be a failure. When sugar beets are grown on this same land the following year without fumigation, the situation may be reversed and the crop may be more severely damaged on the areas that were fumigated the previous year than on those where no fumigant was applied.

A suggested explanation is that during the first season on the unfumigated areas a huge number of nematode larvae hatched, attacked the roots of the plants, destroyed them, and, in so doing, eliminated their own food Supply and thereby destroyed themselves. On the fumigated areas the plants made an abundance of roots, So that the residual population of nematodes not killed by the treatment could build up rapidly. By the end of the season more cysts had been produced in the fumigated than in the unfumigated areas. If this is the correct explanation, these results are scarcely an indictment of soil fumigation. That treatment of the soil with chemicals may permit populations of plant parasitic nematodes to build up by destroying their natural enemies is a possibility. Not a great deal is known about these natural enemies and virtually nothing is known about the extent to which they hold the parasites in check.

Research laboratories are active as never before in a search to find new and more effective chemicals for controlling pests of all kinds and those that inhabit the soil are not being slighted. Compounds under test in different localities show promise of being useful additions to the ones in use, especially for controlling fungi both in seedbeds and in the field. Some can be applied to the soil in powdered or granular form. Others can be mixed with water and applied as a drench, a procedure that may have advantages for certain purposes. Chemicals that will kill some organisms and not others and can be applied around the roots of living plants seem a possibility.

Root diseases are so serious and the need of better methods for controlling them is so pressing that new chemicals are certain to come into use either with or without the sanction of experiment stations. While the purpose of such chemicals is to bring about desirable changes in the biology of the soil they may at the same time have other and undesirable effects. What these effects are, how serious they may become, and how they can be avoided or minimized are questions that provide a research problem for the future.

JESSE R. CHRISTIE, a native of New Hampshire, is a nematologist in the Bureau of Plant Industry, Soils, and Agricultural f Engineering, which he joined in 1922. He is now stationed at the Central Florida Experiment Station, Sanford, Fla., and is in charge of the Bureau's nematode investigations in the Southeastern States. He holds degrees from the University of Kentucky, the University of Illinois, and George Washington University.