HAWAII HAS ALSO PIONEERED in the development of suitable injection machinery. Injection is a problem when large acreages have to be treated and planted in a short season. Probably the first large-scale field fumigation machine was the one engineered by the California Packing Corp. for use with chloropicrin. The development of the field injectors was not easy. D-D is relatively corrosive and requires special metals. Pumps and delivery systems had to be devised; and then redesigned to get the most efficiency. The use of check rows has long been dropped as unnecessary in pineapple fields, but many an example is still provided unwittingly when application is faulty and long rows or partial rows are left untreated. From them the increasing necessity for soil fumigation, as time goes on, is demonstrated.

The methods available for small growers of truck crops have been greatly improved by the development of more effective hand injectors by firms on the United States mainland. With those new methods and new machinery, D-D and other fumigants, such as ethylene dibromide, have been found to be economical and practical as nematocides and as soil amenders.

The use of D-D mixture has become standard practice in Hawaii on pineapple lands. Some 7 million pounds are used in that way each year. The fact that in 1942, when the first results were obtained, only laboratory quantities were available as byproducts from a pilot plant used for other syntheses underscores the remarkableness of the development. Furthermore, the total volume of fumigants used on a field scale is evidence that Hawaii has pioneered in a development of vast significance to agriculture.

Perhaps a more important result of the discovery of D-D mixture was the stimulus given to the whole problem of soil amendment by fumigation for field crops in the United States and in many other countries. Other fumigants, particularly ethylene dibromide, have appeared on the market and are competitive with D-D mixture.

Some ethylene dibromide has been used in Hawaii on pineapple soils as a preplanting fumigant in place of D-D. An exact evaluation of the relative merits of the two compounds for the purpose is difficult because EDB is more sensitive to soil-moisture conditions than is D-D. With appropriate soil moisture, EDB has given excellent response. As most of the pineapple acreage is planted during dry seasons, however, D-D is perhaps the most reliable general preplanting fumigant. EDB has found a place in the post-planting fumigation of pineapple fields. Ethylene chlorobromide (ECB) is also promising for this purpose. The process involves some risk to the growing plant but growth stimulation usually has been pronounced. Sometimes profitable increases in fruit weight have followed.

Methods of testing soil fumigants have been dominated by the microbiologists' need for data on specific organisms, and the small pot test has been standard. New fumigants usually are screened by that method. Quantitative results have accrued, but the interpretation of the results in terms that the grower can use is difficult,for the method at best is artificial and of too short duration. Field-plot tests furnish a more reliable criterion for the growers because ultimate crop yield must determine the economic feasibility of the practice.

Future advances will come by understanding how fumigants affect growth.

There is, first, the effect on specific organism-nematodes, soil insects such as wireworms, and bacteria and fungi, both pathogenic and beneficial.

Second, there is growth stimulation. Plants may be stimulated because the development of root systems is hastened and improved, either by removing root pathogens or by supplying necessary factors for their growth. Possibly there is release of root-promoting hormones in the soil.

Nutrients may be more readily available because of depression of the nitrifying organisms in the soil. That is true of the early stages of growth, but growth stimulation of pineapple plants continues sometimes for the whole 4-year growth period and is often more pronounced in the second crop than in the first. Furthermore, soil fumigation after the plant has been established for several months will favorably affect the root system by stimulating or permitting new active white root tips for that portion of the whole root system that is near the point of injection of the fumigant. This suggests the possibility that soil fumigation makes nutrients available that are needed in small quantity for vigorous plant growth.

These problems of growth stimulation are closely related to a third consideration; namely, the effect of the fumigant on fertilizer practices. That is a practical point because the effects may govern dosages to be used and the economic position of the chemical in the production of the crop.

WALTER CARTER is a graduate of Montana State College and holds advanced degrees from the University of Minnesota. He is head of the entomology department of the Pineapple Research Institute of Hawaii.