For larger installations it is more economical to employ the gravity feed system. The beds or benches are divided into three or four sections, each on a higher elevation and slightly longer than the one following it. Two solution tanks are used in this system. The larger one is located at the end of the beds and is below ground. It is connected with a somewhat smaller tank above the level of the beds by means of a flume. The capacity of the second tank should be approximately one-half the volume of the first sections of the beds. This tank is filled from the larger or sump tank before an irrigation is planned. The nutrient solution flows into the first bed sections by gravity and then successively through the other sections, finally emptying into the sump tank. By this means only the solution for irrigating the first sections of the beds has to be pumped, gravity flow irrigating the rest of the beds. The system was used by the United States Army Air Forces in their operation of soilless-culture gardens at Ascension Island, Atkinson Field in British Guiana, and Iwo Jima.

Benches or beds intended for subirrigation are usually built of reinforced concrete. They should always be coated on the inside with nontoxic petroleum asphalt that is applied hot, as an emulsion, or cut-back in a volatile solvent. The asphalt waterproofs the beds and protects them from the slightly acid nutrient solution. Ground beds of asphalt macadam can be constructed by mixing hot asphalt with sand and molding it into shape while hot. This type of bed was used on Ascension Island.

Prefabricated bituminous surfacing (PBS), consisting of burlap saturated with asphalt, was used successfully for constructing subirrigated beds in the Iwo Jima garden. The material comes in rolls 3 feet wide and has the advantage of being tough, flexible, waterproof, and easily laid. If it becomes generally available, PBS should be satisfactory for waterproofing existing wooden benches for subirrigation.

Several naturally occurring aggregates have been used in the soilless culture of a number of plants. Lava cinder was screened and used in the beds on Ascension and Iwo Jima. Gravel washed free of sand and clay has been widely used in the United States. Sintered shale, a commercial product used in making low-density concrete, is porous, light in weight, and has a higher water-holding capacity than gravel. Calcareous aggregates ( coral limestone) have produced satisfactory crops experimentally after pretreating them with phosphate solutions to stabilize the pH acidity). In tests at Beltsville, we got good results by using expanded vermiculite, a mica-like, hydrated magnesium aluminum silicate used industrially as an insulating material, as an aggregate. Sintered shale and vermiculite contain calcium and potassium and tend to take up phosphates from the nutrient solution that are later available to the plants growing in them; consequently, the pH and nutrient balance of solutions used on them do not fluctuate so rapidly as when the aggregate is gravel.

The size of the particles of the aggregates should be between one-sixteenth and one-half inch in diameter. The frequency of irrigation is determined partly by the water-retaining capacity of the beds. This in turn depends to a considerable extent upon the size of the particles and the porosity of the aggregate.

The nutrient solution supplies water and oxygen as well as mineral elements to the plant roots. Much effort has been expended in attempts to determine the best combination of nutrients for various plants. While many combinations have been proposed, it is now generally recognized that rather wide limits of solution composition can produce equally good growth with many plants. Climatic factors of temperature and the intensity of sunlight, as well as the part of the plant that the grower wants, that is, leaf, root, fruit, or flower, also are determining factors in the composition of solutions for optimum growth.

It should also be recognized that the total volume of the solution in relation to the number of the plants, the particle size of the aggregate, the frequency of irrigation and replenishment of absorbed nutrients, as well as the initial composition of the solution are important factors that govern growth. With small installations the nutrient solution can be replaced at frequent intervals. In larger systems it is more economical to replenish the elements as they are absorbed.

Some technical training and considerable experience are necessary for the efficient management of soilless-culture crop production. Its future development in the United States will probably be confined to the production of crops having a relatively high unit value—ornamentals, out-of-season vegetables, or seedlings for transplanting.

Under favorable conditions, yields may be expected to equal or surpass similar yields in soil, but so far the differences have not been outstanding. The method is also well adapted for specialized studies in plant nutrition, phytopathology, and plant breeding where growth under standard conditions is desired. The indications are that soilless-culture techniques will be more widely employed in the future.

THE AUTHOR

Neil W. Stuart is a physiologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, specializing in research work on effect of light, temperature, and nutrition on floricultural crops. Dr. Stuart is a graduate of Michigan State College.