Seeds absorb or give up moisture until they are in equilibrium with the surrounding air. Eben H. Toole, of the Department of Agriculture, found in a series of studies that, at a relative humidity of 65 percent (80 ), turnip seeds reached an equilibrium moisture content of 8 percent and kidney beans reached 12 percent. Most seeds will reach equilibrium within this range under similar atmospheric conditions.

Temperature has little effect on the moisture content of seeds at a given relative humidity, but it does have a decided effect on the rate of deterioration.

Dr. Toole offered the following recommendations concerning desirable humidity and temperature conditions for vegetable seeds: For seeds stored at 80 , the relative humidity should be no higher than 45 percent and no higher than 60 percent at 70 . Short-lived seeds, such as onion and shelled peanut, should be stored at a lower humidity under similar temperature conditions For seeds in cold storage at 40 to 50 , the air relative humidity should be no higher than 70 percent and preferably no higher than 50 percent. The recommendations also apply to most field seeds.

Conditioned storage rooms usually are designed to provide storage temperatures between 40 and 50 . Relative humidity is controlled in some.

Unit systems the factory-assembled equipment are available in sizes from one-third ton to 20 tons. Often they are placed directly in the conditioned space. Built-up central systems, field assembled, can be designed in shape, size, and capacity for any application.

The size, in tons, of the cooling system needed can be estimated by dividing by 12 thousand the heat gain of the storage heat gained from the stored seeds and through the walls, roof, and windows; expressed in British thermal units per hour.

It is seldom practical to operate a conditioned storage without some insulation in the walls and roof.

A rule of thumb for any storage held at 40 to 50 is to insulate with at least 3 to 4 inches of sheet or block insulation having a conductivity equal to that of corkboard. Unusual conditions of climate or use should modify normal recommendations. The manufacturer's recommendations should be followed.

Attempts have been made to use nuclear magnetic resonance to measure the moisture content of grain. If successful, this could provide a wide-range method that would be useful for the rapid measurement of the moisture content of seeds, the continuous monitoring of moisture during seed processing, and the determination of moisture content without destroying the seed.

If the temperature within a storage cannot be controlled, the operator can do little but work with the atmospheric conditions within and around the storage. Then he limits the moisture content of the seed he stores by receiving dry seed or by drying it after receiving. He also takes advantage of any good weather for ventilating warehouses, for circulating air around the stored bags to minimize extremes of high humidity and temperatures, and for aerating bulk stored seed to prevent hot spots and translocation of moisture.

Most field, grass, and vegetable seeds will store for a season with no serious loss in viability if their moisture content is not more than 12 to 12.5 percent.

For example, we found in studies in Illinois that soybeans stored in bulk at 12 to 12.5 percent moisture maintained their viability for some 175 days; those at 8 to 9 percent, for more than 650 days; but those at 15 percent, for less than 50 days.

Sealed storage, or storage under inert gases, presents some attractive advantages for preventing growth of mold and insect activity in stored seeds.

Some dangers are involved, however. Anaerobic absence of oxygen respiration of the seed can occur. It soon produces dead seed. Damp seeds may be killed as quickly under anaerobic conditions as in free air. Seeds to be stored in sealed containers or bins therefore should be at least 1 percent drier than seed stored under ordinary atmospheric conditions.

Bagged seeds are stored in both single- and multiple-story warehouses. The main requirements are weather-tight roofs and walls; strong, smooth floors and properly spaced columns that permit the efficient use of fork trucks and other machines; and ceiling and truss heights that permit bags to be stacked 16 feet and higher.

Bulk seeds are stored in bins or tanks separate from or within warehouses. The strength required for these storages varies with their size and the kinds of seeds to be stored. Wheat, soybeans, alfalfa, and clover, each weighing 48 pounds per cubic foot, are some of the heavier seeds that are stored in bulk.

Bin walls and floors must be strong enough to support both the lateral (horizontal) and vertical pressures exerted by the stored seed.

The design of structures for bulk seed or grain is complicated, but the following general conclusions can be offered: The pressure of the seed on bin walls and floors follows the law of semi-fluids rather than that of fluids.

The lateral pressure of seeds on bin walls is but 0.3 to 0.6 of the vertical pressure and increases little after the seed depth is 2.5 to 3 times the width or diameter of the bin. The ratio of the lateral to the vertical pressure, which can be determined only by experiments, is not constant but varies with different seeds and bins.

Storages for bulk seeds normally should be designed by an engineer familiar with the structural requirements, or well-tested and recommended plans should be used.

Rodents are a problem in seed storages and warehouses. In one multistory bag storage in Maryland, each floor is isolated so no mice or rats can get onto a floor except in bags of incoming seed. The operator does not depend only on seeing the live rodents. He judges their presence by the signs they leave. An open space about a foot wide is left between the stacked bags and the wall, and this strip of floor is painted white. Rodent tracks and excreta show up easily on it. Traps, poison, or gas are used to get rid of the occasional rodents that do get in. The regular fumigations are depended upon for control in some warehouses and storages.

LEO E. HOLMAN is an agricultural engineer in the Transportation and Facilities Research Division, Agricultural Marketing Service. He began supervising research on handling and storing seed and grain for the Agricultural Marketing Service in 1954. He has written widely on the design and use of aeration systems in maintaining the quality of stored grain. His degrees are from the North Dakota State University.

JAMES R. SNITZLER is an associate member of the W. B. Saunders & Co., Transportation Consultants, Washington, D.C. He was formerly an employee of the Department of Agriculture from 1952 through August 1960. Since 1956 he served as Assistant Chief, Transportation Research Branch, Agricultural Marketing Service. He has written many research publications on transportation on economics.