Another group of plants, including barley, oats, wheat, cotton, tomatoes, asparagus, and alfalfa, respond to sodium when the potassium supply is inadequate.

A third large group of crops respond to sodium slightly or not at all at any potassium level. Among them are corn, rye, potatoes, lettuce, and soybeans. Sodium or potassium generally will depress the uptake of the other cation, but the results of this mutual relationship in any particular situation will depend on the levels of available sodium and potassium and the relative ease of absorption of the two cations by the plant.

THE EFFICIENT MANAGEMENT of soil with respect to potassium must be based on a number of soil-management factors: The kind of crop, the rotation system, the livestock-management system, the nature of the soil, the liming and fertilizer practices, and the weather.

Satisfying the potassium requirements of a cropping system should be based first on the natural potassium-supplying ability of the soil. In this regard soils range from organic and acid sandy soils (which cannot be depended on for any natural reserve supply) to clay soils that contain large amounts of relatively unweathered potassium minerals, which do not have to be supplemented by potash fertilizer.

Soils having little or no reserve potassium supply and low cation-exchange capacities require the frequent additions of small or moderate amounts of potassium. Large single applications to such soils may result in higher losses through leaching and unbalanced nutrient relations in the crop through luxury consumption. Leaching losses from rain can be serious in winter-fallowed soil of regions in which winters are warm. A winter cover crop reduces this loss. The continued removal of hay crops, however, severely depletes the soil potassium, and it must be increased by adding potash.

When rotations include row crops that respond markedly to potash (for example, cotton, tobacco, and potatoes), a potash application should be made to these crops at planting time. Excessive rains after planting may make additional applications necessary to replace leaching losses.

Legume crops that are removed from the land severely lower the available potassium level of soils. Liming of acid soils improves various growth conditions and thereby increases potassium requirements of legumes, but it also reduces the leaching of potassium. In a study at the North Carolina Agricultural Experiment Station, Adolph Mehlich found that even clay subsoils would not retain potassium effectively until they were limed.

If sodium is applied to the soil, either as sodium nitrate or as a treatment for a sodium-responsive crop, the extent of the substitution of this cation for potassium should be considered in estimating potash applications.

Loams and clays containing an abundance of illitic clay minerals or unweathered primary potassium minerals. can be expected to supply from a moderate fraction to all of the potassium required for a cropping system. Younger soils in this group can be cropped for many years before fertilizer potash has to be included in the soil-management program. The exact length of time, of course, depends on the amount of potassium in crops removed from the land and on other losses from the soil.

IN A SOIL-PASTURE-LIVESTOCK management system on such soils, the potassium cycle would be as follows: Mineral, to exchangeable, to soluble, to plant, to animal, to manure, to soluble, to exchangeable, to mineral (fixed). After the forage plants have absorbed soluble soil potassium which has been replaced from the exchangeable form and released earlier from mineral lattice forms, the pastured animals consume it during their feeding. A large fraction of it is returned to the soil in the animal manure, some of which becomes, in turn, soluble, exchangeable, and finally fixed in mineral lattices. The main loss of potassium in this cycle arises from the removal of the animals and animal products from the land.

Byron T. Shaw, of the Department of Agriculture, has estimated that in such a system, 75 to 90 percent of the potassium removed from the soil would be returned to it. Other cropping and livestock systems will conserve a smaller fraction. If sheltered animals are fed hay from the same farm, losses of potassium may occur if the manure is improperly conserved before it is applied to the soil.

If the straw of small grain and corn crops is not removed from the land, only about one-fourth of the potassium in the crop is permanently lost from the soil. The harvested portions of potatoes, celery, other vegetable crops, and tobacco contain much higher fractions of the total crop potassium. Much of the potassium in leaves of deciduous fruit and nut trees that drop on the ground eventually will return to available soil forms. In places where the native potassium supply is inadequate to replace losses, fertilizer potash must be applied. In many soils, a fraction of it will be fixed, and the availability of the application to the current crop will be lowered accordingly. This fixed potassium will be slowly available in the future, however, and so improve the ability of the soil to supply potassium.

The determination of currently available potassium is made by a soil test or by a plant tissue test. Information about the reserve supply is obtained from other laboratory measurements and from the history and general knowledge of the particular soil.