Methods for determining "available" soil calcium

A preliminary study of methods for determining easily soluble calcium is reported. The use of carbonated water as a solvent possesses theoretical significance, but has disadvantages by reason of its instability. Leaching soils with saturated carbonated water in a closed system proved a satisfactory means of using this reagent. A slight modification of the NH1Cl method for determining exchangeable calcium is given. Utilization of the seedling method, advanced by Neubauer, for determining the availability of soil constituents was found useful in indicating the amounts of easily soluble calcium a soil may contain. The chief points of interest brought out by this study are as follows: 1. All the methods show in general a higher calcium extraction from soils growing good alfalfa than from those growing poor alfalfa. The exceptions are probably due to the influence of other fertility conditions. 2. The total amounts of calcium leached out by carbonated water were found to approximate closely the exchange calcium in the soils. While the determination of such total amounts is important in indicating something as to the potential supply of usable calcium a soil may contain, it does not furnish evidence as to that portion which must be available to enable a plant to maintain normal growth. 3. The calcium leached out by the first liter of saturated carbonated water shows something as to potential supply, represents the more easily soluble calcium, and shows considerable correlation to field results with alfalfa. Leaching with saturated carbonated water in a closed system is more accurate than shaking given quantities of soil and N/25 H2CO3 and determining the calcium dissolved. The former procedure approaches more nearly to the hypothecated mechanism of plant feeding and possesses, therefore, more significance as a method for determining so-called available calcium. 4. The extraction of calcium by means of plant seedlings offers interesting possibilities in determining the relative amount of easily soluble constituents in a soil. Used in conjunction with chemical methods, it enhances their significance in determining that portion of an element which is available. Not as wide a range of calcium could be removed from the soil by buckwheat seedlings as by carbonated water, but the results obtained were in somewhat the same order and amounts.