274 QrA\TITATIVE AffRICl'LTUEAL ANALYSIS
Ing ou tin* sieve. The percentage of the. tine portion is determined by
difference.
Preparation of Sample.—Refer to the discussion of sampling, pages 17 to
21. Reduce the remainder of the gross sample, by quartering or by use of
a riffle, to an amount sufficient for analytical purposes (25 to 50 gm), transfer
this to a sieve with 1-mni openings and sift, breaking the lumps with a,
pestle. Grind the part remaining on the sieve in a mortar until the
particles will pass through, mix thoroughly and preserve in tightly stoppered
bottles. Carry out these operations as rapidly as possible to avoid loss or
gain of moisture during the operation.
Moisture.—Loss of weight on drying may be.due to escaped
hygroscopic water, chemically combined water or ammonia or,
to some extent In certain cases, to oxidation of organic matter.
For this reason "moisture7' as usually reported, is not a strictly
accurate term.
Determination of Moisture.—Weigh 2 gm of the sample into wide
crucibles or small dishes and heat for five hours at 100°. In the case of
potassium salts, sodium nitrate aad ammonium sulphate, heat at about 130°
to constant weight. Calculate the loss as percent of moisture.
Phosphorus.—Phosphorus is deficient in soils more often
than are the other necessary elements. The mineral phosphates
form the chief commercial source of phosphorus, although
a considerable amount is obtained from bone, Thomas slag
(from the basic Bessemer steel furnace), tankage and fish scrap.
Calcium orthophosphate, Ca3(P04)23 is the chief constituent of
"raw" rock phosphate. Its solubility in water is very small,
in absence of acids, and therefore it is advisable to use it only
in a soil where there is considerable decaying organic matter to
furnish carbonic acid, as otherwise its availability is small.1
Large amounts of rock phosphate are now commercially
made into acid phosphates by treating the finely ground stone
with sulphuric acid, thus converting the normal phosphate to a
soluble form, suitable for use as a fertilizer. The character
of the result of this treatment depends upon the concentration
of acid and upon the relative amounts of rock phosphate and
acid employed in the treatment. Dicalcium or monocalciuni
phosphate, or even phosphoric acid itself, may be formed, accord-
1 See also HOPKINS, III Exp. Sia. Circ., 167 (1913) and STEWABT, Ibid.,
246 (1920).