The availability of the soil constituents de pends largely upon the extent to which disin tegration has proceeded and the amount of leaching to which the soil has been subjected. Thus in the soils of arid regions in which disin tegration has been going on uninterruptedly for ages with no leaching, the soluble salts are always found in large and often in excessive amounts, as, for example, in the so-called alkali soils, which are simply soils containing an excess of soluble salts accumulated at or near the surface, and which when this excess is removed, dispersed or otherwise rendered in nocuous, are among the most fertile known.
Chemical analysis shows soils to be very variable in composition, and since the processes by which soils are formed and plant food ren dered available are continually going on, fre quent examinations are necessary if accurate knowledge of the chemical properties of a soil at any given time or of the amounts of plant food becoming available during the period of growth of a plant is to be had. While, as already inti mated, chemical analysis must not be taken as an absolute guide to the fertilizer deficiencies and requirements of soils, certain general de ductions have been drawn from soil analyses which may be of practical value if applied with discrimination. For example, German authori ties classify soils on the basis of their chemical analysis as follows: Soils containing less than 0.05 per cent of either nitrogen or phosphoric acid are considered as poor, those containing from 0.05 to 0.1 per cent moderately rich, 0.1 per cent average or normal from 0.10 to 0.15 per cent good, and above 6.15 per cent rich; soils containing less than 0.05 per cent of potash are classified as poor, from 0.05 to 0.15 per cent moderately rich, from 0.15 to 0.25 per cent average or normal, and over 0.25 per cent rich; loam soils containing less than 0.1 per cent of lime are classified as poor, from 0.1 to 0.25 per cent moderately rich, from 025 to 0.5 per cent average or normal, from 0.5 to 1 per cent good, and over 1 per cent rich; sandy soils con taining less than 0.05 per cent of lime are clas sified as poor, from 0.5 to 0.1 per cent mod erately rich, from 0.1 to 0.2 per cent average or normal. Over 0.2 per cent of lime is not often found in sandy soils.
The minimum percentages of the different mineral elements in the soils of the United States which, according to Hilgard, chemical analysis has found to be necessary to the thrifty growth of general crops, may be summarized as follows: Potash may vary widely without ma terial effect but should not as a rule be less than 0.25 per cent. One-tenth per cent of phosphoric acid is adequate and there should not be less than 0.05 per cent of it. The
greater the proportions of lime and humus and the less the amount of iron in the soil the smaller the percentage of phosphoric acid re quired. One-tenth per cent of nitrogen is ade quate. Lime should not fall below 0.1 per cent in sandy soils nor 0.6 per cent in heavy clay soils. As a general rule the larger the propor tion of lime present the lower the percentages of potash, phosphoric acid and nitrogen re quired. Magnesia is seldom deficient.* Iron is usually present in abundance. Humus, which modern investigation has shown to be a variable complex of many substances and not a definite compound, may vary widely in amount and nitrogen content. The humus of humid soils usually contains a smaller percentage of nitro gen than that of soils of arid regions. Humus is not only important as a source of supply of nitrogen, but also of other plant food constitu ents — potash, phosphoric acid, lime, etc. Ac cording to Snyder, the proportion of these con stituents combined with humus represents to a large extent the amounts available in the soil.
An important property of soils, which is partly chemical and partly physical, is their absorptive power for fertilizing constituents. which prevents these constituents from being lost in the drainage or dissipated in other ways before the plant can utilize them. Ot the more important fertilizing constituents soils ap parently have the least retentive power for nitrogen (especially in the form of nitrates) and the greatest for phosphoric acid. Soils ap parently hold lime less tenaciously than potash, since it has been shown that when potassium chloride is added to a soil the potash is retained by the soil and the lime of the soil passes into the drainage in combination with the chlorine.
Physical Properties and Processes.— The physical properties of soils which are of ipecial importance are color, weight, fineness of divi sion or texture, arrangement of particles or structure, adhesiveness, and relations to heat, gases, water and dissolved solids. The physi cal properties of soils are to such a large extent inherent that the common practice of selecting soils with special reference to the suitability of their physical properties to the crop to be grown is usually a wise one. Nevertheless good tilth with good water conditions, aeration and tem perature, as well as an adequate supply of plant food, which are so essential to productiveness, may be modified and improved to a considerable extent by proper management, culture and fer tilizing, and with the improved physical condi tions the chemical and biological processes of the soil come effectively into play.