WATER CAPACITY OF SOILS. 145 



to one-half of what it is at the. sniiace, wbile in loam, owing to smaller 

 capillaries, the reduction is only one-third. The finer the capillaries 

 the more water they can keep up proportionately in balance against the 

 force of gravity, and hence humus and garden mold, with their fine 

 capillaries, show the greatest water capacity. 



The degree of such retention, as has been shown by Prof. C. E. Hil- 

 gard and others, is also somewhat influenced by the temperature of the 

 soil. The least retentive soil is a coarse quartz sand followed by finer 

 sands, and it is increased by an addition of lime loam, or vegetable 

 matter. 



For a "second-class" Florida sand soil the "moisture coefficient" is 

 stated by Hilgard as 1.64 per cent of its own weight, while it is 23 per 

 cent and more in a peat soil; a pure clay rarely exceeds 12, while the 

 moisture coefficient of calcareous clay soils rises to 15 and 20 per cent. 

 The maximum water capacity may be many times that of the absolute 

 water capacity, depending on structure and more or less compact strat- 

 ification of the soil. According to Ebermayer, the amounts of water 

 held may vary between 3 and 88 per cent. Dr. Raman's investigations 

 show the maximum water capacity of sand soils of fine and medium 

 fine texture to be from 3 to 4 per cent. 



According to Prof. Schuebler at Tiibingen, who experimented on 

 soils under natural conditions: 



Sand soil may hold 25 per cent of its weight in crater; loamy soil 40 per cent of 

 its weight in water; clay loam 30 per cent of its weight in water; pure clay soil 75 

 per cent of its weight in water. 



The most impervious soils, as was to be expected, vshowed the greatest 

 retentive power, and since by compacting the soil particles the capillar- 

 ity is increased, the imperviousness of such soils is increased. 



Impermeable soil strata, such as loam and very fine sand, allow, when 

 a surface run-oft" is readily possible, only a passing and inferior retention 

 of water after rainfall, in springtime taking up no more than 10 or 12 

 per cent of their weight, while a stratum of sand of medium grain, 20 

 to 25 feet deep, Avas calculated by Kannxn to be capable of taking up 

 and holding the entire annual precipitation of 24 inches. 



The capillarity of sand soils of the North German plain investigated 

 by Raman was not capable of raising the ground water higher than IJ 

 feet, so that the upper strata of the .soil, which was within reach of 

 ground water, did not show any greater amount of water than the soil 

 which had no ground water to fall back upon. 



Mr. F. H. King ( Agric. Expr. Sta. Wise. Ann. Rept., 1889) has investi- 

 gated the water capacity of soils with special reference to the function of 

 groundwater in plant production. Six thousand observations of ground 

 water stages under varying surface conditions show insufliciency of 

 capillary actions to supply water for plant trans])iration rapidly euough. 

 He found that a column of 1.5 in height of natural soil (consistiug of 

 loamy marl, red clay, sandy clay^ and fine sand) could hold an amount 

 12444— No. 7 10 



