48 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



is of high significance to vegetation. In this respect the various kinds 

 of humus soils are very instructive. Heath-peat (from heath-moors, 

 often composed of the remains of bog-mosses) dries uniformly, and for a 

 long time remains moderately moist internally. The soil of meadow- 

 moors may be as dry as powder at its surface yet greasily wet at a 

 slight depth, for it does not readily permit the equable distribution of 

 water within itself. This property renders it unfit for horticultural 

 usage.^ The factors operating on evaporation are partly internal and 

 partly external. 



Internal factors are those which depend upon soil itself, such as : 

 the structure of soil, the form of the soil-surface (uneven or even), and 

 so forth. From loose soil less water evaporates than from compact soil, 

 because its power of raising water is less ; the formation of compound 

 particles depresses evaporation. Soil with medium-sized particles 

 permits the greatest evaporation ; large-grained soil permits less. 



The colour and kind of soil are of influence. From a darker soil 

 more water is evaporated than from a paler one, because dark soil absorbs 

 more radiant heat ; the order of gradation is : black, grey, brown, 

 yellow, red, white. From quartz-sand and humus soil evaporation is 

 most rapid, from calcareous sand and clay it is slowest ; Masure was 

 able to render sand and humus completely dry in three days, clay and 

 calcareous soil in seven days. But the amount of water evaporated in 

 a given time is greater, the greater is the water-capacity of the soil ; in 

 this respect humus stands at the top and quartz-sand at the bottom. 

 In one experiment by Masure, humus retained 41 per cent., but sand 

 only 2-1 per cent. Evaporation from a soil saturated with water is 

 greater than from an equal water-surface. 



Among the external factors operating on evaporation from the soil 

 must be reckoned : the saturation-deficit of the atmosphere,^ the slope 

 and exposure of the surface, the strength of the wind,^ as well as the 

 vegetation clothing the soil. 



Plants clothing the soil increase the surface exposed, and uninter- 

 ruptedly extract from the soil water which is dissipated by evaporation 

 from their leaves and other organs above ground. A field under cultiva- 

 tion becomes more rapidly parched than does a fallow field (of course, 

 the remaining conditions being the same). Plants clothing the soil rob 

 it of moisture during the vegetative season, but to degrees that vary 

 with the temperature and the kinds of plants present. The temperature 

 of soil determines how much water is taken up by roots.* Herbs parch 

 the soil more than do trees, and grass is particularly active in this respect. 

 Colding's observations showed that at Copenhagen, from April to Sep- 

 tember short grass consumed an amount of water greater than the rainfall. 

 Feilberg^ estimated the daily amounts per 0-55 hectare of land during 

 May, June, July, and August, at about 400, 500, 350, and 300 cubic feet 

 respectively ; these estimations are, of course, only approximate, and 

 vary with the conditions. The amount of water in soil therefore dimi- 

 nishes from spring to autumn ; at this time of the year it is at its 

 lowest, and may be from five to seven per cent, less than in spring ; 

 subsequently it increases during winter until plant-hfe awakens. The 



^ Grabner, 1901. ^ See Chapter VII. ' See Chapter VIII. 



* See Chapter XIII. ' Feilberg, 1890. 



