6o OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



between the temperature by day and by night may be very wide (from 

 forty to forty-five centigrade degrees). As a result, at night-time dew 

 is readily and richly deposited upon sand, whose water-content and 

 vegetation are thus profoundly affected. On the other hand, plants 

 suSer from frost more easily on sandy soil. The sand-flora develops 

 early in spring a feature that recalls the steppe. Plants habitually 

 growing on sand are usually termed psammophytes or psammophilous 



plants-^ ^ 



3. Lime soil. Calcareous sand, consisting of calcium carbonate, is 

 less poor in nutriment than is quartz-sand, has a greater water-capacity, 

 and dries less rapidly, but is nevertheless dry and warm. Mail is an 

 intimate admixture of calcium carbonate (8 to 45 per cent., in calcareous 

 marl 75 per cent.), clay (8 to 60 per cent), and quartz-sand : the lower 

 diluvial marl from the Mark Brandenburg contains calcium carbonate 

 12 to 18 per cent., clay 25 to 47 per cent., quartz-sand 38 to 62 per cent. 

 The characters of marl depend upon the relative proportions of the 

 constituents, and generally stand between those of sand and clay. 



4. Clay soil. This offers an almost complete contrast to sand soil. 

 The particles that are invisible to the naked eye predominate over large 

 grains. Clay consists mainly of kaolin (hydrated silicate of aluminium), 

 and may contain more or less quartz, calcium carbonate, ferric oxide, and 

 so forth. Kaolin is of no nutritive value to the plant, yet the presence 

 of many additional substances may render clay very rich in nutritive 

 bodies ; these are, however, available only with difficulty. With a favour- 

 able admixture of sand, lime, and humus, a clay soil is a fertile soil. 



Clay soil has a large absorbent faculty, and is at the same time very 

 hygroscopic ; it can absorb five or six per cent, of aqueous vapour from 

 the atmosphere. 



Clay soil is tenacious or heavy, as its particles have great cohesive 

 power ; aeration is mostly defective, a circumstance unfavourable to 

 vegetation, and leading to the production of acids and swampiness. 



Clay soil is wet and cold, because its water-capacity is great (up to 

 ninety per cent.), and because its capillary power is great ; it raises much 

 water from the subsoil, and is almost impermeable to water. If we over- 

 load it with water it swells, its volume increases, and its individual particles 

 are forced asunder, so that a paste is formed. Clay soil containing an 

 abundance of water is plastic. After prolonged drought, clay soil acquires 

 a stony hardness, contracts, cracks ; and these occurrences react on 

 vegetation.^ 



The unfavourable characters of clay may be ameliorated by com- 

 mixture with substances of opposite character, such as sand and lime. 



Loam, which may be dealt with in connexion with clay, is weathered 

 marl, the calcium carbonate of which has been more or less dissolved in 

 water, and the ferrous salts of which have been converted into ferric 

 oxide and hydroxide ; the soil consequently becomes brown and essentially 

 contains clay and quartz-sand. 



5. Humus soil.3 Humus is produced from the remains and products 



of plants and animals, often from animal excrement in all stages of 



decomposition, and is mixed in soil with various proportions of mineral 



^ In Section X. " See page 42, Chapter X. 



' Consult the important work by Friih and Schroter, 1904. 



