EFFECTS OF WATER, CLIMATE, ECT. 139 



wants, while the latter supplies the requisite lime and magnesia. 

 Hence, while the granite may be unfruitful, trap soils may be very 

 productive. These soils are however much modified in different coun- 

 tries, by the union of other minerals. Decayed trap is occasionally 

 dug up and successfully applied as top-dressings to granite soils 



It should be remarked in this connection that soil exposed to occa- 

 sional inundations may be entirely changed thereby ; so that whatever 

 may be the rocks below, or in the neighborhood, the sand, clay, etc., 

 brought upon the soil by swelling streams will constitute the soil they 

 cover. This is well known in Egypt and other parts flooded by 

 streams. These streams also convey decayed organic matter, which, 

 with the earthy materials brought from hills and rocks near by, and 

 deposited by them, often produce the most productive soils. It has 

 been by this process, indeed, that most of the horizontal strata have 

 been formed, even to a great depth. Still, in general, the soil may be 

 determined by the underlaying rocks, and especially where soils are 

 not exposed to the floodings alluded to. 



The effects of climate on soil is of much importance. Some soils 

 are heavy, as sands and marls, and others are light, as peaty soils, etc. 

 The latter may, of course, be improved by a covering of clay, sand or 

 lime-stone gravel. Strong clay soils retain three times more water 

 than sandy soils ; hence the importance of draining the former and 

 also of peaty soils. Plants are often quickly burned up in sandy soils, 

 as they retain moisture but slightly and readily yield what they possess ; 

 but, in very rainy weather, they sustain a good vegetation, while plants 

 perish from excessive moisture in clayey soils. Soil shrinks in drying 

 in proportion to their constituents ; so that in peaty soils the roots are 

 compressed and the air is excluded, while sandy soils are not contracted 

 by drying. These absorb but little moisture during the night, whilst 

 clayey soils often absorb a 30th part of their own weight, and a dry 

 peat a 12th part. Thus will be seen the importance of a due mixture 

 of the foregoing constituents of soils, under all circumstances. 



The importance of temperature on vegetation has been noticed else- 

 where, but it will be seen from what has been said that different soils 

 require different degrees of temperature. Some become much hotter 

 than others, so that while the temperature in the shade may be 60 or 

 70 a dry soil may be 90 or 100 (See page 124). The temperature 

 rises more slowly in wet soils, and these are never so warm as dry soils ; 

 hence they are called cold. Brown-red soils are heated more than 

 sand, clay, or garden mould, and peat is heated more than any other. 

 The addition of dark vegetable matter renders soils warmer by the 

 effect of the sun, while the color of dark red marl, from old and new 

 red sand-stone, may aid them in their fertility. 



The chemical constitution of soils may be further considered in rela- 

 tion to their functions. These, in the mass, are to serve as a base in 



