302 SOIL CONDITIONS AND PLANT GROWTH 



any time is the balance of gains over losses. The gains de- 

 pend on the rainfall {i.e. climate), and on the amounts of water 

 derived by drainage from higher land {i.e. topographical posi- 

 tion), or by surface tension from the subsoil ; the losses depend 

 on the extent to which the subsoil facilitates drainage and on 

 the rate of evaporation, which in turn depends on the tempera- 

 ture, the exposure of the soil, the velocity of the wind and the 

 mode of cultivation. Five factors have, therefore, to be con- 

 sidered : (i) the nature of the soil particles, (2) the amount 

 and distribution of the rainfall, (3) the position of the soil in 

 relation to the land round about it, its aspect, shade, and any 

 other factors affecting its relative temperature and water supply, 

 (4) the depth of the soil, (5) the nature of the subsoil, especially 

 its perviousness to air, water, and plant roots. Any of 

 these factors may, within certain limits, dominate the rest and 

 profoundly affect the flora and the agricultural value ; thus 

 a sandy soil may, without any change in type, be a dry and 

 barren heath if underlain near the surface with rock or gravel, 

 a highly fertile fruit or market-garden soil if sufficiently deep, 

 or a stagnant marsh giving rise to peat if so situated that 

 water accumulates and cannot drain away. 



No sharp division can be drawn between the intrinsic and 

 extrinsic properties. The significant units in the soil that 

 determine its intrinsic properties are the compound particles 

 made up of the ultimate mineral particles — clay, silt, sand, 

 etc. — together with calcium carbonate and organic matter de- 

 rived from plant remains. Now the nature and amount of the 

 organic matter are greatly influenced by the extrinsic condi- 

 tions — the temperature and water supply — that have obtained 

 in the past. Moisture, warmth, and aeration favour the de- 

 velopment of a succulent vegetation which sheds easily decom- 

 posable leaves and stems on to the soil ; earth-worms and 

 bacteria can now flourish and produce the normal decomposi- 

 tion products that go to make up " mild humus " and a 

 fertile soil. Dryness necessitates a narrow-leaved xerophytic 

 vegetation, the leathery fragments of which mingle with the 

 soil, but afford a very indifferent medium for the growth of 



