THE SOIL 89 



much water may be present, it is too salt to be of any use. 

 Halophytes can digest Salter water than most plants, but 

 the percentage of salt in ordinary sea-water is too much 

 even for them. Just as with plants in the physical 

 desert, they can only get water when it rains. The 

 fresh water entering the soil mixes with the salt water, 

 and dilutes it sufficiently to make it available to the 

 plants. 



In hot, dry climates, as in Egypt and parts of India, 

 there is a large accumulation of alkali salts in the soil, 

 particularly of sodium carbonate. Sometimes they occur 

 in such quantity as to form a white crust on the surface. 

 Their presence inhibits plant-growth to as great an extent 

 as an excess of common salt, and the natmal vegetation 

 is typically xerophytic. 



2. Chalk. — Chalk — i.e., carbonate of lime — is insoluble 

 in pure water, but when carbonic acid is present, it is con- 

 verted into the soluble bicarbonate. The presence of the 

 bicarbonate makes water " hard," and plants growing in 

 chalky soils absorb it. Although most plants are in- 

 different to soils rich in lime, it seems certain that a few 

 plants prefer them {calciphilous plants), whUe others avoid 

 them altogether {calcifuges). The most pronounced 

 calcifuges are : gorse, broom, bracken, foxglove, ling, 

 heath, bogmoss (Sphagnum), and sundew. These are 

 supposed to die in water containing more than i per 

 cent, of carbonate of Ume. But it has been proved 

 that if no other salts are present, the lime itself does 

 the plants no harm. It seems that here, again, the 

 important thing is not so much the nature of the body 

 in solution as the amount dissolved. Some plants can 

 stand a considerable quantity ; others, like calcifuges, 

 very little. 



It is also stated that the general character of the vegeta- 

 tion living on a soil rich in lime is xerophytic. This is 

 generally true, because most of our calcareous rocks — 

 limestone and chalk — form hiUs or downs, and the rock 

 is porous and consequently dry. If, then, on limestone 

 hUls or chalk downs the flora is xerophytic, it is probably 

 due to physical causes — wind, drought, etc.— and not to 

 the chemical nature of the soil, \mless, of course, the total 

 amount of dissolved salts present in the water reaches a 

 dangerous degree of concentration. 



