66 THE PHYSIOLOGY OF PLANTS 



table substance than in a soil poor in food substances though 

 rich in watei\ The roots must take up a certain fixed amount 

 of nutritive matter for every gramme of dry substance which 

 the leaves have to form. If the solution which the root finds 

 in the soil is very dilute, then a comparatively large amount 

 of this weak solution must be taken into the plant, and much 

 water must therefore be passed out by transpiration through 

 the leaves ; and, as a matter of fact, a square inch of leaf 

 surface of a plant growing in dilute nutritive solution gives ofi" 

 a great deal more water within a given space of time than does 

 a plant grown in a richer solution. After sowing it is very 

 advantageous to roll a dry and warm sandy soil, as its power 

 of retaining water is greatly increased by this procedure. 



The chalky and marly soils are the chief forms of lime-con- 

 taining soils. The chalky soil consists of sandy lime mixed 

 with a varying amount of powdery lime. From the table con- 

 tained in the preceding paragraph we see that chalk can absorb 

 more water than sand, and does not lose it so rapidly by 

 evaporation. It is not so readily heated by the sun as sand, 

 and is only slightly more hygroscopic. Chalk soils must, 

 therefore, be treated very much like sandy soils, but they are 

 able to derive more benefit from manuring, and do so more 

 rapidly than sandy soils. 



Marls contain an intimate mixture of pov^^dery lime and 

 clay ; the properties of the latter predominate and give to the 

 soil its characteristic features. It must, therefore, be treated 

 like a clay soil. It is more fertile than a clay soil, and this is 

 due to the presence of the lime, to which we shall refer again 

 in speaking of the addition of lime to the soil. 



Every field may be said to consist of a clay soil if it contains 

 sufiicient clay, for the properties of this substance to predominate. 

 This may take place when it forms only 30 per cent, of the 

 soil. It is characterised by its extraordinary power of retain- 

 ing water, so that it very rarely drys up. If the atmosphere 

 is moist, its great hygroscopic powers prevent its becoming too 

 dry. If a long and hot drought sets in, the ground cracks, 

 and then of course more water is lost. It is as rapidly heated 

 by the sun as is sand, and has the power of absorbing a large 

 amount of oxygen. We see, therefore, that the clay soil has 



