72 THE PHYSIOLOGY OF PLANTS 



This fact, which has been proved by numerous experiments, 

 shows us how little we are tied to any special soil in the case 

 of potting plants, if we only pay sufficient attention to the 

 aeration of the soil. It is advisable, unless we know by experi- 

 ence that a certain species can stand a heavy soil, to choose in 

 the first instance some light kind of mould. To these belong, 

 in the first instance, heather soil, then follow the soils rich in 

 humus, such as leaf-mould, and lastly loam and clay. 



Besides the costliness of employing heather soil in all cases, 

 the latter has the disadvantage of not absorbing nutritive sub- 

 stances, and of drying up very rapidly. In extensive potting, 

 therefore, leaf-mould is more largely used, as there is less 

 danger of the above-mentioned disadvantages, though in its 

 pure condition it too is liable to rapid drying up. The mould 

 resulting from the decay of leaves and twigs is therefore rarely 

 used in a pure condition, but is mixed according to necessity 

 with some animal manure and with loam. In what way this 

 changes the physical and chemical conditions of the soil may 

 be gathered from our preceding chapters. 



As soon as we have realised that a considerable and per- 

 manent permeability of the soil is one of the chief necessities 

 in pot-culture, we have settled the vexed question as to whether 

 riddled or unriddled soils are most advantageous. By riddling 

 the soil only the finer particles are used ; this decreases the 

 size of the interstices of the soil, increases the capacity of 

 retaining water, and tends to clog up the soil, and thus cuts 

 off from the roots the proper supply of air. These dangers are 

 not counterbalanced by any advantages whatever, for the argu- 

 ment that a closely packed soil offers more nutritive material 

 to the roots than a soil with larger particles, and therefore 

 larger interstices, has no weight when we remember that by 

 a single application of liquid manure more food material can 

 be added to the soil than could possibly be contained in the 

 additional particles of the mould. In all pot-cultures, there- 

 fore, unriddled mould should be used. But we have mentioned 

 before that another point is of the greatest importance for pot- 

 culture, and this one is very often sinned against. We must 

 take into proper consideration the eoneentpation of the nutri- 

 tive solution which is offered to the roots. The water con- 



