GARDENING IN HEAVY SOILS 27 



But the effects of deep digging upon a stiff clay are 

 only transient, unless the clay is mixed with other 

 matter which will prevent it from clogging with the 

 damp and caking with the heat. It must be made 

 porous by the addition of other more porous sub- 

 stances which will both relieve it of moisture and 

 add to its fertility. Of these the most valuable are 

 rubble and humus — that is to say, soil consisting 

 of decayed vegetable matter and, ia particular, leaf 

 mould. There are, of course, many kinds of rubble, 

 but the best of all is mortar rubble, for not only is it 

 very gritty, but it is also full of lime, which in itself 

 is a most valuable form of plant food.. Many people 

 use cinders, and these certainly increase the porosity 

 of the soil, but unfortunately they also impoverish 

 it, as they contain no kind of nourishment whatever. 

 Mortar rubble, therefore, should be used, if possible; 

 and it may be very plentifully mixed with a stiff clay 

 soil with the best results for all except the few plants, 

 such as Rhododendrons and Azaleas, and Kalmias, 

 to which lime is poison. Humus does not, of course, 

 increase the porosity of the soil so much as rubble, 

 but it does make it more porous and also warmer, 

 and it is a most valuable and in a stiff clay an almost 

 essential plant food. The rubble and the humus 

 should be mixed together and dug well into the clay, 

 so that the soil for 2 ft. at least is permeated with 

 them. If further drainage is necessary it should con- 

 sist of a foot or so of broken bricks, &c., the larger 

 the better, about l}/^ ft. below the surface of the 



