26 HARDY PERENNIALS 
Green Manuring 
is very beneficial to sandy and stony soils. Green 
manuring is accomplished by sowing a plot of 
ground with white mustard, rape, or some other 
quick-growing crop that makes plenty of leaf, and 
digging the whole in when growth has well ad- 
vanced. If an ounce each of Kainit and super- 
phosphate per square yard are sown and raked 
in with the seed growth will be very rapid, and 
as the green crop grows plant food is absorbed 
by the leaves from the atmosphere and by the 
roots from the soil, all of which will be eventually 
buried in the soil to become available to the next 
occupants of the ground. 
A frequent mistake when adding humus to a 
hungry soil is to destroy it again by the application 
of quicklime, which burns up the humus since it 
cannot burn the stones and sand. If lime is needed 
on such soils, and it very probably will be, it should 
be provided in the form of chalk which does not 
bum. Stiff clay is unfertile because its particles 
adhere so closely together that air is excluded. 
The exclusion of air renders the natural salts that 
may be present in the soil insoluble, and therefore 
although chemicals required by plants may be 
present in abundance they are locked up and useless. 
Clay also holds water to such an extent that it is 
too cold for the roots of many plants. 
If clay soil is roughly dug and left in clods in 
