HORTICULTURAL SOILS. 
77 
such as nitrate of soda, guano, ammonia salts, or soot if very 
early production of vegetables is desired. 
The change of insoluble into soluble plant-food is always 
going on in the surface soil, especially in rich moulds, and as 
the nitrates are formed they are at once taken up by the growing 
plants ; but if there is no plant at hand, then the soluble con¬ 
stituents are washed away by the rains, and thus a constant 
exhaustion of plant-food in soils that are uncropped is being 
brought about. 
In rich garden soils the production of available plant-food is 
at its maximum, and so is also the waste by drainage if proper 
care be not taken. 
Available Plant-food. 
A large part of the elements of plant-food contained in soils 
is present in such a condition that plants are unable to make 
use of it. For example, it is very usual to find about 015 per 
cent, of phosphoric acid in an ordinary loamy soil. Such a soil 
9 inches deep, in its dry state, may be said to weigh from 1,200 to 
1,500 tons per acre. A soil containing 0-15 per cent, of phos¬ 
phoric acid would accordingly contain somewhere about two tons 
of phosphoric acid to the acre, disregarding the subsoil altogether. 
Such a soil contains as much phosphoric acid per acre as would 
be contained in about seventeen tons of superphosphate or in 
nearly ten tons of bone dust, and yet the addition of a few 
hundredweights per acre of phosphatic manure may make the 
difference between a full crop and a bad one. Similar state¬ 
ments would apply to other constituents of the soil. This leads 
us to recognise the important fact that it is not the total pro¬ 
portion of phosphoric acid, or of potash, or of nitrogen that rules 
a soil’s fertility for horticultural purposes, but the amount of 
each of them that is present in an immediately available 
condition. 
This question of the availability of plant-flood in soils has 
been dealt with more or less fully during recent years by many 
investigators, and to Dr. Bernard Dyer we owe much valuable 
information regarding the subject. By the permission of Sir 
John Lawes, and with the advice and personal assistance of Sir 
Henry Gilbert, Dr. Dyer obtained samples of soils from an ex¬ 
perimental field at Rothamsted, Hertfordshire, which has grown 
