82 
JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
soil texture. Its power of holding water may be improved; its 
supply of available plant food increased; and then by a suitable 
manurial treatment it may be brought into condition to yield 
bountiful returns for all that is done to it. 
The next question very naturally is, What ingredients of 
plant-food are most frequently deficient in horticultural soils ? 
I think we may take it as a pretty-well established fact that 
the only constituents of plant food which need be supplied to 
garden soils are potash, phosphoric acid, lime, and nitrogen. 
When we say these ingredients are lacking, we do not mean that 
the soil does not contain them, but that it does not supply the 
growing plants with as much as they need. It is not so much 
because horticultural soils have been worn out of plant food, but 
rather because the food is locked up in such combinations that 
the roots cannot get at and use it, that an artificial supply of 
soluble food in manure becomes necessary. 
Conclusion. 
In conclusion, a few practical remarks may be made upon the 
three main constituents of plant food in horticultural soils— 
namely, 'nitrogen, potash, and phosphoric acid. 
Nitrogen .—Although the nitrification in rich garden soils, 
leaf-moulds, and peat-moulds may be sufficiently active for the 
gardener to dispense with artificial nitrogenous manures in most 
cases, yet there are certain species of plants which rapidly 
develop a large mass of foliage, and these cause a rapid and 
extensive demand upon the available nitrogen of the soil. For 
such plants it will always be advisable to use nitrate of soda, 
sulphate of ammonia, guano, soot, or similar materials as manure; 
and also for growing very early crops, or plants out of season. 
Phosphoric Acid .—Assimilable phosphoric acid occurs in 
very small actual quantities in most soils, however rich; this 
has been fully illustrated in the tables. It is therefore necessary 
to add this ingredient by a manurial application if full crops are 
to be obtained. The best form in which phosphoric acid may be 
added to horticultural soils is by bone phosphate, bone meal, 
double superphosphate, or basic slag. Superphosphate of lime 
yields a certain proportion of phosphoric acid soluble in water. 
But in rich moulds cheap mineral superphosphates are not to be 
