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 0-15 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 percent, 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 



