44 THE BOOK OF MARKET GARDENING 



ing roots, or abundant, easily-dispersed seeds, must be 

 followed up vigorously. 



2. Persistent Cultivation, i.e. the stirring, breaking, 

 and deepening of soils, whenever the weather conditions 

 permit such operations with safety. The deepening to 

 be preferably effected by the gradual improvement of 

 lower layers, and mixing with the upper portions, not 

 by bringing up large quantities at once. When the 

 subsoil is harsh or unfavourable, bringing it to the 

 surface results in a total deterioration of the whole 

 depth. 



3. Maintaining the fertility. Constantly adding arti- 

 ficial and organic manure in proportion to the character 

 of the soil and the crops removed, so that the essential 

 elements may always be present in abundance, the 

 physical condition being improved as regards aeration, 

 warmth, and the chemical actions in the soil assisted by 

 sufficient quantities of humus, i.e. decaying vegetable 

 matter chiefly. 



4. Protecting crops from insects and diseases. Prompt 

 adoption of measures for the destruction of insects, and 

 the use of remedies for, or preventives against diseases. 

 Equal or even greater attention is needed under glass, 

 but more efficient means are available in the form of 

 fumigators, which cannot be used out of doors with 

 advantage. 



Under Glass Cultivation 



All the essential conditions are under the absolute 

 control of the grower, except sun-heat and sun-light, 

 but with such artificial arrangements the utmost care 

 and the closest supervision are needed to guard against 

 injurious excess or deficit. 



I. Temperature. The conservation of heat is mainly 

 dependent upon sound roofs and thorough glazing. 

 The maintenance of the requisitive temperature neces- 



