126 SYLVICULTURE IN THE TROPICS n .n 



made to run in the best manner possible to secure this 

 infiltration, and either a continuous stream of water 

 must be kept running or the channels are blocked at the 

 lower end until they are filled to the required height. 

 The water is then allowed to stand until it has had time 

 to soak in, when the remainder is allowed to flow oft". 

 The proper degree of saturation can be more quickly 

 obtained when the water is made to meander in the beds, 

 as e.g. is shown in Fig. 23. 



Beds irrigated by flooding must be level. If the 

 nursery is on a slope, the beds must be terraced. With 

 irrigation by percolation, level beds are preferable, but a 



Cross-section of a seed-bed irrigated by flooding 





ft- v-?..-T"V 



; . * J . J 



Cross-section of a seed-bed irrigated by percolation 



Fig. 22. 



slight slope may be allowed. In this case the length of 

 the beds should be at right angles to the slope, and the 

 channels should either be constructed along the upper 

 edge or they should be made to meander through in the 

 manner indicated above. Even beds which are only 

 irrigated by the natural fall of rain should be made at 

 right angles to a slope, especially in tropical countries, 

 where rain falls heavily and there is danger of scour. If 

 the beds are made as indicated, the paths between them 

 can act as drains, to a certain extent at least, and 

 check the rush of water from a tropical downpour. 



The choice of the flooding method of irrigation will 

 depend on the soil and on the species to be raised ; but, 



