Scientific Agriculture. 



458 



[November, 1908- 



number of cells, has a high water- 

 holding capacity. The presence of humus 

 makes the soil dark or black in colour, 

 so that in most countries a dark soil 

 is regarded as a rich soil, but such is 

 not always the case in these Islands, 



All soils in tropical climates are more 

 or less devoid of humus unless they have 

 been recently cleared of their virgin 

 forest or are formed of the washing 

 from surrounding hills. The main cause 

 of this absence of humus, however, is 

 the fact that decomposition goes on 

 constantly, and the excessive rainfall 

 washes out the soluble matter. 



The presence of humus not only adds 

 fertility to the soil, but makes it more 

 porous and open, thus increasing the 

 water-holding capacity, which is an 

 important factor during the dry season. 

 The practice of burning over the fields 

 each year should be stopped, and as 

 much organic matter as possible turned 

 into the soil. The removal of the forests 

 and the burning out of the humus in 

 the soil, and the consequent incrusting 

 of the surface, are the main causes of the 

 rapid rise of our streams and the wash- 

 ing of the land during the rainy season. 



The character of the subsoil has 

 considerable influence on the amount 

 of moisture that will be available for 

 the use of plants. As mentioned above 

 the supply of capillary moisture is the 

 governing factor in plant growth. Also 

 only a limited amount of moisture is 

 held in the soil, hence the store house 

 below the surface must be large, so 

 that capillary water may be drawn 

 up from 6 or 8 feet below the surface. 

 A thin layer of soil underlaid with 

 gravel will suffer much from drought, 

 while on the other hand, a deep layer 

 of clay insures a good supply of 

 moisture during the dry season, but 

 provides poor drainage in the rainy 

 season, when good drainage is essential 

 except for rice and few grass crops. 



In order to furnish a constant supply 

 of moisture during the dry season, it 

 is necessary to irrigate. Irrigation 

 water may be applied in two ways, 

 namely, over the surface or by means 

 of underground pipes, which is known 

 as sub-irrigation, the water being 

 allowed to reach the surface by capillary 

 action. This is by far the best method, 

 because it does not cause "puddling," 

 or baking of the surface, but it is too 

 expensive for general use. It is adapted 

 to garden crops on high-price land. 



In using the surface method, the 

 water is conducted in ditches to the 

 highest point of the field and allowed 

 to run over the land by means of 



ditches between the rows of the grow- 

 ing crops. These ditches should be from 

 4 to 8 feet apart, depending upon the 

 closeness of the soil. If the crop is 

 rice or grass, the water may be 

 allowed to spread over the field until 

 the desired degree of saturation is 

 obtained. These methods presuppose 

 level or nearly level land. 



Surface sprinkling is sometimes prac- 

 tised. This method is in imitation of 

 rainfall, but has several objections, 

 first, the falling water compacts the 

 soil and "puddles" it so that it bakes 

 and necessitates frequent cultivation ; 

 second, it is expensive and wasteful 

 of water because a large percentage 

 of it evaporates before reaching the 

 soil- On very sandy soils, however, 

 where it is impossible to conduct the 

 water over the surface, this method 

 is valuable. 



The best water to use in irrigation 

 is river water, because it carries some 

 plant food in the form of soluble salts 

 or in the sediment and does not con- 

 tain any injurious salts. Artesian water 

 may be used, provided it does not 

 contain anything injurious to the crop. 



The amount of water to be used 

 depends so much upon climatic con- 

 ditions that very little information of 

 much value can be given in a short 

 article. 



Water is measured, as a rule, by acre- 

 inches, which means the amount of 

 water in a basin of one square acre in 

 extent and one inch deep. One acre-inch 

 is equal to 27,111 gallons per acre and 

 means the same thing as inches of rain- 

 fall. In a dry climate where evaporation 

 goes on rapidly, much more water is 

 required for a given crop than in a 

 humid climate. For example, in Hawaii 

 as much as 72 acre-inches of water have 

 been used on sugar cane with good 

 results. The crop, however, used only a 

 small fraction of this water, the bulk 

 being lost by percolation and evapora- 

 tion. 



The probable rainfall during the grow- 

 ing season of the crop must be taken 

 into consideration. If pufficient water 

 is applied to the crop to bring about the 

 best conditions in the soil so far as 

 moisture is concerned and a heavy rain 

 should come just after, the crop would 

 suffer from an excess of moisture the 

 same as in the rainy season. 



The kind of crop grown would also 

 govern the amount of water used. 

 Corn, for example, would require from 

 12 to 20 acre-inches, depending upon the 

 kind of soil. Clay requires less and 



