EROSION AND DRAINAGE. 23 



when the surface is undisturbed the erosion is proportionally very 

 small, and the mountain rivulets are often hut slightly colored from 

 the soil. If the cohesion of the soil particles were destroyed by frost 

 or other agencies, the steep slopes would simply flow down into the 

 valleys and cover the coast plain or go out to sea; but to a less extent 

 this result must follow from any agricultural operations which stir the 

 soil and permit it to be washed away by the torrential rains. Thus in 

 some districts it is extremely doubtful whether the open culture of 

 coffee would be at all practicable, not because the deficiency of shade 

 would 1)0 hurtful, but because the hoeing and weeding or even the sim- 

 ple exposure of the soil to the direct action of winds and rains would 

 undoubtedly result in a rapidly diminished fertility. Moreover, as 

 the soil is relieved of its humus, it becomes the less able to retain 

 moisture and the more liable to drought. To maintain the correct 

 balance of moisture in the soil is, indeed, one of the more difficult 

 problems of the culture of a plant which requires at once effective 

 drainage and continuous humidity of soil. In heavy cla} T s like those 

 of Porto Rico special provisions for drainage may be necessary even 

 on steep hillsides, particularly in localities where the rainfall is evenly 

 distributed so that there is no danger of drought. Soil soaked full of 

 water admits but little air, which is now known to be as necessary to 

 the normal activities of the roots as to those of the leaves. 



In planning drainage it is desirable to avoid as far as possible the 

 erosion of the surface soil or the washing away of the vegetable debris 

 from which humus is formed. Ditches are commonty dug across or 

 oblique to the general slope. This decreases the fall and necessitates 

 frequent cleaning, but the fine soil and vegetable matter deposited are 

 eminently worth saving and serve instead of more expensive fertilizers. 



In some districts of Central America a system of transverse pits or 

 trenches is in use for preventing the loss of the loose soil and vegetable 

 debris on steep hillsides. In the next rain}^ season these pits are filled 

 by surface soil and other washings from above, and in the succeeding 

 dry months other trenches are dug immediately above the preceding. 

 As the coffee tree increases in size and the roots push farther out they 

 meet a succession of these accumulations of rich soil which serve in 

 lieu of commercial or other fertilizer. The application of this s} r stem 

 in Porto Rico has been advocated by Mr. J. D. Sulsona, of Mayaguez, 

 but it is not known that the suggestion has been acted on, even to an 

 experimental extent. The practicability of the plan on many of the 

 steeper slopes is not certain, but its value might be found to be increased 

 if used in connection with a reasonable number of leguminous trees, 

 which would help to break the force of the winds and torrents. More- 

 over, the roots would tend to prevent deep washing by heavy storms, 

 and those cut away in digging the trenches would form with their 

 tubercles a valuable contribution to the fertility imparted by the sedi- 

 ment caught by the trenches. In Venezuela similar transverse trenches 



