298 



[May 1907. 



SCIENTIFIC AGRICULTURE. 



THE IMPORTANCE OF HUMUS IN TROPICAL AGRICULTURE. 

 With Special Reference to Cacao Cultivation. 



The recently-issued report for 1905-08 on the Botanic Station of Dominica 

 (Imperial Department of Agriculture for the West Indies) contains a summary 

 by Dr. Francis Watts, CM. G., the Superintendent of Agriculture for the Leeward 

 Islands, of the results obtained on a series of plots devoted to manurial experiments 

 with cacao. These particular experiments, inasmuch as their results indicate a 

 remunerative method of manuring cocoa trees, are naturally of great local value. 

 They also have a wider interest as throwing light on one of the fundamental problems 

 of tropical agriculture, namely, the preservation of humus. 



In the scheme of experiments, some If acres, bearing cacao trees about 10 

 years old, were divided up into five plots, which were treated as follows from 

 1900 onwards :— 



A. No manure. 



B. Basic phosphate, 4 cwt. per acre. Sulphate of potash, 1^ cwt. per acre. 



C. Dried blood, 4 cwt. per acre. 



D. Basic phosphate, 4 cwt. per acre. Sulphate of potash, \\ cwt. per 

 acre. Dried blood, 4 cwt. per acre. 



E. Mulched with grass and leaves. 



The individual trees were thus manured year after year in exactly the same 

 way, and the cumulative results are comparable to those which would be obtained 

 in actual estate practice. The first point established was that the treatment with 

 nitrogenous manures was remunerative. Thus, to take the last year, the net 

 financial result with plot B, which received phosphate and potash and no nitrogen, 

 was a loss at the rate of £4. 4s. 3d. per acre ; on the other hand, on both plots to 

 Avhich nitrogen was added there was a gain, that is to say, the increased value of the 

 crops more than compensated for the cost of the manure ; plot C gave a net gain on 

 manuring of £4 8s. per acre, and plot D of £8. 0s. 4d. per acre. 



Still more striking, however, was the result obtained on the fifth plot, E, 

 manured with grass and leaves, the gain on manuring being at the rate of not less 

 than £20. 16s. 6d. per acre. That this was no accidental result is well brought out in 

 the diagrammatic summary of the experiments accompanying the report, in which 

 the yields of " wet " cacao are plotted by means of lines on a uniform scale for the 

 years 1903-06. From this diagram it can clearly be seen that the mulch of grass and 

 leaves, whilst of slower action than more readily available manures, is more 

 lasting in its effects. The mulched plot, E, was beaten by C and D in 1903, tied with 

 C in 1904, beat both in 1905, and altogether outdistanced them in 1906. 



The material employed for mulching consisted chiefly of lawn mowings ana 

 the fallen leaves of neighbouring trees, particularly PitJiecolobiutu saman, the well- 

 known " Guango" or " Rain " tree. The general suggestion is made that grass, etc., 

 from uncultivated lands might commonly be used for mulching cacao, or if no waste 

 land is to hand, that certain areas should be set aside and used for the production of 

 material for mulching. 



These results are worthy of serious consideration in many parts of the tropics 

 where the provision of chemical manures is often expensive, and where also the 

 humus of the soil rapidly disappears and needs replenishing. The beneficial nature 

 of such manuring will probably be apparent not only with cacao, but also other 

 crops, especially those which owing to their casting a deep shade or for other 

 reasons do not allow a mulch to grow under them.— Bulletin of the Imperial Institute' 

 Vol IV, No, 4, 1906. 



