MANURES FOR SPECIAL SOILS 213 



Tropical Climates. A characteristic of all tropical 

 regions is the great rapidity of nitrification in the soil. 

 Nitrates are very easily lost by drainage should the rain- 

 fall be heavy, hence sulphate of ammonia and organic 

 manures are often very valuable. As the climate induces 

 rapid ripening, there is not the need for the use of phosphates 

 as is felt in colder climates, though there are many soils 

 which need considerable amounts of phosphatic fertilizers.* 

 Phosphates always need replacing, when they have been"* 

 removed from certain areas since prehistoric times byS 

 excessive grazing. The results of experiments in many > 

 tropical places show that where the climate and soil lend ' 

 themselves to large crops, heavy applications of fertilizers 

 are often extremely profitable, but where the crops are not in 

 any case large the use of these expensive manures is often 

 economically unjustifiable. For example, the yield of maize 

 on one experimental unmanured plot was comparatively 

 low arid no improvement was induced by the introduction of 

 superphosphate on a parallel plot ; in a district where the un- 

 manured plot was of a somewhat more fertile nature, owing to 

 the soil and climatic conditions, the use of artificial manures 

 was very striking. Where the water supply is good, very 

 large crops can be obtained in warm climates, and where the 

 amount of water is insufficient small top dressings of easily 

 soluble fertilizers may help the crop over a difficult and 

 droughty time. Drought causes premature ripening ; the 

 use of sulphate of ammonia in such cases, by extending the 

 period of growth, may produce very valuable results. 



Wet and Dry Climates. The relative effects of wet 

 and dry years on crop yields are shown admirably by Hall 

 in the Rothamsted experiments. A comparison of a wet year 

 with a dry year and with an average of many years, on the 

 wheat field at Rothamsted, shows that in a dry season farm- 

 yard manure is very good indeed, but that in a wet season 

 the minerals show up well. It is very striking in these 

 experiments how materials that are not considered of high 

 manurial value produced special results in the wet years. 

 In the wet year of 1879 sulphate of soda and sulphate of 



