222 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



than the air passing over a mineralized soil ; the rainfall will there- 

 fore be greater. If the soil all over the country were richer in humus 

 the rainfall would be more uniform, as in countries covered with forests. 



The soil, being more open, is better aerated, and consequently 

 more plant food is set free. 



Much manurial matter that would otherwise leach away is 

 retained. 



The plants are healthier ; a soil deficient in humus is always more 

 likely to support disease : clover sickness and finger-and-toe are 

 never so prevalent on soils showing a high humic content. 



Of this fact the Indian coffee planters are well aware ; as long as 

 their soils were well stored with the vegetable matter of the primeval 

 forests which previously occupied the site of their plantations, there 

 was very little disease among the plants ; as soon as the humus 

 became exhausted diseases increased, but have again been reduced 

 by adding the top soil from the neighbouring forest lands. 



The importance of a good tilth and a good seed-bed cannot be 

 over-estimated ; in fact, if the weather conditions are adverse to the 

 start of a crop, the eventual yield will often depend more on the 

 physical condition of the seed-bed than upon any other factor. No 

 better proof of this fact could be given than that afforded by Table I. 

 from the Rothamsted experiments given above. As has been 

 previously pointed out, there are always more roots on the rape cake 

 plot than on the others, showing that the conditions are far better 

 for the young plants ; in fact, the physical condition of the soil where 

 only minerals are used has become so bad that only in favourable 

 seasons is a good plant obtained and on three occasions the sowings 

 have completely failed, whereas the rape cake plot always starts 

 regularly. 



Against these many advantages of the humus-containing organic 

 manures we have to place the distinctly bad effect that most mineral 

 manures are known to have on the physical condition of the soil. 



Probably nitrate of soda is the worst, owing to the fact that 

 the residue it leaves behind in the soil — caustic soda — deflocculates 

 the clay, which consequently becomes puddled. 



Sulphate of ammonia and superphosphate leave an acid residue 

 which is harmful to plant life, and still more so to bacterial 

 activity. 



It is true that those minerals which leave lime as a residue in the 

 soil, such as basic slag and nitrolim, are not subject in the same 

 way to this objection, but it is undoubtedly true to say that one of 

 the great advantages of organics over inorganics is that the former 

 materially improve the texture of the soil by increasing the supply 

 of humus, and thus enable a good seed-bed to be obtained and a good 

 tilth maintained, whereas the latter, generally speaking, have an 

 opposite effect. 



2. Chemical. — Under this heading I want to consider the chemical 

 form in which the food materials exist in the two classes of manures, 



