224 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



3. Biological. — Lastly, let us consider the effect of these two 

 classes of manures on the bacterial life of the soil. This is, perhaps, 

 the most important aspect of all. For some time it had been diffi- 

 cult fully to explain the effect of organic manures on the growth 

 of the crop, and to account for the fact that plants so fed seemed 

 to have more " life " in them. One could only say that decayed 

 animal and vegetable matter was the natural plant food, and that 

 there was probably some affinity in the life of the organic materials 

 — as evinced by the growth of fungi &c. that they support (by some 

 erroneously considered a disadvantage) — and the life of the plant ; 

 though, needless to say, this was scoffed at by the holders of 

 the mineral theory. 



Recent work on bacteriology undertaken and ably carried out, 

 among others, by Russell and Hutchinson of Rothamsted, has 

 thrown much light on the subject. They have shown that 

 bacteria are absolutely dependent upon humus in the soil, that 

 they feed on it, use it as their fuel, and get their energy and power 

 from its combustion, and it is thus absolutely essential for their 

 life. 



We now know that the soil is not an inert mass acting merely 

 as a medium to hold plant foods, and an anchorage for their roots, 

 but that it is indeed alive, that it is in fact a huge factory peopled 

 by millions of minute organisms that are there to prepare the food 

 already in the soil and supplied as manure for the delicate root-hairs 

 of the plant. 



It has been shown that these workers are wonderfully organized, 

 that there is a perfect division of labour among them, each group 

 having its appointed task ; when each particular group has done 

 its part, it, so to speak, hands it on to another for the next opera- 

 tion, and so on until the insoluble material on which they are working 

 has been completely broken down into a form readily assimilable 

 by the plant. 



When we realize that such substances as farmyard manure would 

 not only be useless, but actually harmful to the plant, were it not 

 for the action of these beneficial bacteria, we begin to see the im- 

 portance of the part they play in plant nutrition and in the great 

 scheme of life. 



It is impossible in the short time we have at our disposal to go 

 into this fascinating study, but as an instance of how these minute 

 organisms work it is interesting to note that every organic manure 

 that is applied to the soil has to be attacked by at least three sepa- 

 rate groups of bacteria before the plant can get at the ammonia it 

 contains, the nitrogen being first turned to ammonia, then to nitrite, 

 and lastly to nitrate, which is soluble and can be taken up by 

 the plant. 



Another bacterium has the power of fixing the free nitrogen of 

 the air and bringing it into use for the plants. 



Bacteria also play an important part in bringing phosphates 



