84 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



practically unanswerable except by supposing something of this kind 

 does take place. 



The first problem is, Why cannot we grow some crops year after year 

 on the same soil ? and the second, Why do some crops grow so splendidly 

 in one place and do so badly in another ? 



Take for example a Tomato crop. If this is grown for more than two 

 years in the same soil, it is almost a failure the third year. Chemists 

 have analysed the Tomato plant and told us exactly what chemical 

 elements they remove from the soil, but no fertiliser or combination of 

 them can replace what has been removed. The soil is still fertile as 

 regards other crops, for if they be planted in it they will grow freely 

 enough. We can only suppose that there was either some organic com- 

 pound in the soil when the Tomatos were planted, and that by constant 

 cropping it is exhausted, or that the microbes are present in a healthy 

 condition and make the food they need. We can see that when the 

 supply of food is exhausted or the microbes become enfeebled the plants 

 suffer, and is this not exactly what does take place ? 



A soil in a district that grows excellent crops may be one teeming 

 with suitable bacteria, and these may be only sparingly found in, or 

 entirely absent from, soil in which the same crops do badly. 



It is said that each of the leguminous plants has a different microbe 

 causing the nodules on their roots. Thus the microbes on the roots of 

 Peas are different from those on Beans, and those on Clover different from 

 those on Lupins. A soil gets sick of growing Clover, but it will grow good 

 crops of Peas or Beans. This is an analogy worth noting, for we cannot 

 suppose that the leguminous plants are the only class of plants having 

 microbes making food for them, for different groups are constantly being 

 added to them. It is true that their presence is known by the effect they 

 cause, viz. the nodules ; but may not each group of garden plants have a 

 special microbe either attached to their roots or in close contact with them, 

 making something for them ? 



Nothing suits crops so well as good farmyard manure, and may not 

 its value lie in the fact that it is full of microbes ? Organic manures are 

 better for crops than inorganic manures : the first encourages the soil life, 

 the other retards it. Burnt soil is not so suitable for plant growth as 

 unburnt, even when nitrogen is added ; the burning destroys the life. 

 Scores of facts could be given, but let these suffice. 

 I said in the early part of this paper that plants absorb some of their 

 nitrogen in the form of ammonium nitrate, for there is plenty of indirect 

 evidence to prove that such is a fact. What is ammonium nitrate ? It 

 is a chemical substance formed by the union of ammonia with nitric acid ; 

 consequently it is a salt very rich in nitrogen. To show the comparison 

 of this with another rich nitrogen salt, viz. nitrate of soda, I may say 

 that 100 11). of ammonium nitrate would contain 35 lb. of nitrogen, while 

 the same weight of nitrate of soda would contain a little more than 15 lb. 



It has been proved that there is a class of microbes in the soil which 

 change organic nitrogen into ammonia, and others which change ammonia 

 into nitric acid. Ammonia and nitric acid are both being formed in the 

 soil at the same time, and there is nothing unreasonable in supposing that 

 they unite and form ammonium nitrate. 



