SEPTEMBER 



IRISH GARDENING 



35 



increase. By various and conclusive experi- 

 ments it was proved beyond doubt that by far 

 the greater part of the increase of nitrogen 

 was brought about by the agency of bacteria 

 in the soil. These free-living bacteria are 

 present in practically all soils, but their rates of 

 activity differ considerably with the conditions. 

 In order to encourage their multiplication and 

 growth, and consequent addition of valuable 

 nitrogenous matter to the soil, it is of the 

 greatest importance to make the conditions as 

 suitable as possible for their development. 

 They are retarded in their action, if not killed 

 out, by the presence of acids in the soil, while 

 plenty of air, a sufficiency of moisture, a good 

 supply of lime, phosphates and potash are all 

 essential to their active existence. Drainage 

 and liming to remove acidity and superfluous 

 water, good tillage to ensure aeration, and 

 dressings of phosphates and potash, are the 

 chief means by which these minute organisms 

 can be encouraged in their valuable work of add- 

 ing a valuable fertilizing ingredient to the soil. 

 {b) Amongst green plants there is an exten- 

 sive order known as the Leguminosae, so called 

 because the seeds are borne in legumes or pods. 

 Peas, beans, vetches, clover, lucerne, and sainfoin 

 are the chief cultivated plants belonging to the 

 order. From experiments carried out at 

 Rothamstead, in England, and at various 

 centres in Germany, it has been clearly shown 

 that the soil on which a leguminous crop has 

 been grown contains more nitrogenous matter 

 after the crop has been removed than was there 

 before it was planted ; and this notwithstanding 

 that th"e leguminous plants are highly nitro- 

 genous, and consequently use large quantities of 

 nitrogen in the building up of their own tissues. 

 Leguminosae are unique amongst green plants. 

 In the case of all others the nitrogen they con- 

 tain is derived from the supply already in the 

 soil, and it can never exceed this amount. The 

 power of increasing the store of nitrogen in a soil 

 is possessed by all leguminous plants, but as the 

 quantity added by any given crop varies with 

 the conditions, it becomes of interest to find out 

 the causes which are most helpful in building up 

 this valuable store. It has been known for a 

 long time that the frequent growth of legumi- 

 nous crops increases the fertility of the land, but 

 it is little more than thirty years since it was 

 shown how this was brought about. If you 

 pull up a large clover plant vith its roots you 



will invariably find numerous little roundish 

 growths or nodules at intervals on the root- 

 fibres, while a weakly plant will have few or 

 none of these nodules. When grown on ordi- 

 nary soil clover plants almost always bear 

 nodules, and when the crop is harvested the 

 soil will show an increase of nitrogen. On the 

 other hand, if the plants be grown on sterilised 

 sand no nodules appear on the roots, and all the 

 nitrogen required by the plants must be applied 

 artificially. These results led to the suggestion 

 that the nodules were caused by living bacteria 

 in the soil, and that they were also in some way 

 connected with the increase of nitrogen already 

 spoken of. Further and numerous experiments 

 have shown the suggestion to be true. It is 

 now well established that the nodules are 

 caused by bacteria which are present in almost 

 all soils where legumes are grown, even at long 

 intervals. When the delicate root-hair of a 

 leguminous plant comes in contact with one of 

 these minute bodies the latter enters into the 

 cells of the root, and by its presence causes an 

 irritation which leads to rapid growth at the 

 part, resulting in the formation of the nodule. 

 Inside the nodule the organismlrapidly multiplies, 

 so that in a short time all the cells are filled 

 with the bacteria. During their growth they 

 take in nitrogen from the air and utilize it in 

 building up the substance of their own bodies. 

 After a time, and when the cells constituting the 

 nodule have become choked, as it were, with 

 the bacteria, the latter become disorganised ; 

 they get dissolved, and the material of their 

 bodies is carried to the seeds of the plant on 

 which they have been growing. This dissolved 

 material derived from the bodies of the bacteria 

 is highly nitrogenous, and is of very great help 

 in supplying the nitrogen of which leguminous 

 seeds need so large a proportion. The nodules 

 are now of no further use, they shrivel up and 

 disappear ; but some of the bacteria which they 

 contain escape destruction and make their way 

 into the soil, where they remain waiting for 

 another leguminous crop. 



As the nodule-forming bacteria are found in 

 almost all soils it is seldom necessary to add 

 them artificially. In reclaimed land, where no 

 leguminous plants have previously grown, it 

 will be found beneficial to add the bacteria. 

 This may be done either by adding a little soil 

 from a field where the nodule -bacteria are 

 known to be, or, better, by using one of the 



