1 10 The Nitrogen Problem in Crop Production. 
the loss of feeding material in the silo is so great that the 
silage costs more than it is worth. In any attempts to extend 
the leguminous crops they must not be brought into com- 
petition with roots. 
The second drawback is more serious because more obscure. 
Many arable soils will not profitably carry clover oftener than 
once in six or eight years ; if it is tried more frequently the 
yield falls off and the land is said to become tk clover sick.” 
No cure is known, and even the nature of the “ sickness ” 
is not understood. It is known, however, that land which 
fails to carry one leguminous crop will not necessarily fail to 
carry another, and the practical inconvenience arising from 
“ sickness ” can be obviated to a certain extent by changing 
varieties, so that each sort has a long interval before it is sown 
again. The relative value of the different varieties of legumi- 
nous crops depends on local conditions, and would form a 
very fruitful line of trial for those in a position to carry out 
agricultural experiments. 
It has been stated above that the free nitrogen of the 
air forms the food of leguminous plants. The nitrogen is, 
however, not taken up directly by the plant but by minute 
bacteria associated with the little lumps — the nodules — on the 
roots. Leguminous plants therefore depend for their food 
on these bacteria, and it has been urged that farmers have 
only to add bacteria to the soil in order to secure heavy and 
valuable crops for a trifling outlay. The scheme is certainly 
attractive, and appeals strongly to the natural human desire 
to achieve great results at a small cost, so that “ soil 
inoculation ” is always sure to arouse very widespread interest, 
and has even formed the subject of questions in the House 
of Commons. On going carefully through the experimental 
evidence, two facts stand out clearly : 
(1) Soil inoculation has proved successful on poor vii*gin 
soils that are being put into leguminous crops for the first 
time ; 
(2) It has also succeeded on older arable land when a 
completely new variety of leguminous crop is being grown 
for the first time. 
But when we turn to the common British case of legu- 
minous crops that have been grown fairly frequently on the 
same soil, we find so few successful instances that the plan 
cannot at present be recommended. The problem is, however, 
under investigation in several laboratories, and a practical 
method may at some time be evolved. 
5. Nitrogen fixed by bacteria without leguminous plants . — 
Bacteria capable of taking up free nitrogen and making it 
into plant food occur in many, if not most, soils, but as they 
