60 the farmers' handbook. 



Absence of Elements essential to Fertility. 



In addition to the presence of the actively injurious substances enumer- 

 ated above, soil infertility may be caused by the absence or deficiency of 

 certain essential soil ingredients. 



Want of lime, for instance, as we have seen, may induce sourness and a 

 disturbance in the ratio between lime and magnesia. Its absence also affects 

 the growth of many crops for which it is an essential plant-food, particu- 

 larly leguminous crops, lucerne, clovers, &c. 



Deficiency in humus is a common cause of infertility. A soil deficient 

 or wanting in humus is less able to withstand droughty conditions, lacks 

 cohesion, and is easily blown or washed away, and is unfavourable to the 

 growth of micro-organisms. The subject is more fully dealt with on 

 page 70. 



Absence of bacteria, particularly of the nitrifying organisms, is prejudicial 

 to the satisfactory production of crops. The cause is generally one or other 

 of those discussed above, either want of aeration, lack of lime or vegetable 

 matter, sourness, bad tillage or drainage, &c, and when such soils are 

 restored to good condition the development to the nitrifying organisms will 

 proceed normally. 



Want of plant-food is, of course, a common cause of infertility, especially 

 in the case of land which has been exhausted by repeated croppings without 

 manuring or rotation. Proper manuring, having due consideration to the 

 requirements both of the soil and of the crop, is the remedy, provided that 

 the land is in good condition ; but the important fact must not be lost sight 

 of that the mere addition of plant-food is not sufficient unless the soil is in 

 such mechanical condition that it can make good use of the manure applied. 



Manuring alone is not likely to be of any benefit on land that is badly 

 drained, sour, or in bad tilth. 



Presence of Organisms which destroy the Nitrogen-forming Bacteria. 



In connection with one of the causes of sterility noted above, namely, the 

 absence or deficiency of bacteria, the results of an investigation of the 

 greatest interest and importance has recently been published by Messrs. 

 Kussell and Hutchinson from the Rothamsted Laboratory. These investi- 

 gators find that probably in all soils the development of these bacteria is 

 kept in check by the presence in soil of certain larger unicellular organisms 

 (Protozoa) which feed on the bacteria concerned in the formation of soluble 

 nitrogen compounds. 



If the soil is partially sterilized by heating for a short time to the tem- 

 perature of boiling-water, or by subjecting it to the action of vapours such 

 as chloroform, bisulphide of carbon, toluene, &c, such vapours being sub- 

 sequently removed by spreading the soil out in a thin layer and allowing 

 the vapour to evaporate, the effect is to destroy the protozoa and probably 

 most of the bacteria as well, but not the spores of the ammonia-producing 

 bacteria. These spores subsequently develop, and in the absence of the 

 hostile protozoa, their development proceeds with increased activity, the 

 result being a considerable increase in the soluble nitrogenous plant-food 

 and a more vigorous crop growth. 



These experiments have so far been carried out in the laboratory. If 

 means are discovered of partially sterilizing the soil in the field a most 

 valuable method of increasing the fertility of the soil will be placed at the 

 disposal of the farmer. 



