58 THE farmers' IIANDHOOK. 



Magnesia. 



Although lime is one of the most important elements of soil fertility, 

 magnesia, which is a very similar compound, and which is frequently found 

 associated with lime, is by no means desirable in any considerable quantity. 

 Although limestone rocks almost invariably produce the most fertile soils, 

 those formed from dolomite (magnesian limestone), which is carbonate of 

 magnesia, are of poor quality, and even the presence of any quantity of 

 magnesia in the limestone renders the soil a poor one. 



The general result of experimental work in the field and in pots tends to 

 show that when the magnesia in a soil exceeds the lime the soil is an 

 unproductive one. Loew, who has particularly studied this matter, finds 

 that a certain ratio must be preserved between the lime and the magnesia in 

 the soil. This ratio appears to vary with different crops; some crops do best 

 when the amounts of these compounds are equal, whilst others require about 

 two or three parts of lime to one of magnesia. 



The remedy most likely to be effective in the case of soils whose infertility 

 is due to excess of magnesia is the addition of lime, in order to restore the 

 balance, and such soils come under the heading of those which are deficient 

 in lime, although actually the amount of lime salts present might be suffi- 

 cient provided magnesia was absent. 



Toxic Substances Secreted by the Plant. 



It is well known that when certain crops are grown continuously on the 

 same ground without fallowing or rotation, the land becomes unsuited to 

 that particular crop. The cause usually assigned to this phenomenon, 

 namely, that the continued growth of the crop in question removes certain 

 elements of plant-food necessary to that crop, requires some modification 

 in view of recent investigations. 



The United States Bureau of Soils has advanced the suggestion that the 

 plant during its growth secretes toxic material which is injurious to the 

 succeeding similar crop, though not to crops of a different nature, and a 

 good deal of work has been done in trying to isolate and identify these toxic 

 substances. A corollary of this theory is that the beneficial action of 

 fertilisers is due less to their actual value as plant-food than to the fact that 

 they neutralise or otherwise alter these poisonous substances. 



Another theory is that these plant toxins are formed not by the direct 

 secretion of the growing plant, but by the action of bacteria upon the 

 residues left in the soil by the crop. Such a theory would afford an explana- 

 tion of such phenomena as clover sickness, &c, where it is found that bad 

 results follow the continuous growth of the same crop on the same land, 

 although the soil itself may show no appreciable loss of plant-food. 



It must, however, be stated that these theories have not yet advanced 

 much beyond the speculative stage; and that, although a considerable 

 amount of work has been done, and several definite organic substances have 

 been isolated, the toxicity of these substances in the field has not yet been 

 established conclusively. 



Whatever may be the real cause of the trouble, whether it is due to soil 

 exhaustion or to the production of toxins, either by the growing plant or by 

 bacterial action on crop residues, or to a combination of these causes, the 

 remedy indicated will in every case be the same, namely, crop rotation or 

 fallowing. 



