Building up the Fertility of the Soil. 757 



stioiigly acid in reaction and hurtfnl to plants, and they must be 

 decomposed with lime. Ammonium sulphate is broken up and 

 oxidized and becomes a source of sulphuric acid and nitric acid. 

 These acids must be neutralized by lime. Potash salts have their 

 potash removed and fixed by the crop. The sulphuric or hydrochloric 

 acid with wliich it was combined is set free in the soil and must be 

 neutralized by lime. In neutralizing' these sources of acidity the lime 

 is not regenerated, except in so far as the crop may assimilate the 

 acidic radicals of the salts formed. This is the case with the nitrate, 

 and to a less extent with the sulphate. 



(b) Lime provides a neutral medium in wliich bacteria favourable 

 to farm crops can flourish. The productivity of the soil is dependent 

 largely on the activity and multiplication of useful micro-organisms, 

 which fix atmospheric nitrogen, produce nitric acid, and generally 

 give powerful aid in the dissolution of soil material. 



(c) Lime prevents the harmful effects of a high proportion of 

 magnesia in the soil. Land in magnesian limestone districts is some- 

 times found that is more or less infertile and that does not respond 

 properly to fertilizers. The magnesium carbonate is a very effective 

 neutralizer of acids, but if lime is not abundant it proves dangerous 

 where fertilizers are used. Magnesia is required as food by plants 

 to a slight extent; they can find sufficient for their needs in all soils, 

 and it is not, therefore, required in fertilizers. 



Where the magnesia in the soil exceeds the lime in amount, crops 

 sometimes make poor growth in spite of fertilizers. If lime is applied 

 the soils recover. The reason is to be found in the greater solubility 

 of magnesium compounds, especially magnesium sulphate (epsom 

 salt). This salt is very soluble in water, and, where soils contain more 

 magnesia than lime it is formed by double decomposition whenever 

 fertilizers containing sulphates are used. Sulphates are the com- 

 monest and most useful of all salts in fertilizers. They occur plenti- 

 fully in all dissolved phosphates. There is no other obtainable acid 

 that can be used or that is more suitable for use for this purpose than 

 sulphuric acid. The principal reason for this is that its use results in 

 the formation of gypsum, which is an excellent drying agent only 

 very sparingly soluble in water, and, therefore, harmless to plants. 

 Kow, where magnesia occurs too plentifully in soils, the application 

 of most fertilizers results in the formation of sulphate of magnesia. 

 This is a very effective plant poison, and its formation always leads 

 to a reduced crop. 



Soils, therefore, containing magnesia in notable amounts are 

 particularly in need of lime, so that when fertilizers are applied the 

 liarmless gypsum may be formed and not the poisonous epsom salt. 

 The farmer may generally best give a dressing of a ton or more of 

 ground limestone per acre to the crop following potatoes in the 

 rotation. 



III. A third requirement is nitrogen. This is very costly and has 

 often to be purchased. It is not easily retained by the soil and is 

 liable to loss in several ways. Therefore, except in so far as humus 

 may help, no attempt should be made to accumulate nitrogen in the 

 soil, as loss and disappointment would follow any such course of 

 action. In the matter of nitrogen farmers have to live from hand 

 to mouth. 



