88 the farmers' handbook. 



The case of legumes is, however, quite different, and may be referred to 

 more fully. It appears from experiments of Hellriegel and Willfarth, and 

 later, of Lawes and Gilbert, on peas, lupins, vetches, and lucerne, that there 

 exist in the root-nodules, or small excrescences which are found on the roots of 

 these plants, certain minute organisms, which are capable of assimilating free 

 nitrogen and of rendering it in an available form to the plant. These 

 organisms act therefore, as carriers of nitrogen between the air and the plant, 

 and the root-nodules become a store-house from which the leguminous crop 

 derives its nitrogenous food. The air from which the nitrogen is thus with- 

 drawn is the air in the interstices of the soil, and as this is continually 

 renewed, especially in the case of a porous soil, from the outside air, the supply 

 of nitrogen is practically an inexhaustible one. The form and appearance of 

 those nodules vary somewhat in the different plants, being generally large or 

 small swellings on the root or root-fibres, sometimes single and sometimes 

 agglomerated. Though leguminous plants exhibit this peculiarity in a special 

 degree, there is reason to believe that other plants are able to fix nitrogen 

 from the air in a similar manner, though in a much smaller degree. 



On account of this property possessed by leguminous plants, such plants 

 derive little or no benefit from the application of nitrogenous manures — in 

 fact, it has been observed that the formation of root-tubercles is reduced, or 

 even stopped entirely by the addition of much nitrate. 



The manures that particularly benefit leguminous crops are such as contain 

 potash and phosphoric acid. The latter especially seems to have a most 

 favorable effect, and hence superphosphate, both when sowing and as a 

 top-dressing during the life of the stand, has proved most beneficial. 



Composition and Action of Manures, 



We have now to consider the. composition and the specific action of the 

 several substances used as manures ; and for this purpose it will be con- 

 venient to classify them according as lime, phosphoric acid, nitrogen, or 

 potash is their dominant constituent. 



MANURES CONTAINING LIME. 



The manures which depend for their action upon the presence of lime are 

 (a) burnt-lime, (6) carbonate of lime, and (c) gypsum. 



(a) — Burnt-lime, quicklime, or stone-lime. 

 When liming is recommended for a soil, it is always either burnt-lime 

 (powdered or freshly slaked), that is intended or else carbonate of lime. The 

 • following remarks apply to burnt-lime or quicklime. Many substances used 

 as manures contain lime, but in these cases the lime is in combination with 

 other substances, and has not the same action on the soil as burnt-lime. For 

 example, bone-dust and superphosphate both contain considerable proportions 

 of lime in combination with phosphoric acid as phosphate of lime. Neither 

 of these substances, however, has any effect in lightening clay soils, or in 

 sweetening sour ones. In the same way gypsum (sulphate of lime) is a 

 substance rich in lime, and a valuable addition to the land in certain cases, 

 but its action is not that of burnt-lime, and it is not to be used when 

 liming is recommended. Wood-ashes also contain carbonate of lime and 

 have a considerable value as fertilisers. Thomas' phosphate contains free 

 lime, and there is no doubt that it has a considerable effect in altering the 

 texture of heavy clay -soils, but none of these substances are to be substituted 

 for burnt-lime. 



