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08 MARYLAND AGRICULTURAL EXPERIMENT 8TATION 



RELATION OF LEGUMINOUS PLANTS TO SOIL FERTILITY. 



There seems to be an erroneous belief among- those not thoroughly 

 conversant with the subject, that a legume will increase the fertility of 

 any soil, by taking up nitrogen from the air, regardless of prevailing- 

 conditions, and previous treatment of the soil ; and that all green ma- 

 nuring crops are legumes. Therefore at the beginning it may be well 

 to explain the soil conditions which are necessary in order that legumes 

 may gather nitrogen from the air, and also make a distinction between 

 those green manuring crops which gather atmospheric nitrogen, and 

 those which either consume or only convert the soil nitrogen, or, 

 strictly speaking, the leguminous and non-leguminous green manures. 



In order that a legume may assimilate nitrogen from the atmos- 

 phere, the soil must primarily contain or be inoculated with certain 

 bacteria, whose presence is manifested by the growth of nodules on the 

 roots, through which it is believed that the atmospheric nitrogen is 

 obtained. These germs are usually found abundantly in most well- 

 tilled soils. 



Any crop may serve as a green manure, ^but leguminous crops 

 possess a greater value for this purpose, than others, because they can 

 obtain certain of their constituents from sources not accessible to all 

 plants. Therefore in order to show the value of legumes as green 

 manure, it is necessary here to separate them from other green manur- 

 ing crops, which, instead of increasing the supply of soil nitrogen, 

 actually decrease it . 



FERTILIZING POWER OF LEGUMINOUS AND NON-LEGUMINOUS PLANTS. 



The most important legumes available for use as green ma- 

 nures, are crimson clover, red clover, cow peas, and soja beans. They 

 are not only valuable on account of their nitrogen-gathering property, 

 but their period and time of growth make them very convenient crops. 

 The quantity of nitrogen which these crops gather from the air, depends 

 largely on the amount of nitrogen stored in the soil, for they will gather 

 at least a part of their nitrogen from the soil, in preference to 

 that of the air, unless starved of soil nitrogen. Therefore the exact 

 amount of nitrogen which a plant gathers from the air, cannot be de- 

 termined by the content of nitrogen in the plant. But it has been de- 

 termined by experiment that they do gather nitrogen from the air, and 

 store it in their own tissues which, by decaying, allow it to be used by 

 other crops, which can obtain this valuable element only from the soil. 

 For this reason, they can, by judicious growing, be made a very potent 

 factor in the economical production of crops. 



The principal non-leguminous green manures are rye, buckwheat 

 and mustard. They do not increase the supply of soil nitrogen, but by 

 their time of growth, prevent the loss of this element by leaching, 

 which is very liable to occur if the soil is left naked. They improve 

 the mechanical and physical condition of soils, and conserve the soil 

 nitrogen. But while they retain the supply of nitrogen in the soil, they 

 convert it from the immediately available to the less available organic 



