SOILS 



1847 



any rank-growing farm crop may be used. 

 The more succulent or juicy plants are 

 best, as tbey decay much more quickly 

 and are more easily incorporated in tbe 

 soil. If, however, the supply of nitrogen 

 in the soil is small and its increase is 

 either the chief necessity or a desirable 

 addition to the increased humus content, 

 then some leguminous crop must be used, 

 as no other farm crop has the power of 

 utilizing atmospheric nitrogen or of re- 

 turning to the soil any essential element 

 of fertility which it did not draw from 

 it. If it is desired to bring up from be- 

 low some of the mineral plant food which 

 is present in deeper layers of soil, then 

 a deep-rooting crop should be used. 



Legumes as Oreen Manure 



The legumes, or leguminous crops, are 

 a group of plants which are characterized 

 by growing their seed in pods and by hav- 

 ing peculiar knots or nodules on their 

 roots. These nodules are formed by the 

 action of a certain group of bacteria, im- 

 mense numbers of which are found in 

 each nodule, which have the peculiar 

 property of being able to use the gaseous 

 nitrogen of the air for their own growth, 

 and supplying this element as they die 

 and decay to the host plant on whose 

 roots they are located. Included in this 

 group are alfalfa, all the clovers, vetches, 

 peas, beans, etc. No other group of 



plants or animals, so far as is now 

 known, is thus able to make use of at- 

 mospheric nitrogen. Legumes may grow 

 in soils which are rich in available nitro- 

 gen without the presence of the nodule- 

 producing bacteria, deriving their nitro- 

 gen supply directly from the soil as do 

 other crops, but have the distinctive pow- 

 er of being able to flourish in soils poor 

 in nitrogen if the proper bacteria are pres- 

 ent to grow upon their roots and supply 

 them with nitrogen from the air, and 

 when so grown to increase the supply of 

 soil nitrogen when plowed under as green 

 manures. 



The amount of actual gain in nitrogen 

 to the soil from the growing of legumi- 

 nous cover crops depends upon several fac- 

 tors, such as: (1) the kind of crop used; 

 (2) the amount of nitrogen already pres- 

 ent in the soil, as this affects the propor- 

 tion of nitrogen which the crop will take 

 from the air, and (3) the proportion of 

 the crop which is returned to the soil. 



Mtrogen Content of Different Legumes 



Analyses have been made by the writer 

 in the laboratory of the State Experiment 

 Station of the different parts of various 

 leguminous crops — ^the samples being 

 taken from adjacent plots where each 

 legume was given equal conditions of soil, 

 moisture-supply, etc., for its growth. The 

 results of some of these analyses are 

 shown in Table IV. 



TABLE IV 

 Mtrogen Content of Different Legumes 



CROP 



Per Cent Nitrogen in Dry Matter 



Tops 



Roots 



Nodules 



Whole Plant 



Tangier Pea 



Field Pea 



3.63 

 2.68 

 2 61 

 2 96 



1 72 



2 30 

 1.87 



2.47 

 2 38 

 2.54 

 2.45 

 1.27 

 1.91 

 1.73 



4.00 

 2.84 

 5.09 ^ 

 5.07 "« 

 6.92 

 5.97 ' 

 5.86 ' 



3 SO 

 2 62 



Spring Vetch. 



Hairy Vetch 



Alfalfa 



Red Clover 



White Clover 



2 58 

 2.80 

 1.60 

 2.18 

 1.82 







As has been pointed out, the proportion 

 of this nitrogen which the crop will gath- 

 er from the air, through its bacteria, 

 depends upon the supply of available ni- 

 trogen present in the soil. On average 

 soils, with a generous supply of nodules 



developing on their root systems, it is 

 estimated that about one-fifth the total 

 nitrogen content of the crop comes from 

 the soil and the other four-fifths from 

 the atmosphere. 

 If the tops, or foliage growth, of the 



