PLANT NUTRIENTS AND THEIR ABSORPTION 115 



a sod including legumes is maintained. There are conditions under 

 which it is difficult or impracticable to grow legumes in the orchard; 

 nevertheless their special value should not be overlooked, particularly 

 where there is need of increasing the available nitrate supply. Their 

 judicious use in place of some of the other cover or mulching crops or 

 in the place of some other system of orchard management often obviates 

 the necessity of supplying the trees with nitrogen through mineral or 

 animal fertilizers. 



An instance of the results that can be obtained by the use of leguminous plants 

 as cover crops is described by Coville.'*^ " The trees in one newly planted orchard 

 of Grimes apples have been kept in a remarkable condition of growth by one 

 initial application of manure in the year of their planting, succeeded by the 

 following rotation: In May the ground is sowed to cowpeas. These are plowed 

 under in September and followed immediately by the sowing of rye mixed with 

 hairy vetch. In the following May the mixed crop is plowed under. The 

 same 1-year rotation has been followed year after year. Under this treatment 

 the soil, which has the appearance of almost pure sand, has become so fertile 

 without the application of lime, commercial fertilizer or manure that an occa- 

 sional crop of cowpeas has been cut for hay without serious interference with the 

 progress of the orchard." The successful use of such a system would depend 

 upon an abundant water supply. 



Were it possible to maintain permanently a good stand of clover, 

 vetch, alfalfa or some other leguminous crop in the orchard and to leave 

 the growth that it produced on the ground for a mulch, it would afford 

 an almost ideal sod system of management — from the standpoint of 

 maintaining Soil fertility — though water competition between the trees 

 and the intercrop would make it entirely impracticable under many 

 circumstances. Under average conditions, however, the maintainance 

 of such a sod is next to impossible because bluegrass or other species 

 crowd out the legumes. Where such a legume sod can be maintained 

 and the competition for moistm-e can be largely eliminated by irrigation, 

 a system of soil management is possible that affords the trees excellent 

 nutritive conditions for vigorous growth and heavy production and is 

 at the same time economical. Various fungi found in the roots of certain 

 heaths (Ericaceae), are likewise capable of fixing nitrogen. It is probable 

 that the cranberry and blueberry obtain at least a portion of their nitro- 

 gen supply through similar agencies. 



Soil Reaction : Acidity and Alkalinity. — The absorption of available 

 inorganic salts by the root is affected to an important degree by acidity, 

 concentration, toxicity, aeration and temperature of the soil and of the 

 soil solution. The reaction of the soil solution is of great importance. 

 Most plants thrive best when the soil is very weakly acid. Many water 

 plants live better in a very weakly alkaline solution, while land plants 

 show marked differences in the amount of acidity which they will endure. 



