PASTURE CULTURE 31 



however, that liberal applications of phosphate fertilizers need not be continued, 

 for many of our most desirable forage crops, particularly the legumes, are raven- 

 ous consumers of phosphorus as well as of calcium, magnesium, and potassium 

 and they must have adequate supplies of all of these elements. 



Nitrogen. — Organic matter is the natural repository of soil nitrogen. The 

 original reserve of nitrogen in Massachusetts soils was contained in the accumu- 

 lated layer of organic matter or vegetable mold and as long as appreciable quanti- 

 ties of this original supply of organic matter persisted in the soil, there was no 

 great need for additional nitrogen. As soon as the supply was exhausted, how- 

 ever, another source of nitrogen was necessary. For many years this need in 

 pastures was supplied principally by the legume, white clover. Where legumes 

 constitute a substantial part of the vegetation today, they can still be relied upon 

 to supply a considerable portion of the necessary nitrogen. Where grasses pre- 

 dominate, on the other hand, the use of nitrogen fertilizers is obligatory, since 

 grasses to be productive must have large quantities of this element. 



Minor Elements. — The lack of some of the minor elements is indicated by the 

 more frequent occurrence in recent years of a number of different "deficiency 

 diseases." To prevent this occurrence, it now appears that, in addition to mag- 

 nesium, boron and perhaps other elements must be supplied to many Massachu- 

 setts soils. 



Organic Matter. — Soil organic matter, the material which played such an 

 important soil fertility role in the past in Massachusetts, is at the present time 

 a very important factor in soil fertility relationships. The principal difference 

 between present and former times is that today the nature of organic matter and 

 its behavior are better understood and the desirability of periodically replenishing 

 the organic matter supply of cultivated soils is more widely appreciated. 



Soil organic matter is one of the most active constituents in the soil. If present 

 in adequate quantities in soils well supplied with plant food it serves to hold 

 certain of the more soluble nutrients in such a way as to make them readily 

 available to growing plants but yet to prevent excessive losses through leaching. 

 This function is also carried on by colloidal clay, an inorganic material, but since 

 the clay content of most Massachusetts soils is quite low, the action of organic 

 colloids in this respect is of much greater importance. 



A great many of the favorable effects of organic matter result not from its mere 

 presence in the soil but from its being continually broken down into its simpler 

 constituents. Under favorable circumstances complete breakdown of this material 

 releases quantities of plant food nutrients which can be readily taken up by grow- 

 ing plants. In this way not only nitrogen is released but also calcium, potassium, 

 magnesium, and many other essential elements. 



The relationship of decomposing organic matter to the development and 

 maintenance of good soil structure is important, but this subject will receive fuller 

 treatment in the discussion on tillage. 



In spite of the many "positive" effects of soil organic matter, Albrecht.i^** in 

 a recent enlightening discussion on this subject, points out that organic matter 

 under certain conditions may be injurious to plant growth. Such can be the 

 case if plant nutrients are physically held through absorption so securely as to be 

 unavailable to growing plants. Injurious effects can also result if, during the 

 process of complete breakdown, the microorganisms which bring about this 

 transformation are not supplied with a "well-balanced bacterial ration." The 

 microorganisms may withdraw nutrients from the soil solution and from the 



ll'Soil Science, LI (1941), 487-494. 



