134 MASS. EXPERIMENT STATION BULLETIN 162. 



The value of an application of lime in some form to manj^ of our soils 

 is fully recognized. It is frequently an essential for successful crop pro- 

 duction. The importance of organic matter is admitted. It is useful 

 not alone in promoting the availability of such elements as potassium and 

 phosphoric acid, and as a source of nitrogen to the growing crop, but also 

 for the maintenance of satisfactory soil texture. Without a fair propor- 

 tion of such matter in the soil good tilth is impossible, and on the lighter 

 soils, especially, extreme injury to crops in periods of drouth is a cer- 

 tainty. These phases of the general subject of fertility will not be dis- 

 cussed in this paper. It is proposed simply to study the question of the 

 applicability of the Hopkins theory in relation to the use of phosphorus ^ 

 to Massachusetts conditions. 



The quotation above cited makes it apparent that Hopkins regards 

 the application of fertilizer nitrogen or potash under normal soil conditions 

 as superfluous, and that he believes that a suitable application of phos- 

 phorus (in addition to lime and organic matter) is all that will be found 

 needful. 



The conclusions of Dr. Hopkins are further emphasized and the reasons 

 therefor more clearly brought out by the following quotation from the 

 book above referred to : ^ — 



Phosphorus is the only element that must be purchased and returned to the 

 most common soils of the United States. Phosphorus is the key to permanent agri- 

 culture on these lands. To maintain or increase the amount of phosphorus in the 

 soil makes possible the growth of clover (or other legumes) and the consequent 

 addition of nitrogen from the inexhaustible supply in the air; and, with the addi- 

 tion of decaying organic matter in the residues of clover and other crops and in 

 manure made in large part from clover hay and pasture and from the larger crops 

 of corn and other grains which clover helps to produce, comes the possibility of 

 liberating from the immense supplies in the soil sufficient potassium, ^ magnesium, 

 and other essential abundant elements, supplemented by the amounts returned 

 in manure and crop residues, for the production of large crops at least for thou- 

 sands of years; whereas, if the supply of phosphorus in the soil is steadily decreased 

 in the future, in accordance with the past and present most common farm practice, 

 then poverty is the only future for the people who till the common agricultural 

 lands of the United States. 



And this does not refer to the far-distant future only, for the turning point is 

 already past on most farms in our older states and on many farms in the corn 

 belt; and lands that have passed their prime with sixty years of cultivation will 

 decrease rapidly in productive power and value during another sixty j^ears of 

 similar exhaustive farm practice. 



1 Phosphorus is the element of value as plant food supplied by the compound phosphoric acid, 

 which is the most valuable constituent of acid phosphate, dissolved bone black, basic slag meal , 

 fine-ground rock phosphates and raw and steamed bones. In the publications of this experiment 

 station and in agricultural literature in general the name "phosphoric acid" is usually used. 

 Figures indicating the amounts of phosphoric acid can be converted into approximate equiva- 

 lents in phosphorus by multiplying by .44; and figures for phosphorus into substantial equiva- 

 lents in phosphoric acid by multiplying by 2.3. 



2 Hopkins' "Soil Fertility and Permanent Agriculture," page 183. 



' Potassium is the name of the element of plant-food value in the compound potash or potassium 

 oxid, under which names in our station publications and in agricultural literature in general this 

 plant food is usually referred to. Figures for potash (or potassium oxid) can be converted into 

 approximate equivalents in potassium by multiplying by .83 ; figures for potassium can be con- 

 verted into potash (or potassium oxid) by multiplying by 1.2. 



