158 MASS. EXPERIMENT STATION BULLETIN 162. 



and Permanent Agriculture,"' published in 1910, Dr. Hopkins says: 

 ". . . no provision was made for maintaining organic matter in the soil." 

 In view of the facts that a heavy crop of winter rye was plowed in in 

 1901, and after three years in hay (1905-07), a heavy growth of grass 

 before late cabbage in 1908, it is beheved the supply of organic matter had 

 been well maintained. Certainly in our experience we have not only 

 fully maintained, but actually increased, productiveness on soils of similar 

 character by use of fertihzers only, under systems of management less 

 favorable to the maintenance of the humus content. It is not beheved 

 there could have been a shortage of organic matter in the soil of these 

 plots at the time when Dr. Hopkins wrote. Wishing, however, to create 

 conditions as favorable as possible to the action of the raw phosphates, 

 two heavy green manure crops have since been grown and plowed in, — 

 buckwheat in 1912 and winter rye in 1913. It has been shown that in 

 spite of this treatment not only is the increase in crops from the raw phos- 

 phates still less than from the others, but it seems to be falhng still further 

 behind. 



INDIRECT OR SECONDARY EFFECTS. 



The no-phosphate plots in this series of experiments have given crops 

 which, as shown by calculation on the basis of average composition, have 

 carried away nearly as much phosphoric acid as has been carried away 

 in the crops of the phosphate plots. The totals of this element for these 

 plots exceed the totals for the plots receiving no phosphoric acid, as 

 follows : — 



Phosphoric Acid in the Total Increases in Crops. 



Per Plot (One-eighth Acre) 



(Pounds). 



Natural fine ground rock phosphates, . . . . . . . 1 . 06 



Slag and bone meals, . . . . . . . . . .8.43 



Dissolved phosphates, . . . . . . . . . .8.63 



If the crops on the phosphate plots have drawn upon the natural soil 

 supply of phosphoric acid as largely as those on the no-phosphate plots, 

 then the proportion of the phosphoric acid apphed in these experiments 

 which has been removed by the crops is extremely small, — indeed quite 

 insignificant. As phosphoric acid is not subject to much if any loss from 

 soils by leaching, it would seem that nearly all of this element which has 

 been apphed must still remain in the soil, even in those plots to which it 

 has been applied in the more soluble forms. 



In spite of this fact the use of the slag, bone meals and dissolved phos- 

 phates has given increases in crops which much more than cover the cost 

 of the phosphates used as shown by the table on page 156. In view of 

 this fact it appears probable that the benefits following their use must 

 have been due in considerable measure, to indirect or secondary effects 

 rather than to the direct plant-food action of the phosphoric acid they 

 contained. One of these indirect effects — the stimulation to rapid 



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