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phorus contained in the soil is not in the form of acid phosphate, 

 but largely, at least, in the form of pulverized or disintegrated 

 rock; and yet it is the common experience that this phosphorus 

 can be made available by large use of clover and other green ma- 

 nures. It is an interesting and absolute fact, too, that phosphatic 

 marls, containing phosphorus in the ordinary insoluble mineral 

 form, have been much used for centuries for direct application 

 to the land. It is recorded by writers that, when the Romans 

 first invaded Britain, " the natives were found using phosphatic 

 marls to obtain better crops." 



The United States Bureau of Soils states that millions of tons of 

 the greensand marl of New Jersey have been used " as a natural 

 fertilizer"; and, according to the Bureau's analysis of a specimen 

 " collected as a sample to show the amount of plant food in ma- 

 terial actually used as a fertilizer," this marl contains less than 

 i per cent (18 pounds per ton) of acid-soluble potassium, and but 

 little more calcium and magnesium than could be combined in 

 the phosphates present. Evidently, the fertilizing value of the 

 marl is due very largely to its phosphorus content, which amounts 

 to 28.6 pounds per ton. In comparison it may be noted that one 

 ton of the most common corn-belt soil contains about 1.2 pounds 

 of phosphorus, 8 pounds of acid-soluble potassium, and 35 pounds 

 of total potassium; and that 200 pounds (the average application) 

 of the most common " complete " commercial fertilizer contain 

 about 8jjr pounds of total phosphorus and 3^ pounds of potassium. 



An analysis by the Bureau of Soils of the greensand marl of 

 Prince George County, Maryland, shows about ^ pound of phos- 

 phorus and 42.6 pounds of acid-soluble potassium, in one ton. 

 The following statements are quoted from the Report of the Bureau 

 of Soils for 1901, pages 186-187: 



"It is probable that the New Jersey greensand marls would, on the average, 

 have a phosphorus content fifty times as great as the corresponding marls from 

 Maryland. " 



"In the Prince George area this greensand marl, which occurs along the 

 numerous stream cuttings and natural cliffs, has only been used to a slight 

 extent as a source of fertilizer. ... In other areas, both in the United States 

 and foreign countries, the greensand marl has long been utilized as an inex- 

 pensive though effective medium for restoring impoverished soils. " 



