142 ACTION OF SOIL ON FOOD OF PLANTS IN MANURE. 



But to thrive well, plants do not require a soil saturated 

 with nutritive substances ; for, the absorption numbers 

 we have quoted sufficiently show how far the arable 

 soils are from a state of complete saturation. All that 

 plants need for their proper nutrition is that theu^ roots, 

 downwards in the soil, should come in contact with a 

 certain quantity of saturated earth ; and the mechanical 

 operations of tillage have the important object of con- 

 veying earthy particles satm^ated with nutritive substance 

 and of mixing them with others, which by preceding culti- 

 vation have become poorer in those constituents. 



The average crop from a hectare of wheat (2000 kilo- 

 grammes=4400 lbs. of grain, and 5000 kilogrammes = 

 11,000 lbs. of straw) contains 52 million milhgrammes 

 (=114-4 lbs.) of potash, 26 milhon milhgrammes ( = 57-2 

 lbs.) of phosphoric acid, and 54 million milligrammes 

 (=118-8 lbs.) of nitrogen. Assuming the nitrogen to be 

 supplied by the soil, the wheat plants growing on a square 

 metre (=10*75 square feet) receive the ten- thousandth part 

 of the potash, phosphoric acid, and nitrogen, or altogether 

 13,200 milligrammes (=203-3 grains). Supposing 100 

 plants to grow upon a square metre, each of these takes 

 up from the soil 132 milhgrammes of these constituents, 

 or 54 milligrammes of nitrogen=65 milhgrammes or 1 

 grain of ammonia, 52 milhgrammes (;=0-8 grain) of pot- 

 ash, and 26 milhgrammes (=0'4 grain) of phosphoric acid. 



Each cubic centimetre (=-06 cubic inch) of Bogen- 

 hausen loam absorbs to saturation 2*6 milhgrammes 

 ( = •04 grain) of ammonia, 2*3 milligrammes (=0-35 grain) 

 of potash, and 0-5 milhgrammes (=-008 grain) of phos- 

 phoric acid ; therefore, to restore a sufficiency of these 

 constituents which the wheat plant has taken from the 

 soil, would require a supply of 25 cubic centimetres of 

 the saturated earth, and 25 milligrammes of phosphate of 



