144 ACTION OF SOIL ON FOOD OF PLANTS IN MANURE. 



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 their roots, down- 

 wards in the soil, should come in contact with a certain 

 quantity of saturated earth ; and the mechanical opera- 

 tions of tillage have the important object of conveying 

 earthy particles saturated 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 Ibs. of grain, and 5000 kilogrammes 

 = 11,000 Ibs. of straw) contains 52 million milli- 

 grammes = 114*4 Ibs.) of potash, 26 million milli- 

 grammes (= 57'2 Ibs.) of phosphoric acid, and 54 mil- 

 lion milligrammes (= 118-8 Ibs.) of nitrogen. As- 

 suming 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 pot- 

 ash, 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 Io2 milligrammes of these constituents, or 

 54 milligrammes of nitrogen 65 milligrammes or 1 

 grain of ammonia, 52 milligrammes (= 0-8 grain) of 

 potash, and 26 milligrammes (= 0'4 grain) of phos- 

 phoric acid. 



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

 hausen loam absorbs to saturation 2'6 milligrammes 

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

 grain) of potash, and 0-5 milligrammes (= '008 grain) 

 of phosphoric 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 cen- 

 timetres of the saturated earth, and 25 milligrammes 

 of phosphate of lime for each square decimetre of the 

 field. Calculated upon a square decimetre ( = 15|- 

 square inches) of surface and a depth of 20 centimetres 

 ( =7'8 inches), these 25 cubic centimetres constitute the 

 eightieth part of the entire mass of earth. 



The experiments of ISfageli and Zoeller, before de^ 



