AND AGRICULTURE. 37 



least one- twentieth, or five per cent., of organic 

 matter. 



If land is ploughed and planted year after year, 

 and but little manured, the organic matter is ex- 

 hausted, and the crops are poor ; but when the 

 soil is put to permanent pasture, or sufficiently 

 enriched with farm-yard or other appropriate 

 manure, as peat, compost, or guano, it is not im- 

 poverished, because the crops do not exhaust it 

 of its organic matter. 



The quantity of organic matter plants take up 

 from the soil differs greatly, depending on the 

 kind of land, the nature of the plant, and the 

 season ; but they always take up a good deal, and 

 it is essential for the health and perfection of the 

 plant. 



The requisite supply of organic matter in the 

 soil may be insured by ploughing in green crops, 

 by sowing clover, and the grasses, which leave 

 abundant roots in the soil, and by restoring the 



Questions How much organic matter should land con- 

 tain, to produce good crops? What impoverishes land? 

 How can it be kept from being exhausted by crops ? Do all 

 plants take the same quantity of organic matter from the 

 soil ? Why do they thus differ ? Why is organic matter 

 essential for the plant ? How may the requisite supply of 

 organic matter be retained in the land ? 



