302 SOILS AND MANURES 



soil by this means. The physical properties of very light 

 and very heavy soils may be greatly improved by green 

 manuring, but under ordinary circumstances, unless the 

 land is conspicuously deficient in humus, it is generally 

 more profitable either to cut the crop or to feed it off. 

 In the latter case most of the plant foods are returned 

 to the land, but as the organic matter is consumed Jby 

 the animals, the land is not greatly enriched in humus. 

 If the crop is carried off the land, the effect is to im- 

 poverish the soil as previously explained. This, of 

 course, is the reverse of manuring. 



Seaweed. The nitrogenous matter and other plant 

 food that is constantly being poured into the sea in 

 sewage and in other ways is not altogether lost. It is 

 taken up by marine plants, and considerable quantities 

 of it may be recovered in this form and used as manure. 

 The manurial value of seaweed is no modern discovery. 

 It is, and has long been, used in large quantities on 

 farms near the coast, but being very bulky, it cannot be 

 profitably carried any distance inland. In the fresh 

 state it contains a large proportion of water, and the 

 cost of transportation can be greatly reduced by dry- 

 ing it, when it is possible to do so, before carting. 



It appears from numerous analyses which have been 

 published that the composition varies considerably. The 

 general averages are as follows : 



Per cent, 



Water . 70 80 



Organic matter 

 Ash 



Nitrogen 

 Phosphoric acid 

 Potash . 



10 20 

 5 10 

 0-4 0-8 

 0-1 0-2 

 1-0 2-0 



Compared with farmyard manure the fresh substance 

 generally contains about the same quantity of nitrogen, 

 and is rather richer in potash, .but poorer in phosphates. 

 The tissues are soft and cellular ; it decomposes rapidly 



