CH. vi] Manuring of Sandp Soils 115 



it seems probable that more use might with advantage 

 be made of the system and that the organic manure 

 added to the soil should not be limited by the number 

 of animals the farmer may find it convenient to keep. 



Whatever the system of agriculture, it is desirable to 

 crop as frequently as possible because sandy soils lose 

 a great deal of their fertilising constituents if left bare 

 and exposed to the rain. The cultivations must also be 

 thorough to keep down weeds: no soils are so prone to 

 be smothered with weeds as are sands. 



The manuring has to be decided by the crop : reference 

 has already been made to the paramount importance of 

 organic matter and of lime : potash is wanted for many 

 cropS; especially potatoes., while phosphates are usually 

 needed to prevent rankness in the grain crops taken 

 after green crops have been fed off and also to secure the 

 maximum feeding value of the green crops themselves. 



Soluble manures must only be added in small quan- 

 tities at a time because of the ease with which they are 

 washed out. Sandy soils have less capacity than clays 

 for absorbing soluble substances: this can be demon- 

 strated by the following experiment. Dissolve 0-3 gram 

 of superphosphate, dilute to 500 c.c. and divide into two 

 lots of 250 c.c. To one add 50 grams of a light sandy- 

 soil, and to the other 50 grams of a heavy clay soil, 

 shake both solutions well for 3 minutes, allow to settle 

 -for 3 minutes, shake, and settle again. After 15 minutes 

 filter. To 50 c.c. of each filtrate add 10 c.c. of ammonium 

 molybdate solution (p. 233). Much more precipitate is 

 obtained from the sand than from the clay. 



A similar experiment with a weak solution of am- 

 monium sulphate (2-5 grams per litre) in place of super- 

 phosphate shows that clay also absorbs ammonia more 



82 



