THE WEEDS OF CULTIVATION 79 



Now roots form a regular element in the rotation of crops ; the 

 order, however, varies in different parts ; in Norfolk, Turnips are 

 followed by Barley, Barley by Clover, and Clover by Wheat. 



In order to ascertain the substances that a plant requires for 

 food, its ash maybe analysed. Then the soil in which it is proposed 

 to grow it should also be analysed, and any deficiency of the 

 substances necessary to the plant should be made good by appro- 

 priate manure. The amount of humus in soil may be ascertained 

 by first drying and weighing the soil, then burning it, and weighing 

 again. By means of sieves, with meshes of a different size, the 

 amount of silt-clay, gravel, and sand can be roughly estimated. 

 Silt-clay passes through a mesh of 0-05 mm., leaving the sand and 

 gravel in the sieve. This residue is then sifted in a sieve with a 

 mesh of i mm., the sand passing through whilst the gravel remains. 

 A definite weight of dried soil must be taken to begin with, and at 

 each stage the residue must be dried and weighed. In this way 

 the percentage of sand, clay, etc. can be determined. The terms 

 " marl " and " loam " are constantly used in any rough classifi- 

 cation of soils. A marl is a mixture of lime and clay ; a loam, of 

 clay and sand. 



All good soils contain a certain amount of clay, which is richer 

 in plant food than any other part of the soil, and it also has the 

 power of fixing certain substances required by the plant. Clay 

 also retains water, and needs good drainage and admixture with 

 sand, or lime, or ashes, etc. to increase its porosity. 



CERTAIN WEEDS CHARACTERISTIC OF CERTAIN SOILS. The 

 majority of plants seem able to grow on most soils, provided they 

 can get the water they require. This seems to show that it is the 

 physical, rather than the chemical, properties of the soil which are 

 of the greatest importance to the plant. The one exception to this 

 rule is lime, which many plants appear either to love or hate ; it is 

 for this reason that it is preeminently possible to speak of a chalk 

 flora. On light calcareous soils it is found that Sainfoin and 

 Lucerne are valuable crops for purposes of fodder. Amongst the 

 weeds, which are commonly found on soils with a good proportion 

 of calcium carbonate, may be mentioned Fumitory (Fumaria 

 officinalis) and the Dove's-foot Geranium (G. molle). 



