78 THE BOOK OF NATURE STUDY 



some use, for being a leguminous plant it enriches the soil with 

 nitrogen, and may in this way help to supply the want of manure. 

 In appearance it is not unlike the small yellow Clover, but 

 may be distinguished from it by the fruit, which is black and 

 spirally twisted. Another difficulty in associating certain weeds 

 with particular crops is caused by the impurity of seed. Weeds 

 are constantly introduced with the seed sown. Thus Charlock, so 

 common in Potato, Corn, and other tields, has a seed very like that 

 of the turnip or rape, and might be easily introduced with either 

 crop ; the Dodder is so often mixed with Clover that in England 

 an ounce of Dodder in a ton of clover is considered permissible. 

 In most continental countries there are seed-testing stations under 

 Government control, in order to test the germinating power of the 

 seed, and to ensure its purity and genuineness. This is of great 

 practical use to the farmer, for if he knows that the seed he is 

 sowing has only a germinating value of 75 per cent, instead of 95, 

 he can increase the quantity sown in a given area and thus save 

 loss. At the seed station in connection with the Royal College 

 of Science, Dublin, some fifteen hundred samples a year are tested, 

 and the quality of the flax seed sown in Ireland has distinctly 

 improved since the establishment of the station. 



ROTATION OF CROPS. The rotation of crops in farming is due 

 to the fact that different plants make a different demand on the 

 soil. There are certain substances that all plants require for food ; 

 some are obtained from the air, others from the soil, but the 

 quantity required by each plant varies. Clover needs a great deal 

 of potash, wheat comparatively little ; on the other hand, wheat 

 needs silica. As long as the " open-field " system lasted it was 

 impossible to have a rotation of crops, in which roots, such as 

 Turnips, should form an element. Up to the end of the eighteenth 

 century each village farm had attached to it three great tillage 

 fields, and the usual practice was to sow one with Wheat or Rye ; 

 the second with Barley, Oats, Beans or Peas, whilst the third lay 

 fallow. Thus, even then, there was a rotation of crops, but a 

 limited one. Arthur Young, writing in 1768, remarks that Clover 

 and Turnips were unheard of in many parts of the country, and 

 even as late as 1811 they were still almost unknown in Wiltshire. 



