274 MATERIALS OF INDIRECT VALUE [chap. 



manured with basic slag and potash salts, until a soil 

 had been built up. 



Curiously enough, on the sandy soil at Woburn, 

 Voelcker has always obtained better crops after mustard 

 than after vetches, despite the fact that the vetches had 

 contributed a greater weight both of dry matter and 

 nitrogen to the land. The vetch compounds may decay 

 the more slowly, but Voelcker further showed that the 

 land was left drier by the vetch crop ; that this was 

 the cause of the superiority of the mustard as a 

 green manure is rendered more probable by the fact 

 that the result was reversed on the strong Rothamsted 

 soil, where the vetches are the better preparation for a 

 succeeding wheat crop. The real difficulty experi- 

 enced in utilising green manuring and catch crops 

 generally on many soils in this country, to which they 

 are otherwise most admirably suited, is the way they 

 deplete the water-suppl)- for the succeeding crop. For 

 example, a crop of vetches or crimson clover may be 

 sown on the stubble in August or September and 

 harvested in May, in j>lcnty of time to prepare the land 

 for turnips, but in many cases the soil and subsoil will 

 be left so dry that the turnip crop will fail or be greatly 

 reduced, unless the incidence of rain be unusually 

 favourable. The difficulty of starting the catch crop 

 after the drying effect of the harvested corn, and the 

 dryness of the land which again ensues after the 

 catch crop in spring, form the great objection to catch- 

 cropping, which indeed only flourishes where the annual 

 rainfall is well over 30 inches. In this respect mustard 

 is the least objectionable crop, since it will grow in 

 six or eight weeks under good conditions in autumn, 

 and can then be turned in, leaving the ground broken 

 to catch the late winter rainfall. On the light soils 

 it is more general to fold sheep on the catch crops 



