THE ROTATION OF CROPS. 65 



Fertilisers. — 1.1 received eight tons stable manure per acre 

 once in five years applied to the maize crop. 



4.11 timothy top-dressed, after mowing first hay crop. 



6. IV. Same as 1.1. 



The legumes, plants which gather atmospheric nitrogen 

 through the nodular bacteria attached to their roots, increase, 

 or at least maintain , the nitrogen content of the soil , and usually 

 because of their extensive root systems give an increase in 

 humus content even when the aerial parts are harvested, and 

 this has a very beneficial effect on the tilth of the soil. 



Certain crops again are regarded as " gross-feeders " and 

 others as " delicate feeders." The former are crops whose 

 extensive much-branched rooting systems make them more 

 effective in reaching through the soil for plant food and soil 

 moisture. Some gross-feedmg crops seem to possess a greater 

 power of assimilating plant food which to delicate feeding 

 crops would be less easily assimilated; thus, maize, rye and 

 oats will yield profitable crops on soil often too poor for wheat 

 and barley, which are generally classed as " delicate-feeders." 

 The latter should therefore be placed in the most favoured posi- 

 tions as regards available plant food, in rotations. The prin- 

 cipal gross-feeding crops in South Africa are maize, rye, some 

 varieties of oats, sorghums and the coarser millets ; while among 

 those having comparatively fine, slender and restricted root 

 systems may be mentioned wheat, barley, some varieties of 

 oats and potatoes. 



Again crops differ radically in being either shallow or deep- 

 rooted, and in general correspond, but not always, to the deli- 

 cate and gross-feeding crops just mentioned. Barley, teff' and 

 flax are crops whose roots are more or less confined to the furrow 

 slice, while lucerne, cowpeas and soybeans have strong laterals 

 and taproots penetrating to considerable depths ; lucerne roots 

 have been found sixty feet below the surface. Many shallow- 

 rooted crops, when grown continuously, soon lower the produc- 

 tivity of the upper layer of the soil, and for this reason are 

 looked upon by farmers as exhaustive crops. 



The effect of growing deep-rooted crops is to improve the 

 physical condition of the soil ; the sub-soil is opened up, and the 

 soil rendered more porous and receptive of moisture ; the feed- 

 ing area for succeeding crops is markedly improved. Certain 

 heavy clay soils in England on which only pasture grasses could 

 formerly be grown have now been reclaimed and can success- 

 fully grow ordinary crops. This has been done by persisting in 

 the growth of the deeper rooted crops. 



