NATURAL INDIGO 6i 



It, however, makes a considerable demand upon the available 

 phosphates, and continuous growing of indigo on the same land 

 has in many cases resulted in such a depletion of the soil in 

 this constituent that the crop no longer flourishes. This has 

 been emphasised by Davis in several of the Indigo Publications 

 of the Pusa Research Institute. Abundant phosphate in the 

 soil promotes root growth, and, as has been pointed out by 

 Howard and Howard in publications of the Pusa Botanical 

 Series the wilt which has caused so much damage, especially 

 to khoonties, is accompanied by failure to develop new rootlets 

 after their destruction as a result of the cutting back of the 

 plant. 



The wilt disease was first reported in the Sumatrana plant 

 in Behar in 1907, and the khoonties were entirely lost. Accord- 

 ing to Davis it was first observed in plants grown in soils 

 which were abnormally deficient in available phosphate (below 

 0-0005 per cent,), and did not show itself at all in soils rela- 

 tively better supplied with phosphate (above o-ooi per cent.). 

 The Java plant was, however, equally affected. Now, whereas 

 the Sumatrana indigo is shallow-rooting, the Java is deep- 

 rooting and derives its nourishment mainly from below the 

 top six inches of soil. Davis has found that, in cases in which 

 the khoonties failed or in which August sowings for seed were 

 poor, the subsoil was even more deficient in available phos- 

 phate than was the surface layer. Thus both on the score of 

 better aeration, which favours nodule activity as emphasised 

 by Howard and Howard, and on that of phosphate supply, 

 surface rooting forms have advantages under these special 

 soil conditions. On the other hand, when first introduced 

 into Behar the Java plant grew luxuriantly, and both in its 

 yield of plant per unit area and of indigo per unit weight of 

 plant it far surpassed the Sumatrana. Davis considers that 

 this was due to the fact that it drew on the deeper layers of 

 the soil which had not been exhausted by cereals or the shal- 

 lower rooting Sumatrana indigo. Quite apart from this, 

 however, in equally advantageous soil conditions the Java 

 plant is taller and more vigorous than the Sumatrana, and 

 has a higher leaf percentage. 



As far back as 1900 Rawson had drawn attention to the 

 deficienc}'- of available phosphates in some of the indigo soils 

 of Behar. In 1907 Leather reported that they were almost 

 uniformly deficient in it. Neglect to remedy this has resulted 

 in still further depletion of phosphates, and Davis has urged 

 the necessity for manuring with superphosphate. 



Though phosphate deficiency is doubtless the limiting 

 factor in the growth of indigo in many places, it does not follow 

 that it is the only one that is ever operative. Thus, under 



