HOWARD AND HOWARD 33 
Soil aeration. 
From the time the ordinary crop is sown in September or early October 
to the following May, indigo shows a remarkable response to soil aeration. 
After germination and while the seedlings are small, anything in the nature 
of a surface crust is fatal and constant harrowing is essential. During the 
cold weather and the hot months of March, April and May, indigo shows a 
remarkable response to repeated harrowings with the lever harrow. ‘These 
implements were originally introduced into India by the Botanical Section 
of the Pusa Institute and are now widely used on the indigo estates. They 
enable the surface soil to be broken up to a depth of nearly two inches and the 
mulch produced preserves the soil moisture during the hot weather. In 
addition, aeration is improved and the large supply of air needed by the intense 
nodular development which takes place at the break of the rains, is provided 
for. During the early monsoon, soil aeration is maintained by the rain- 
fall which is a saturated solution of oxygen and experiments show that culti- 
vation at this period is unnecessary and does more harm than good. In the 
later rains, the marked aerotropism of the roots (caused by the rise of the gases 
of the deep soil layers which follows the upward movement of the ground 
water) combined with the destruction of the absorbing root system of the 
subsoil, places cultivation out of the question. This should, therefore, stop at 
the break of the rains in May. 
In addition to improved aeration by means of surface cultivation, the 
indigo plant shows a marked response to a more open soil texture and also to 
the aeration of the subsoil. Asan example of the effect of altered soil texture 
the results obtained on growth by the addition of sand or potsherds to the soil 
are of interest. (Table II.) 
Taswe II. 
The effect of improved soil aeration on the growth of indigo. 
Kind of soil No. of plants Average length | Percentage 
measured in cm. increase 
Soil only a as aA 33 36°7 ae 
50%, soil 50% sand is os 36 516 40 
90% soil 10% potsherds ae 33 48:3 31 
70% soil 30% potsherds aye 35 50°9 38 
The substitution of only ten per cent of the volume of soil by inert potsherds 
sufficed to increase growth by over 30 per cent. 
Equally striking are the results obtained by waterlogging the subsoil 
before the indigo crop is sown. The effect of waterlogging the heavier soils 
at Pusa during September, has been found to result in extensive losses of 
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