THE MANURING OF THE CANE 105 



Where the sugar cane forms the main crop in India, the following typical 

 rotations, amongst others, are given by Mukerji : 35 



Bengal. High and light soils. Rice (May to September) ; pulse or oil 

 seed (October to March) ; jute (April to September) ; pulse or oil seed 

 (October to March) ; rice (May to September) ; potatoes (October to 

 February) ; sugar cane (February to February) ; rice (May to September) ; 

 pulse (October to March), &c. 



Punjab. Dhaincha (Sesbania aculeata )or sunn hemp (Crotalaria juncea), 

 or cow peas (Vigna unguiculata) cut in bloom in August ; potatoes (October 

 to February) ; sugar cane (February to February) ; pigeon pea (Cajanus 

 indicus) or rice ; potatoes ; sugar cane. 



Whenever practised the absence of a rotation is a weak point in sugar 

 cane culture ; the rich fertile soils which are often met with in the tropics 

 lor a number of years support a continuous unvaried crop, but eventually 

 they tend to become barren. Jn certain countries, as Demerara, where 

 abundance of virgin soil awaits cultivation, proprietors can continually 

 empolder new lands and allow that which has become barren to lie fallow, 

 and after a space of time, during which by the continued disintegration of 

 the soil plant food has become available, again plant the old abandoned land. 



The effect of continuously growing cane on the same soil has not been, 

 so far. as the writer is aware, distinctly studied, but the following quotation 

 from A. D. Hall 36 with reference to the Rothamsted wheat experiments 

 seems broadly applicable also to cane culture : 



" Plot 10 has received an annual dressing of nitrogen only, in the shape of 

 -ammonium salts since the earliest dates of the experiments. It will be evident from 

 the curve showing the crop production that, despite this long-continued use of a 

 manure supplying but one element of plant nutrition, the crop has been wonderfully 

 maintained. Whereas the average production over the whole period is increased by 

 the supply of minerals to the extent of i 8 bushels, the nitrogen alone has produced 

 an average increase of 7- 6 bushels, the unmanured plot being taken as the standard 

 in each case. The curve, however, shows that the production on this Plot 10 is 

 declining, notwithstanding the great reserves of mineral plant food with which the 

 soil started . At the present time also the crop on this plot presents a very unhealthy 

 appearance, is very slow to mature, and is extremely liable to rust. 



We thus see that it is possible to grow a cereal crop like wheat, year after 

 year, on the same land for at least sixty years without any decline in the produc- 

 tiveness of the soil, provided an appropriate manure be supplied to replace the 

 nitrogen, phosphoric acid and potash removed by the crops. There is no evidence, 

 in fact, that the wheat gives a smaller yield when following a long succession of 

 previous wheat crops than when grown in rotation, although the vigour of the plant 

 does not appear to be so great. The real difficulty in continuous corn growing is to 

 keep the land clean ; certain weeds are favoured by the wheat and tend to accumu- 

 late, so that the land can only be maintained clean by an excessive expenditure in 

 repeated land hoeing. Notwithstanding all the labour that is put on the plots, the 

 ' Black Bent ' grass, Alopecurus agrestis, has from time to time become so trouble- 

 some that special measures have had to be taken to eradicate it and to restore the 

 plots to a reasonable degree of cleanliness." 



It does not seem then altogether unreasonable to attribute in part the 

 damage done by fungus and insect pests to the continual growth of cane on 

 the same soil, as in this way the pests have a continuous habitat. 



. In discussing rotations it may not be out of place to refer to the toxic 

 -excretion theory ; it was originally suggested by De Candolle that plants 

 excreted a toxic substance which prevented the continual growth on the same 

 soil, and in this way explained the benefits of rotation. After definite 

 abandonment this idea has been revived, mainly by Whitney and Cameron, 

 but its discussion lies altogether without the limits of the present textbook. 



