TOXIC ACTION OF ONE CROP ON ANOTHER 303 



grass goes on growing in the fields for ever : primroses, fox- 

 gloves, wood anemones, and a host of other wild flowers, seed 

 themselves and come up year after year in the same woods and 

 dells. Braconnet was among the first to give voice to this feel- 

 ing of dissatisfaction, but the careful experimental inquiry came 

 in the 'thirties and early 'forties, when Daubeny, Sibthorpian 

 Professor of Rural Economy at the University of Oxford, con- 

 ducted a classical series of experiments in the Physic Garden to 

 test the hypothesis. 



A series of thirty- two plots was laid out : sixteen had certain 

 different crops grown on them continuously, whilst on the other 

 sixteen the same crops were grown, but in rotation, so that no 

 plot bore the same crop twice. With two possible exceptions, 

 no differences were apparent between the continuous and rota- 

 tional cropping other than might be attributed to exhaustion 

 of the soil. Thus no evidence in favour of the excretion postulated 

 by De Candolle was obtained. 



More definite evidence against such an hypothesis, however, 

 was obtained by Lawes and Gilbert in later years. 1 Five out 

 of the six crops tested barley, wheat, oats, swedes and mangold 

 grew well enough continuously, but clover failed after a very 

 short period. This, however, did not prove either the exhaustion 

 of the soil or the contamination by poisonous excretions, for on 

 examination the dying plants were found to be infested by two 

 pests, an eel worm and a fungus. 



At a later date 2 the subject was again taken up at Rothamsted, 

 and in a series of pot experiments wherein any disturbing element 

 introduced through the exhaustion of the soil was eliminated 

 by adding whatever nutrient the previous crop had removed, 

 it was proved that there no lasting toxic effect was produced 

 on the soil by either rye, buckwheat or spinach, and that, if 

 anything is excreted by the roots, the amount of 'it which 

 accumulates during six years is insufficient to cause any 

 appreciable depression in the next crop. 



The Woburn experiments emphasise and extend this con- 

 clusion, for they show that such toxic matter as may be formed 

 in the soil as a result df plant-growth has only a very temporary 

 existence, and does not accumulate in the soil at all. It is 

 only while the surface crop is actually growing that trees are 



1 Gilbert, Results of Experiments at Rothamsted on the Growth of 

 Leguminous Crops for many years in succession on the same land. 1889. 

 Rothamsted Mem., VI, 15, pp. 1-60. 



2 Russell, Soil Conditions and Plant Growth, 1917. pp. 150-151. 



