264 POPULAR SCIENCE MONTHLY 



ents, but lies rather in the production of unfavorable soil conditions 

 by the excretion of substances from the roots of preceding crops of 

 plants. 



The effect of root excretions is strikingly shown by some experi- 

 ments in which wheat was grown in pure quartz sand. The sand con- 

 tained a very insignificant amount of plant nutrients, but the young 

 wheat seedlings were able to draw sufficient nourishment from the seed 

 to maintain growth during the time of the experiment. Pots of fresh 

 quartz sand were planted with wheat simultaneously with an equal 

 number of pots containing quartz sand which had previously grown 

 wheat for twenty-one days. The growth of the wheat in the " ex- 

 hausted " sand was only about 45 per cent, of that in the fresh sand, 

 in spite of the fact that both sets of plants were supplied with nutri- 

 ents from the seeds. It would be obviously incorrect to ascribe the 

 " exhaustion " of the sand to a depletion of plant nutrients at the out- 

 set. The harmful effects following a previous crop appear to be more 

 probably due to the presence of deleterious substances arising in the 

 sand during the growth of that crop. 



Evidence from another experiment showed that the roots leave sub- 

 stances in the soil which may be removed by proper treatment. Three 

 crops of wheat seedlings were grown in pots and showed decreasing 

 yields.* When the last crop was removed, an aqueous extract of the 

 soil was prepared and used as a nutrient solution in which a fourth set 

 of wheat plants was grown. A portion of the soil extract was shaken 

 with carbon black, which acts as a strong absorbing agent, and filtered 

 free from carbon black at the end of a half hour. When a soil extract 

 possessing deleterious properties is given this treatment with an absorb- 

 ing agent, it is almost invariably improved, on account of the removal 

 of the harmful substances. In this " exhausted " soil the same results 

 of the carbon black treatment were noticed as in the case of deleterious 

 soil extracts. The extract of the " exhausted " soil produced as good 

 plants, after treatment with carbon black, as the extract of fresh soil 

 treated in the same way. 



The antagonistic action of one plant upon another was shown by 

 the harmful effect of trees upon wheat. Small trees were transplanted 

 into paraffine wire pots and kept growing in a greenhouse where opti- 

 mum conditions of light and moisture could be obtained. Wheat was 

 also planted in each pot and allowed to grow about three weeks. As 

 soon as the first set of wheat plants had been cut and weighed, a second 

 set was planted and this was repeated at intervals of about three weeks, 

 until nine successive crops had been grown, always growing control 

 crops in pots without trees. The growth of the wheat in the pots con- 

 taining trees was poorer than the controls during the summer while 

 the trees were actively growing, "but when autumn came and the trees 



