CROP NUTRITION 273 



4. In connection with tea pests, mention needs to be made of 

 the now well-known experiment by Andrews (1923), who showed 

 that a high ratio of available potash to available phosphoric acid 

 in the soil, in which tea bushes are growing, greatly reduces attack 

 by Helopeltis theivora. The latter is a Capsid bug, and a persistent 

 enemy of tea, against which spraying appears to be an 

 impracticable proposition. Andrews found that where a constant 

 supply of soluble potash is directly applied to the roots of the 

 bushes, this substance is absorbed and the bushes are freed from 

 the pest, remaining free from attack the rest of the season. This 

 would lead to the obvious conclusion that the addition of a potassic 

 fertiliser to the soil was needed. Success, however, cannot be 

 regularly obtained by such treatment, for the reason that the 

 plants are apparently unable always to utilise the added potash. 

 The effect of the potash in manurial experiments is transient, and 

 some factor, operating to cause the original shortage in available 

 potash obtained by the plant, intervenes to prevent its continued 

 absorption. Andrews' inquiry showed that the composition of the 

 leaf of the tea plant shows differences corresponding with those 

 deduced from observation of the soils, and that comparative 

 immunity from attack accompanies an increase in the proportion 

 of potash, as compared with phosphoric acid, in the leaf. The 

 factors controlling this ratio, however, are not understood, since 

 the chemical composition of the leaf may be quite different when 

 grown in the same soil under different cultural conditions, with a 

 resulting difference in liability to attack by Helopeltis. It would 

 appear, therefore, that before practical application can be made of 

 this discovery, an inquiry into the behaviour of the substances 

 mentioned in the soil under different conditions is called for. 



5. In South Australia the springtail Sminthurus viridis has, 

 during the last forty years, become one of the most important pests 

 of field crops in that country, particularly to lucerne and clovers. 

 The damage is caused by the insect gnawing the epidermis and 

 underlying mesophyll of the leaves of the young plants, giving a 

 field a scorched appearance as the result of a bad attack. 



This insect has recently been the subject of a comprehensive 

 ecological study by Davidson (1934), while earlier contributions 



