34 



THE AGRICULTURAL NEWS 



Febhuakv 4, 1911. 



it is due to the poisonous action of one ur more of its 

 constituents. Experiments to solve this problem were 

 undertaken, as well as to determine the ma.ximum 

 amounts of manures for citrus trees of different sizes, 

 in known quantities of soil. 



Before the work that was done is de.scribed, it is 

 pointed out that the term maximum quantity of manure 

 means the largest amount that can be applied to a plant, 

 and at the same time allow it to live and thrive. This 

 varies naturally under different conditions, notably 

 those of availability of the manure and the state of 

 the soil. Other maxima exist, in regard to any 

 given manure, depending upon the other manures 

 with which it may be applied: the usual effect of 

 these is to enable the plant to thrive when given 

 larger amounts of the original manure — higher maxima 

 are obtained. There is a third maximum, in regard to 

 mixed manures; this is the largest amount of the 

 mixture that can be used without killing the plant. 

 It is a maximum that may depend on two conditions: 

 the maximum of some one of the ingredients, or on 

 the total concentration of the manure: that is to say 

 this concentration may reach such a degree as to cause 

 the cells of the roots to colla])se and become useless, 

 even before the (juantity of any one of the ingredients 

 has reached a proportion in which it is poi.sonous. 



The account of the investigations gives a descrip- 

 tion of two exjjeriments. There were four lots, each of 

 three budded orange plants, in the first trial, three of 

 which were treated with certain amounts of sulphate of 

 potash, superphosphate and nitrate of soda, respectively; 

 while the fourth lot did not receive any manure. The 

 amounts of the manures mentioned, when added 

 together, equalled the (juantity that had caused the 

 plants to die, in a former experiment, when they were 

 applied as a mixed manure. Observation of the plants 

 from day to day, showed that those which had been 

 treated with nitrate of soda first exhibited signs of 

 dying; while later, crimping of the leaves took ])lace 

 where the other manures had been apjjlied, and they 

 were somewhat dwarfed, but normal in colour. The 

 plants that had received no manure grew well. 'I'he 

 result of the experiment was to indicate that the harm 

 in the mixed manure had been the quantity of nitrate 

 of soda that it iiontained. 



The second cxpirimeiit was really an extension of 

 the first. Four lots, each of three budded orange ])Iants, 

 were taken, as before, three of which were traated 



with the manures mentioned above, in pairs, so that no 

 two pairs wire alike, while no maniwe was given to the 

 fiiurth. Thus one lot received nitrate of soda and super- 

 phosphate, the Second sulphate of potash and nitrate of 

 soda, and the third sul})hate of potash and superphos- 

 phate, the quantities being the same a.s tho.se in the 

 former expeiiment The first plants to exhibit falling 

 leaves were those which had received sulphate of potash 

 and nitrate of soda: later, all these plants died back to 

 the bud. Injury was next shown by those manured with 

 nitrate of soda and sui)erphosphate, though not to the 

 same extent. Some injury was shown in the third case, 

 where no nitrate of soda was used: the leaves were 

 crimped, one plant was partly defoliated and no new 

 growth was being produced by it; the other two, how- 

 evei', were growing new tissue. The unmanured plants 

 made yood ijrowth. 



The Conclusions to be derived from this experi- 

 ment must betaken in connexion with the results given 

 by the first. The most obiious matter is that the great- 

 est amount of injury was caused where nitrate of soda 

 was present. The least harm was done by the mixture 

 of suljihate of potash and superphosphate: though even 

 here, there were indications that the concentration of 

 the manure had nearly reached the maximum. When 

 the results of using suj)erphosphate and nitrate of soda 

 are compared with those where the latter manure was 

 employed alone, it is seen that the harmful effect of 

 nitrate of soda is decreased to some extent by the 

 presence of superphosphate. Finally, referring again to 

 the trial with superp.hosj)hate and potash, there were 

 unmistakable indications that the amount of the mix- 

 ture ajjplied was greater than that which could be 

 endured easily by the plants. 



It is evident that the results of these experiments 

 have relation solely to the application of manures in 

 large amounts. In the tropics, more attention is given to 

 the provision of humus and the maintenance of the soil in 

 a good state of tilth, than to high manuring with artificial 

 manures. 



This does not detract from the suggestive inter- 

 est of work of the kind just described. Little is 

 known concerning the effect of manures on the sap and 

 tissues of plants, or on the organisms, beneficial and 

 otherwise, in the soil, .ind much investigation will be 

 required before practical information can be given con- 

 cerning these. This is particularly the case in relation 

 to the connexion between the manurial treatment of 

 plants and the incidence of disease. 



