650 



Journal of Agriculture, Victoria. [10 Nov., 1917 



whilst the percentage of magnesia remains practically constant through- 

 out. 



It must not he inferred that a migration of plant food to the soil has 

 occurred. There are two reasons for the decrease in the percentages. In 

 the first place the young and tender plant is always richer in the essen- 

 tials than the older or coarser plant. This is due to concentration. On 

 the other hand there is a migration of plant food to another part of 

 the plant, viz., the tubers. All the coiupounds necessary for tuber for- 

 mation are derived from the laboratory of the plant, viz., the haulm. 



A glance at Table VII., giving tlie amounts of plant foods contained 

 in the haulms of an acre of potatoes (as exj)erimentally grown) throws 

 additional light on the question. The amounts of the bases (potash, 

 lime, and magnesia) increase right through "to the end of the third 

 month, and in the case of lime even to the end of the fourth month. 

 The latter fact may be due to the absorption of lime possibly to 

 neutralize the haulm acidity. On the other hand, it will be observed 

 that the amount of nitrogen and phosphoric acid contained in the haulm 

 increases at a great rate until the end of the second month, and then, 

 especially in the case of the latter cdnipound (phosphoric acid), the 

 content shows a remarkable decrease. Apparently these two essentials 

 play an important part in tuber fonnation and development. 



The fact remains, and it furnishes food for thought. Once again 

 nature tells us to help the helpless — as the mother nourishes the suckling 

 babe, so, too, should the potato farmer nourish the crop in the early 

 stages of growth, so that in the presence of a sufficiency of plant food pro- 

 vided by good tilth and liberal manuring, strong, healthy plants con- 

 taining a rich flow of elaborated sap will greet his eye and, incidentally, 

 fill his pocket. 



In a normal season, the critical stage of the potato plant is during 

 the first six or eight iveels' growth. 



A word on the plant food ratio of the haulm. The outstanding 

 feature is the two constants. Firstly, the practically constant ratio of 

 the first two months; and, secondly, the practically constant and dis- 

 tinctly different ratio of tbe final two months. 



Apparently, as previously mentioned, the bases (lime and magnesia) 

 are absorbed to neutralize acidity, even after the migration of the phos- 

 phoric acid, nitrogen, and, to a lesser extent, of potash from the haulm 

 to the tuber. 



There seems to exist a definite ratio between lime and magnesia. 

 This can hardly be by accident, for the soil used was much richer in lime 

 than magnesia. 



Table VIII. 



Percentages on absolute dry matter. 



