786 REPORT—1904, 
decline, and usually terminates quite suddenly. The causes of this cessation have 
been thoroughly worked out, and are found in the great increase of parasitic in- 
sects, and insecticidal fungi, including bacteria. I believe it will be found that the 
almost sudden cessation of our periodic visitations of the diamond-back moth is 
due to a similar cause. 
The failure of turnips is apparently largely, if not entirely, due to the increase 
of insects and parasitic fungi. 
The subject of harmful excretions has recently obtained renewed attention 
through the work being done at the Woburn Fruit Station. No point has 
received more striking demonstration there than the harmful influence that 
growing grass exerts on fruit-trees. It has been shown that this prejudicial 
influence is not due to the withdrawal of moisture, to the curtailment of supplies 
of plant food, to interference with aeration, or to modifications of temperature. 
In Mr. Pickering’s opinion, ' ‘the exclusion of all these possible explanations drives 
us to believe that the cause of the action of grass is due to some directly 
poisonous action which it exerts on the trees, possibly through the intervention of 
bacteria, or possibly taking place more directly.’ It is satisfactory to know that 
the subject, which is of considerable scientific and practical importance, is likely 
to be vigorously followed up. 
In the early forties attention was being directed to a subject that even now 
has a great attraction for agriculturists, namely, the stimulating and exhausting 
effect of artificial manures, especially nitrate of soda. The principle that 
‘stimuli lose their full effect upon living matter when frequently repeated’ was 
generally held to account for the want of response that crops exhibited to 
repeated dressings of nitrate of soda; but Professor Daubeny in 1841? pointed out 
what is now generally accepted as the true cause, namely, the exhaustion of the 
soil of other substances. This, he said, can be counteracted by giving other 
manures, of which he instanced bone meal. His suggestions for future investiga- 
tions have been largely followed, though, as we now know, they are of theoretical 
rather than practical importance. He proposed the alternatives: 
1. Analysis of the soil, discovery of the amount of available plant food, and 
the application of the substances found to be deficient up to the probable measure 
of the crop’s requirements. 
2. Discovery, by analysis of the yield, or estimation by calculation, of the 
amount of plant food removed in the produce, and the application to the soil in 
the form of manure of what was withdrawn by the crop. 
Daubeny suggested that manuring should be undertaken on a system of book- 
keeping—on the one side being entered all the items of plant food taken out by 
crops, and on the other all that is applied in the form of manures, the two sides 
of the account being made to balance. This theory of manuring is distinctly 
suggestive, and often fits in rather remarkably with actual practice, though the 
comparative agreement between theory and practice is due to causes that the 
author of the theory probably hardly contemplated. Take, for instance, the case 
of wheat. An average crop removes from an acre about 50 lbs, nitrogen, 
30 Ibs. potash, and 20 lbs. phosphoric acid. This loss would be restored by the 
use of some 8 cwt. nitrate of soda, 2 cwt. kainit, and 13 cwt. superphosphate ; 
and on many soils wheat could, no doubt, be grown continuously for many years 
on such a mixture, aided by good tillage, without the yield suffering materially. 
But we now know that much of the plant food offered in manure never enters the 
crop at all, so that the balancing of the account is due almost as much to chance 
as to calculation. This becomes more apparent when we regard such a crop as 
meadow hay, which in actual practice is often grown for a long series of years on 
the same land. To balance the withdrawal of phosphoric acid by an average 
yield of this crop only about { cwt. of superphosphate per acre is theoretically 
necessary, but on most soils an average yield would not be maintained by the use 
of so small a quantity. 
' The Effects of Grass on Apple Trees. Journal R.A.S.E. Vol. Ixiy. p. 365, 
* On Manures considered as Stimuli to Vegetation, 
