CRITICAL PERIODS IN PLANT LIFE 539 



stimulant or in a system of permanent soil improvement, may some- 

 times be the means of bringing this critical period at the time when 

 the weather conditions are most unfavorable, while the untreated 

 land may mature a larger crop at the more favorable time. 



An instance has been reported of a field treated with half a ton 

 per acre of raw phosphate having produced a crop of 45 bushels of 

 oats free of rust, whereas only 20 bushels of badly rusted oats were 

 produced from similar seed on adjoining untreated land. Two 

 influences may help to produce this difference: the added phos- 

 phorus tends to balance the food ration and thus to strengthen the 

 oats against the fungous disease (and against lodging, as well), 

 and also to hasten the maturity by which the crop escapes the rust 

 which might attack the plants maturing later and perhaps under 

 weather conditions more favorable for the development of the 

 disease. As was stated by the author to the farmer who reported 

 this experience, the marked difference in yield is not to be credited 

 even largely to the phosphorus because of the plant food for its 

 own sake, but rather to a combination of influences to which the 

 added phosphorus proved to be the key. 



While such examples may serve temporarily as good advertise- 

 ments for the treatment applied, they are just as misleading for 

 wide application as are the occasional reports of damage to crops 

 produced by applying manure. All of this serves to emphasize 

 the importance of having some fundamental knowledge upon which 

 to base definite systems of permanent agriculture. For this pur- 

 pose we must rely primarily upon the absolute facts furnished 

 by chemistry and mathematics and be guided only by the results 

 of carefully conducted and long-continued experiments. Single 

 examples can be found in support of almost any practice or theory 

 that can be advanced; but a mere experience, though it be repeated, 

 invariably with the same result, for fourscore times, furnishes no 

 proof whatever that the octogenarian will live to celebrate another 

 birthday. 



A small amount of readily available plant food, such as 50 pounds 

 of sodium nitrate per acre as a top dressing for wheat on poor land 

 in a cold spring, may produce a sufficient increase in yield to more 

 than pay the cost of the nitrate. Likewise, 100 pounds of " am- 

 moniated bone and potash," carrying perhaps 2 pounds of nitro- 



