TO June. 1910.] Limiting Factors in JigricuUure. 355 



quantity of phosphoric acid than it would otherwise need. Now silica 

 or sand is the commonest constituent of all soils. Traces of it are brought 

 .slowly into solution bv the decompositions which are continually in progress 

 in a " live " soil and by the presence of water containing ca.rbon dioxide 

 in solution. Hence, in a well aerated soil, rich in humus, these solvent 

 actions would be especiallv prominent and a less quantitv of phosphoric 

 acid should be necessary than on a dead, inert, or l)adly aerated soil. 



It is hardly necessary to say that difierent crops have different ash 

 requirements. Grain crops have especially high ash requirements, par- 

 ticularly as regards phosphoric acid. Hence, the use of mineral manures 

 is especially important for all crops where the part removed from the soil 

 and from the farm is the seed. In the case of leafy or .root crops, the 

 ash requirements are not so great relatively to the weight of the crop and 

 the element which is apt to i^e reduced bv such crops to an unduly low ebb, 

 is potassium. Naturally, however, a statement such as this is a general 

 one and will not apply to everv soil and everv crop. 



In anv case, sufficient has been said to show that the chemistry of the 

 soil, so far as these ash constituents are concerned which the plant uses 

 as food, is less important than the physics of the soil. Chemical deficien- 

 cies in the soil mav be made good naturallv by substitution, and bv the 

 power of selection the plant possesses, or thev may be made good artificially, 

 bv the selection of suitable crops or crop rotations, and bv the use of 

 chemical manures. Nothing can, however, make good a deficiency of 

 water, a low temperature or a deficient aeration in the soil, and these three 

 factors are all capable of considerable amelioration by suitable working 

 of the soil. 



One of the most important factors in the development of crops is entirely 

 b.eyond our control, that is, the percentage of carbon dioxide present in 

 the air. As this forms the source of the whole of the carbon in the plant, 

 and as carbon is the most important element and forms the greater part 

 of the drv weight of all ordinary crops, this is a fact to be regretted. 

 It has been found that small increases in the percentage of carbon dioxide 

 present in the air, increase the amount of food assimilated, and hence 

 also the amount of growth in a given time. The limit is soon reached, 

 however, at ordinary temperatures. In the same way^ increases in the 

 intensity of the light make the plant feed and grow more and more rapidly. 

 The limit is reached in the case of plants growing in the open when the 

 light has about the intensity of ordinarv bright diffused daylight. Beyond 

 this, no further increase in the rate of feeding is shown. If, however, the 

 temperature is raised and the amount of carbon dioxide increased at the 

 same time, the amount of food assimilated goes on increasing as the light 

 inc.rea.ses in intensity, up to, or even bevond that of strong, direct sunlight. 

 Evidently, the temperature and the amount of carbon dioxide are limiting 

 factors to the effects produced bv increases of light. An analogy with this 

 is afforded bv a locomotive which is stoked up with the steam only partly 

 turned on. No amount of stoking can increase the speed beyond a certain 

 limit, but as soon as the steam-cock is turned fully on, the train may 

 attain its maximum speed. Naturally, if there is no water in the boiler 

 or not enough, stoking is useless just as it is in the plant. 



The following simple experiments with maize illustrate these facts and 

 show that it is no use attending to a minor factor (mineral constituents 

 in soil) in plant nutrition until the more important major factors, (water, 



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