AYEKAGE PRODUCE PER ACRE. 123 



powers of exhalation must be kept in action ; it is impatient of 

 excessive or long-continued heat without moisture and free 

 currents of air. .Its active foliage has also the power of absorb- 

 ing moisture when the air supplies it in excess, as it does in 

 summer nights, in the shape of dew. That dew is richer in the 

 compounds of nitrogen referred to than ordinary rain. The 

 turnip is, moreover, the plant of a comparatively cold and moist 

 climate. It cannot stand a southern sun ; its rapid growth 

 durin^ a brief summer must ever render it the sheet-anchor of 

 the northern farmer. Such are its powers of rapid development 

 that, within a space of four months, a crop of turnips will gather 

 up from an acre of land and (with the aid of carbon from the 

 atmosphere) form into a mass of vegetable cells and tissue fit for 

 animal nourishment, twice as much nitrogen, and — leaving the 

 silica of straw out of consideration — more than twice as much 

 valuable mineral matter as a crop of any of our cereal plants can 

 collect and assimilate from the same extent of land. And this 

 brings us back to our subject from the digression into which we 

 have been slipping. 



Potatoes, hay, and pease approach the turnip crop, and the un- 

 certain and variable bean crop occasionally equals it in respect 

 of nitrogen ; but none of these, not even the potato crop, 

 approaches it in the amount of mineral matter gathered up from 

 an acre of land. Something of the same considerations which we 

 have here indicated with the respect to the turnip crop could be 

 shown to apply quoad the means of obtaining nitrogen, to the 

 clover and pulse crops. But let what has been said suffice here 

 to indicate the importance of a point which deserves more in- 

 vestigation than it has yet received. Certain it is that, though 

 the turnip crop requires a great amount of nitrogen, it can in a 

 large measure dispense with that element in the manure applied 

 to the soil. 



But, with respect to the purely mineral constituents of plants, 

 it is manifest that the case is different. No such considerations 

 can apply to any of them, — except, indeed, sulphur can be held 

 to be, in some slight degree, an exception. They must neces- 

 sarily exist in the soil ; or, if they do not exist there in sufticient 

 abundance, or in an available form, they must needs be supplied 

 in the manure. And, consequently, the composition of the ash 

 of a plant does afford an indication of the sort of manure that 

 the plant requires, but with this inq)ortant qualification, that 

 the soil comes in as a factor to be considered in the case. If a 

 plant or crop requires a large amount of potash, as turnips, and 

 in a somewhat less degree, potatoes, require, then that potash 

 must exist in the soil, or be au]t])lied in the manure. Not to 

 speak of the imperfect indications which analyses of soils 

 supply, we know from experience that many soils do contain 



