CHEMISTRY IN RELATION TO HORTICULTURE. 



109 



Plant Food. 



We come next to the consideration of those ingredients which are 

 -supplied to the plant through the medium of the soil. These are, mainly, 

 the mineral ingredients. They form, as we have seen, a relatively small 

 proportion of the bulk of plants, and yet their importance is of the highest, 

 both to agriculturist and horticulturist. We have spoken of the atmo- 

 sphere as supplying the carbonic acid out of which the plant's structure, is 

 mainly built, but this supply is one that never fails, whereas the soil may 

 be very variable in the amount of mineral food it contains. Without the 

 supply of each essential mineral ingredient, and this in sufficiency accord- 

 ing to the requirements of the individual plant, there can be no proper 

 growth, and so it comes about that soils frequently require supplementing 

 in respect of one or another of their mineral constituents. These, together 

 with nearly all the nitrogen and water required by plants, are obtained by 

 the roots, which take them up in a soluble state, and also (by virtue, it is 

 generally believed, of the acid sap contained in the rootlets) attack the 

 insoluble forms of plant-food, rendering them in turn soluble and capable 

 of transmission to the various parts of the structure. 



Of the mineral elements usually found in soils six may be considered 

 as essential to plant-life, viz. potassium, magnesium, calcium, iron, 

 phosphorus, and sulphur ; others such as sodium, silicon, manganese, and 

 chlorine, are generally found, together with aluminium, and occasionally 

 with other ingredients in small quantity. That plants take these up severally 

 by a selective power which they possess has been made abundantly clear, 

 as, for example, in the taking up of potash salts in preference to soda 

 salts when the two are present. Upon this, indeed, is based the whole 

 system of manuring for the particular needs of particular crops. The 

 analysis of the non-combustible or " ash " constituents of plants does not 

 necessarily give the clue to the proper manuring to adopt for it, though 

 it may afford useful information. But each plant has to be studied for 

 itself and in relation to the power which it possesses of taking hold of and 

 utilising the food constituents presented to it. Thus it has been shown 

 that the wheat plant, though containing in its structure relatively large 

 amounts of potash and phosphoric acid with comparatively little nitrogen, 

 is really dependent mainly on the supply of nitrogen, and not on that 

 of potash and phosphoric acid. So, too, the turnip, though it has very 

 little phosphoric acid in it, has but small power of abstracting this in- 

 gredient from the soil, but depends largely upon its artificial supply in a 

 soluble form. The same is the case with garden crops, the development 

 of starch, e.g., in the potato, or of sugar in the beet, being largely 

 dependent upon the mineral and nitrogenous ingredients supplied in an 

 available form. Nitrogen, moreover, is essential for the assimilation of 

 carbon to go on. 



As to what actually takes place within the plant in regard to the 

 action of each mineral ingredient, but little is known. In the corn crops 

 met with in agriculture- we know that there is a gradual transference from 

 the green parts of the plant, the stalk and leaves, to the flower and seed, 

 and that these are, as the crop ripens, stored up mainly in the seed or 

 grain, the stalk or straw then withering. But in horticulture — with but 



