THE NUTRITION OF THE ROOT 69 



over the fields a considerable time before sowing, as soon, 

 indeed, as the soil has become slightly dry. Small quantities 

 (5 to 10 cwt. per acre) added after every five years are more 

 beneficial than a single heavy addition of lime, as the decom- 

 position of humus becomes so active that very little is left for 

 subsequent harvests. Of course the above-mentioned quantity 

 must only be looked upon as an approximate value ; the actual 

 quantity will depend greatly on the nature of the soil. Great 

 care is needed in treating light sandy soil with lime, while on 

 a tenacious clay soil lai'ge quantities may be used. The most 

 conspicuous and rapid results of this treatment will be shown 

 by a soil rich in humus but poor in lime upon which the 

 Sheep Sorrel {Bumex Acetosclla) grows abundantly. In such 

 soil the lime will act directly as food material. 



As in the process of marling the lime contained in the 

 marl is also the active principle, it is evident that a soil rich 

 in humus and in clay will stand it much better than a light 

 sandy soil. To the latter a larger amount of clay marl than 

 of calcareous marl or shell marl may be added. The latter 

 two forms of mai'l are more advantageous for clay soils. The 

 much-dreaded over-marling will only occur when too small 

 an amount of stable manure is added ; we must remember 

 that this substance is the basis of the permanent fertility of 

 the soil. 



Besides the increased decomposition of organic substances 

 by the lime, which results in an enormous increase in the 

 production of carbonic acid, the lime fixes many free acids 

 which are injurious to vegetable life, transforms the ferrous 

 into ferric oxide, and causes a greater absorption of basic food 

 substances. The bases are contained in the soil in the form 

 of hydrated silicates and humus salts. The bases, if they are 

 to combine with the humus compounds, must be united to 

 carbonic acid. Now the lime promotes the formation of such 

 carbonates. Besides these indirect effects of marling, there 

 are some direct actions which must be mentioned. A direct 

 enriching of the soil is effected by the potassium, phosphoric 

 acid, and magnesia which every kind of marl contains. 



The addition of gypsum, which was already practised by 

 the Greeks and Romans, is effective in making the potassium 



