142 



Necessary and available Mineral Matter. 



The first seven of these substances so extracted from the soil are essential for 

 healthy plant life, and the others are not indispensable. 8ome of the seven 

 essential substances are in the soil in abundance, but nitrogen, phosphoric acid, 

 and potash, and in some soils lime, are present only in exceedingly small propor- 

 tion ; and accordingly the farmer or gardener who practises frequent cropping 

 has to supply these substances to the soil directly and indirectly to avoid a 

 deficiency of any one of them. A fairly fertile soil may have in it in the first 

 nine inches in depth only about 15 lbs. of nitrogen, 15 Its. of potash, and 10 lbs. 

 of phosphoric acid per 10,000 lbs. of soil ; and of these amounts only a very small 

 proportion may be immediately available as plant food. In an acre of soil to 

 that depth the nitrogen and potash if they could be isolated would weigh about 

 two tons each, and the phosphoric acid about one and a half tons. 



When a Soil is Exhausted. 



But a soil shows signs of exhaustion long, long before these siibstances are 

 drawn on in an appreciable proportion to their weight, for only the soluble frac- 

 tion is available. Ultimate exhaustion of a soil is therefore altogether impossible. 

 When a soil has been temporarily exhausted — and all exhaustion, I will say 

 again, is only temporary — the farmer may if he can afford to wait let his land lie 

 in pasture until the natural agencies always at work have rendered more of the 

 imexhausted reserve available, or he may hasten the action of such agencies by 

 fallowing or repeatedly working the land without cropping. If he cannot wait, 

 or finds it unprofitable to do so, he has to 



Resort to Manuring, 



either directly by supplying and distributing over the land manures, natural or 

 artificial, or indirectly by growing leguminous and other crops, to be ploughed 

 in, or by purchasing artificial foodstiiffs, with the view of carrying more stock, 

 increasing accordingly the size of his manure heap, or enriching the land right 

 away by their droppings. 



A Law which Altereth Not. 



In the practice of manuring we have to bear in mind that it is one or more 

 of these substances nitrogen, potash, or phosphoric acid, and occasionally lime, 

 which may become deficient, and that a deficiency of any one of them cannot be 

 made up by an excess of another. When anyone of these substances is not 

 available, the land becomes barren, and farmers have, therefore, to endeavour to 

 maintain the natural balance. Further, the crop is stunted or unhealthy accor- 

 ding as one or more of these indispensable substances becomes scarce, or avail- 

 able in too small proportion. 



Farmyard Manure. 



Now a general manure such as farmyard manure supplies more or less, 

 according to its quality, of each of these comparatively scarce but all-important 

 substances. 



A ton of it contains : — 



8 to 16 lbs. of nitrogen, 

 8 to 17 lbs. of potash, and 

 4 to 9 R)s. of phosphoric acid, 



and, considered with its other recommendations, if it can be readily obtained in 

 quantity, and at moderate cost, there is little need of further dressing with, 

 artificial manures, except perhaps a little of the phosphates, in which, as the 

 figures above show, it is relatively weak. 



Artificial Manures. 



However, it ia with artificial manures that we now propose to deal. 



Phosjyhoric Acid. — Scarcity of phosphoric acid can be relieved by using bone 

 dust, bone meal, bone charcoal, mineral superphosphate, dissolved bones, basic 

 slag, or phosphatic guanos, &c. 



Nitrogen. — Deficiency of nitrogen can be made up by artificial dressings to 

 the land of nitrate of soda, sulphate of ammonia, blood manure, nitrogenous 

 guanos, &c. 



Potash. — Weakness in potash can be avoided by the use of wood ashes, 

 muriate of potash, kainit, &c. 



