THE AORICUL TUBAL JOURNAL. 



Agricultural Chemistry for Beginners. 



CHAPTER VIII. 

 By Archibald Pbarcb. 



IF some fresh wood-aslies are shaken 

 lip with a little water, we shall ap- 

 parently notice that they do not dissolve 

 at all; but let us apply a few tests to try 

 and pa-ove the truth of otherwise of our 

 observation. First, strain off the water 

 through cotton wool or, better still, white 

 blotting-paper; we shall obtain a liquid 

 nearly as clear as water. But let us 

 taste it; it has a kind of soapy flavour; 

 test it with red litmus paper, the colour 

 changes to blue; drop a few drops on to 

 hot iron, a whitish deposit remains. We 

 must therefore conclude that wood ashes 

 contain a white alkaline substance which 

 can be dissolved out by water, leaving 

 the larger portion of the ash as an in- 

 soluble residue. This substance is one 

 which for many years was manufactured 

 in just this way; in countries where wood 

 is cheap and plentiful, like Ihissia and 

 Canada, large quantities of timber were 

 collected and burned, the ashes soaked 

 in water and strained off, and the liquid 

 boiled down in large iron pots till only 

 the solid substance remained, and to this 

 the name pot-ash was given. If this 

 potash is treated with an acid, it effer- 

 vesces and gives off carbon dioxide; it 

 is therefore a carbonate, and we now 

 know that it is the carbonate of the rare 

 metal potassium. If this potash is 

 boiled with lime, its alkaline properties 

 are very mucK'fncreased, in reality it has 

 become a new potassium compound called 

 caustic potash, which is made in large 

 quantities for the manufacture of soap 

 and other purposes. Its chemical name 

 is potassium hydrate. Now, unfortu- 

 nately, the term potash is used very 

 loosely in ordinary language, no less than 

 three different substances being spoken 

 of by this same name, which is confusing 

 and misleading. First, there is the pot- 

 ash which is obtained by washing wood 

 ashes, and which is really the carbonate, 

 as explained; secondly, the hydrate, made 

 by boiling the carbonate witli lime, is 



often spoken of simply as potash; while 

 the compound which properly and cor- 

 rectly has the name of potash given to 

 it is the basic oxide of potassium, from 

 wbich all the potassium or potash salts 

 are formed. We must remember then 

 that when we speak chemically of potash, 

 we are always referring to this oxide, be- 

 cause we compare the value of all potash 

 salts by calculating how much of this 

 oxide each of them contains. 



If the ashes of any plant, or any por- 

 tion of a plant, are examined, we are 

 always able to detect the presence of po- 

 tassium in the form of its carbonate, 

 and hence it follows that all plants must 

 have potash salts in some form or other 

 for their growth. But all plants do not 

 require the same amount, nor are all 

 parts of the same plant equally rich in 

 potash. We find that the asEes of the 

 trunk of a tree contain very much less 

 than the same weight of ashes derived 

 from the twigs or leaves. As a g'eneral 

 rule the harder and older the parts of a 

 plant are the less potash they contain. 

 Again, some kinds of plants can get on 

 with comparatively little, while others re- 

 quire a large aniount. Of plants of the 

 latter kind, the potato is pre-eminent, 

 and this crop may be said never to fail 

 to give an increased return for manuring 

 with potash. Tobacco also has almost 

 always a potash dressing applied to it. 

 Root crops and leguminous crops, too, 

 like a good supply, while cereals only 

 need an a])i)lication when the soil is very 

 poor in this form of plant-food. On 

 such soils fruit trees are gralcful for an 

 additional ([uantity; it is said to have a 

 tendency to produce a growth of fruit, 

 while nitrogenous manures help to make 

 the trees run to wood. Tlie question 

 then arises as to what soils need potash 

 manuring most. Of course a definite an- 

 swer with respect to any special soil can- 

 not be given without an analysis, but it 

 is possible to some extent to judge from 



