IMPROVING THE SOIL. 45 



compounds of phosphoric acid or phosphates, such as we find 

 in bones; compounds of potash, such as we find in wood 

 ashes; compounds of lime, of iron, of magnesia, etc., etc. 

 Now the point to be noted here is that the plant must 

 have every one of these compounds, and growth will not take 

 pla'ce if even only one be lacking. Nearly every soil contains 

 lime ; it is a very common substance in rocks and soils, there- 

 fore we do not need to supply that food. Magnesia and iron 

 are quite common, and much of either is not required. When 

 we feed the soil, or fertilize it, we have mainly to consider this 

 whether the soil needs nitrogen, phosphates, or potash. 

 These are the three main constituents of fertilizers, and they 

 largely fix the values of those that are applied. 



When we apply barnyard manure to a soil, we add a bulky 

 fertilizer that, in addition to increasing the plant food, has an 

 important effect upon the texture of the soil. For instance, 

 light, sandy soils are apt to be poor in plant food, and to 

 be too loose or porous the rain runs through them. You 

 notice that as the barnyard manure becomes older in the pile 

 it becomes darker through changes that we call fermentation. 

 This dark color is due to the changing of the straw or litter 

 into humus ; and when this is applied to the light soil the tex- 

 ture of the soil is improved, the sandy soil becomes more 

 loamy. When applied to clay soil its sticky quality is more or 

 less overcome, and the heavy clay changes towards a loose 

 loam. One of the main benefits of applying barnyard manure 

 to a soil, then, lies in its effect upon the texture or physical 

 quality of the soil. This same effect is produced by green 

 manuring, that is the plowing under of a green growing crop 

 such as clover, tares, rye, or buckwheat 



The barnyard manure contains compounds of nitrogen, of 

 potash, and also phosphates, so that in it we apply the different 

 kinds of food that plants must get out of the soil. Barnyard 

 manure is called a general or complete fertili/er. 



