50 AROUND THE YEAR IN THE GARDEN 



acid and potash existing in the soil, and many of those 

 added to it in manures or fertilizers, have to undergo cer- 

 tain changes, which take place in the soil, before plants 

 can use them. Certain degrees of heat, of moisture and of 

 air make these changes. 



All plants require for their sustenance a number of 

 different elements in the soil. Of these the only ones that 

 are likely to become deficient are the three already men- 

 tioned nitrogen, phosphoric acid and potash. The worst 

 of it is that if any one of these begins to get used up the 

 plant will stop growth instead of using more of the other 

 things. The kind of plant food that has become exhausted 

 is termed the " limiting factor." 



A " complete fertilizer" is one that contains all three of 

 the plant foods mentioned, though they may not be in the 

 proportions required. As a matter of fact most of the com- 

 plete ready-mixed fertilizers to be bought, and especially 

 the cheaper grades, do not contain the plant foods in the 

 best proportions for general garden use. A good general 

 garden fertilizer contains four per cent of nitrogen, eight 

 per cent of available phosphoric acid and ten per cent of 

 potash. Potash is temporarily scarce at the present time, 

 as no potash salts can be imported from Germany, and 

 the percentage of potash in all fertilizers is being cut down. 



Both Water and Air Needed in the Soil 



After a soil has been well supplied with plant foods in 

 available forms the plants cannot grow unless they can 

 absorb the various elements in the form of solutions. That 

 means that a certain amount of moisture must be present 

 in the soil. Conditions are most favorable to growth when 

 the soil is about half saturated. Air must be present as 

 well as moisture. If the soil is wet the air is excluded. If 

 it is dry the plants cannot get hold of the food lying about 

 them. 



The practical problem remains of just how to make the 

 garden properly rich. Manures vary so much that they 



