120 PLANT FOOD 



Let us examine two common crops as corn and clover hay 

 and note to what extent they withdraw fertility from the 

 soil. A 50-bushel crop of corn removes about 74 pounds 

 of nitrogen, 12 pounds of phosphorus, and 35 pounds of 

 potassium. A ton of clover hay removes about 40 pounds 

 of nitrogen, 5 pounds of phosphorus, and 30 pounds of 

 potassium. By a simple calculation we can determine how 

 many years we could expect these yields if we knew our 

 soil resources. But, unfortunately, the question is not so 

 simple. A soil may be non-productive and yet be very 

 fertile so far as these chemical elements go. That is, there 

 may be an abundance of these elements present, but other 

 substances, acids, alkalies, vegetable poisons, or an excess 

 of free water perhaps, may by their harmful activity ren- 

 der the soil non-productive. 



The Germans have a proverb which says, " Kalk macht 

 die Vater reich, aber die Sohne arm." (Lime makes the 

 fathers rich, but the sons poor.) This means in a general 

 way that lime liberates plant food and tends to exhaust 

 soil fertility. That is, lime is an active substance, as 

 are phosphoric acid and potash. The amount of potential 

 plant food in the soil is not so important as the process 

 at work there in forming and accumulating available 

 plant food. 



79. Available Plant Food. Available plant foods are 

 such substances as can be utilized immediately through 

 natural processes. The foregoing soil analyses tell us the 

 total quantity of plant food in the soil. This knowledge 

 has a bearing upon the permanency of agriculture, but it 

 has little meaning so far as this year's or next year's crop 

 is concerned. The practical question concerns the avail- 

 able supply. How much of what is in the soil is the plant 

 able to get ? This problem is taken up in the next 

 chapter. 



