536 VARIOUS FERTILITY FACTORS 



of five tons of farm manure about once in ten years (spread very 

 uniformly, and an occasional dressing of burned lime, would make 

 this system of ultimate land ruin very complete. 



A mixture of 300 pounds of acid phosphate and 100 pounds of 

 kainit in five years would, with the manure system, furnish about 

 5 pounds of nitrogen, 5 pounds of phosphorus, and 6 pounds of 

 potassium per acre per annum; whereas, crops as large as we ought 

 to try to produce would remove from the soil as a yearly average 

 about 100 pounds of nitrogen, 20 pounds of phosphorus, and 80 

 pounds of potassium (see Table 13). When crops one half as large 

 are produced under such a system of fertilization, the soil under the 

 action of these stimulants must furnish about nine tenths of the 

 nitrogen, half of the phosphorus, and six sevenths of the potassium 

 required for the crops. An invoice of his stock of fertility will help 

 the landowner to plan wisely for the future, because he can thus 

 know in advance what the ultimate effect must be of such systems. 



Protective agents. As protective agents we may include materials 

 which tend to ward off disease or insect enemies; and the effect 

 may be produced by substances destructive to fungi or repellent 

 to insects. Kainit is thought to act sometimes as a fungicide, and 

 tankage is held by some to prevent attack from certain insects. 



Any treatment which hastens the normal growth of the plant 

 usually helps the plant to resist or overcome the attack of insects 

 or disease; and it is apparently true that imperfect or abnormal 

 plants are more likely to suffer from such attacks than normal, 

 healthy plants. It has been suggested that sucking insects prefer 

 the concentrated sap of weak or somewhat withered plants to that 

 of vigorous succulent plants. Doctor Forbes has suggested that 

 the very dilute juice of a rapidly growing plant may constitute a 

 starvation diet for healthy insects; in other words, that the 

 capacity of the insect for such juice is not sufficient to furnish 

 it with the amount of nutrition necessary for maintenance and 

 reproduction. 



It is common observation that chinch bugs may attack and 

 destroy wheat that would otherwise yield 10 or 15 bushels per acre, 

 while wheat growing in the same field on land capable of producing 

 30 bushels or more per acre is not attacked. The author has noted 

 in several different seasons that corn growing on land that will 



