FERTILITY AND HOW TO MAINTAIN 69 



Drainage Increases Fertility. — Fertility is increased by drainage, 

 especially underdrainage, which lowers the water table, increases aeration, 

 and causes plant roots to go deeper in the soil. The amount of plant 

 food that plants can secure is approximately proportionate to the volume 

 of the soil. to which they have access. Drainage virtually deepens the 

 soil. 



Manure is the Best Fertilizer.— Manures increase fertility by the 

 direct addition of plant food and by increasing the organic matter of the 

 soil. Manures increase the water-holding capacity of the soil, improve 

 its physical condition, introduce various forms of bacteria and encourage 

 the multiplication of desirable bacteria. 



Commercial Fertilizers Add Plant Food Only. — Commercial fertilizers 

 increase fertility by the direct addition of the plant food elements they 

 contain, but, as a rule, have very little if any other effect. Commercial 

 fertilizers are expensive and call for an intimate knowledge of the require- 

 ments of the soil and the form and availability of the constituents in the 

 fertilizer. The factors above mentioned in relation to soil fertility will 

 be more fully discussed under the several chapters pertaining to them, 

 which follow. 



The Limiting Factor. — There is always a limiting factor in crop pro- 

 duction, and it is the business of the farmer to ascertain his limiting 

 factor or factors. In many cases the limiting factor in the growth of a 

 crop will be the supply of water. This may be a deficient supply or it 

 may be an excess. If water is the limiting factor it may be due to a low 

 rainfall during the crop season and the low storage capacity of the soil. 

 The farmer has no control over the rainfall, but he should endeavor to 

 increase the water storage capacity of his soil by such means as are 

 economical. Deeper plowing, the addition of organic matter, thorough 

 tillage to conserve soil moisture or the application of water in the form 

 of irrigation are all of them means to such an end. If the limiting factor 

 is due to an excess of water, thus preventing plant growth, the problem 

 becomes one of land drainage and the removal of the water. 



The limiting factor may be a deficiency in phosphorus. This being 

 the case, it is important that the farmer know the truth in order that he 

 may supply the deficiency by the application of a phosphatic fertilizer." 

 When the limiting factor or deficiency has been supplied, something else 

 may then become a limiting factor. For example, the limestone soils of 

 Pennsylvania are generally deficient in phosphorus. Such soils, when 

 cropped with a four-year rotation of corn, oats, wheat and mixed clover 

 and timothy, will show a steady decline in crop yields if no manures or 

 fertilizers are applied. Experiments with fertilizers on limestone soil and 

 for the crops mentioned show that when nitrogen alone is applied it has 

 no effect. Potash applied alone is likewise ineffective. When phosphorus 

 is applied there is a marked increase in the yield of crops. Phosphorus, 

 however, will not fully maintain the fertility of the soil. Its yield will 



