Higher Crop Yields From Improved Soils
Extension Circular 474
Fertilizing Farm Fields in Indiana
by
S. A. BARBER and R. K. STIVERS Department of Agronomy
PURDUE UNIVERSITY Agricultural Extension Service, Lafayette, Ind.
Fertilizer Need
FERTILIZERS SUPPLEMENT the nitrogen, phosphorus, potassium and minor elements available from the soil so that the crop will have an adequate supply. The amount of fertilizer needed depends on:
t the crop to be grown.
•	the yield level of the crop.
•	the supply of nutrients already available in the soil.
Crop to be Grown
Crops vary in fertilizer requirement. Alfalfa requires large amounts of potassium while wheat does not; wheat requires large amounts of phosphate. Corn requires large amounts of nitrogen. Soybeans obtain nitrogen from the air through the nitrogen bacteria in the nodules on their roots and generally do not need any added fertilizer nitrogen.
figure l. Wheat shows a marked response to phosphate in the early growth.
Yield Level of Crop
Where the possible yield is high, more nutrients will be needed than where it is low. The change in rate owing to yield level varies with the nutrients concerned. The need for nitrogen increases in proportion to the yield. A 100-bushel corn crop may need to have available a total of 200 pounds of nitrogen from soil, previous legume crop and fertilizer. A 150-bushel corn crop may need to have available a total of 300 pounds of nitrogen. The extra 50 bushels requires 100 pounds of nitrogen.
The need for additional phosphate and potassium does not increase as much with the yield increase as does the need for nitrogen. When the corn yield expected is 150 bushels instead of 100, the increased need for phosphate and potash will be only 20 to 40 pounds of each per acre.
In this case the level of the nutrient in the soil determines its effect on plant growth. The plant may use only 10 or 20 percent of the amount of available phosphate or potash in the soil. A high
Figure 2. Wheat shows a marked response to phosphate later in the season.