Won't using such a small amount of fertilizer eventually "wear out" my soil?

Short answer

       All plant tissue is composed of 96 to 97% carbon (C), hydrogen (H), and oxygen (O). 

       Let's grow 150 bushels per acre of corn.  According to a current study ("Nutrient Removal by Corn Grain Harvest"); we would need 219 pounds of nutrients.  (Remember the three most abundant elements: carbon, hydrogen, and oxygen are free)  Growers would recommend 8 gallons of Growers Mineral Solutions (GMS), which is 91.2 pounds with another 80 pounds of nitrogen to give us 171(91+80) pounds of applied elements. 

       We now have a gap of nutrients of 48 pounds per acre (219-171).  Using the Growers Composition of Soil chart (Section 2) for a silt loam soil, we arrive at 61,725 pounds of elements for an acre of soil that is 7 inches in depth. 

       So, in conclusion if we are going to "wear out" soil by keeping roots in the top 7 inches of soil, we need 1,286 years (61,725/48) for that "wear out" to occur.

Origin

       Studying the early basic scientific research, Dr. Tiedjens realized that all plant tissue was predominately carbon (C), hydrogen (H), and oxygen (O) (96 to 97 percent).  He believed that a very competitive crop could be produced using a small amount of added fertility elements if they were used at the correct time in the plant's life.

       Shortly after World War II, the sale of chemical elements as fertilizer became big business.  The sales tool of the fertilizer companies was to use plenty of fertilizer so as not to "wear out" the soil.  There was very little early research on nutrient removal of crops from the soil. 

       Employing Dr. Kenneth C. Beesen's analysis, Dr. Tiedjens explained to farmers that the elements present in a crop can vary tremendously in amounts ("Crop Yields and Mineral Content").  In addition, Beesen's work demonstrated that if some principle nutrients were low in the soil, the crop would "hog" feed on the other nutrients to fill the deficient nutrient's place. 

       From this, Dr. Tiedjens concluded that the fertilizer industry which was selling nitrogen, phosphorus, and potassium, wanted to substitute the elements they were selling for the elements they were not selling, such as calcium or magnesium.  Thus, if the soil nutrients vary and the fertilization varies, crop removal of nutrients will vary.

Recent studies

       More recently nutrient management studies have focused attention on crop removal of nutrients.  Those studies also see a large variation in nutrient contents of crops ("Nutrient Removal by Corn Grain Harvest").  The authors say the "grain nutrient concentrations can be highly variable even for a given corn hybrid grain in different environments." 

       These differing environments make nutrient removal numbers suspect especially when dealing with high input agriculture.  The same crop yield can give you different extraction numbers.

Run the numbers

       To do some comparing we can use the numbers of the latest study and find out exactly what you may need to extract in nutrients from the soil profile over time. 

       For an example, let's grow 150 bushels per acre of corn.  From Table 3 "Nutrient Removal by Corn Grain Harvest", our extraction includes 11 elements (remember the three most abundant elements carbon, hydrogen, and oxygen from Section 2 are free), so, for 150 bushels per acre we would need 219 pounds of nutrients for our crop (Table A). 

Table A: Pounds of Nutrients Needed per Acre to Grow 150 Bushels per Acre of Corn According to "Nutrient Removal by Corn Grain Harvest"

¹This number represents pounds of P as P₂O₅
²This number represents pounds of K as K₂O

       The numbers in Table 3 "Nutrient Removal by Corn Grain Harvest" are the middle extraction values; farmers on the Growers Program would have lower extraction values for nitrogen, phosphorous, and potassium than the minimum values in Table 4 "Nutrient Removal by Corn Grain Harvest".  The other elements for the Growers Program farmer would probably be nearer to the maximum values. 

       Now for the Growers Program to grow 150 bushels per acre we would recommend about 8 gallons per acre of GNS.  In addition, for a grass, such as corn, we would suggest some extra nitrogen if the field's biological life is not active. 

       A normal suggestion for nitrogen by a GMS representative would be 80 pounds of nitrogen per acre if the preceding crop was a legume.  (If the preceding crop was a grass, the nitrogen rate would have to be increased.) 

       So, now we have 8 gallons of GMS, which is 91.2 pounds with another 80 pounds of nitrogen to give us 171 pounds of applied elements.  Thus, we now have a gap of nutrients of 48 pounds per acre (219-171). 

       Now, the agricultural establishment says that the soil nutrient reservoir must make up the gap.  Using the "Growers Composition of Soil" chart (see section 2) for a silt loam soil, we will total up the same elements that are listed in Table 3 "Nutrient Removal by Corn Grain Harvest" of the nutrient removal paper. 

       When we total up those values from our composition chart (Table B), we arrive at 61,725 pounds of elements for an acre of soil that is 7 inches in depth.  So, to fill our deficient of 48 pounds we have a potential in the acre of 61,725 pounds.  Now remember if you are able to get your roots to go deeper than 7 inches into the soil, the reservoir of possible nutrition will also become larger.

¹This number represents pounds of P as P₂O₅
²This number represents pounds of K as K₂O

       We realize that not all  elements in the soil will always be available.  However, we believe that using GMS and or the Growers Program will allow the soil to release more of those nutrients through a better physical and biological environment.

       In conclusion, if we are going to "wear out" the soil by keeping the roots in the top 7 inches of soil, we need 1,286 (61,725/48) years for that "wear out" to occur (see Tables A & B for calculations).