A commonly asked question by crop producers is, "how many pounds of plant nutrients do my crops take up?" While the absolute answer to this varies by yield and soil fertility, useful estimates can be made from research data to enable a producer to have a "ballpark" value for nutrient up-take by some of the major crops of Kentucky. The following tables contain estimates of nitrogen (N), phosphorus (P),and potassium (K) contained in certain crops and the amount of N, phosphate (P2O5), and potash (K,O) taken up by tobacco, corn, soybeans, wheat, alfalfa hay, clover grass hay, and pastures. Values are based on good yields for tobacco, corn, soybeans, and wheat, and are expressed on a per ton of production for hay and pasture (example: a ton of alfalfa contains about 50-14-55 lbs/A of N-P2O5- K2O, respectively). A grower with 5 T/A yield of alfalfa would thus have removed 250-70-275 lbs/A of N-P2O5-K,O per year). For cattle grazed land, it is important to note that most of the nutrients taken up and consumed are recycled back to the surface of the field (in a non-uniform pattern, particularly with low to normal stocking rates). Beef cattle retain only about 25-20-15% of N-P2O5-K2O intake, respectively. For dairy cattle, retention of N-P2O5-K2O is about 35-30-20%, respectively. Nutrients contained in stalks, stover, and straw are also of note. If recycled back onto fields, these plant residues can be a valuable source of nutrients.
Wells, Kenneth L. and Thom, William O., "Estimated Nutrient Content and Uptake by Kentucky's Crops" (1994). Soil Science News and Views. 142.