AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 2001, No. 3
As our population increases, production efficiency through innovation and new technology has kept pace to provide us with a continuing supply of abundant and nutritious food. Fertilizer products and practices are important contributors to this success.
High yields come at a cost. Essential elements such as phosphorus and potassium are absorbed from the soil by plant roots and exported in the harvested crop; the higher the yield, the greater the removal. Yields of many crops grown today have doubled in just the past few decades. Such high nutrient demand requires skillful management to avoid yield limitations developing because of an induced nutrient deficiency.
Nutrient uptake and nutrient removal are not the same. Nutrient uptake refers to all the nutrients taken up by the crop as it grows and matures. This would include the nutrients in both the vegetative and reproductive (seed or grain) portions of the crop, including the roots. Nutrient removal, on the other hand, refers only to that portion of the crop actually hauled from the field. In some cases uptake and removal are similar. For example, harvesting hay or silage removes almost all of the above-ground portion of the crop, so removal would approach that of total uptake. However, uptake and removal are quite different when only the seed or fruit is removed and the vegetative portions (that is, leaves, stems and stalks) are plowed down or remain on the field surface.
Phosphorus plays a vital role in genetic transfer and is part of the DNA molecule. It is not surprising, therefore, to find it concentrated in the seed. Although overall plant content of phosphorus is only about 10 percent that of potassium, harvests of grain from such crops as corn, wheat and rice remove substantially more phosphorus than potassium. Actual removal will vary somewhat depending on the individual variety, and environmental and management factors, but the following illustrates the point. Each 100 pounds of corn removes an estimated equivalent of 0.79 pounds of P2O5 compared to only 0.52 pounds of K2O….or 52 percent more P2O5 compared to K2O. So growers producing grain and returning the vegetative material to the soil are removing a lot more phosphorus than potassium with each harvest even though it takes substantially more potassium than phosphorus to actually grow the crop.
Important functions of potassium in the plant include a role in the synthesis of carbohydrates and protein. Therefore, harvests that remove a lot of protein, such as hay, silage and forage, remove a lot of potassium. Alfalfa, for example, removes the equivalent of about 15 pounds of P2O5 per ton and 60 pounds of K2O…or four times more potassium relative to phosphorus. The same is true for carbohydrate. Such crops as potatoes, sugar beets, grapes, tomatoes and fruit also remove a lot of potassium. One final example…potato tubers remove the equivalent of about 0.15 pounds of P2O5 per hundred-weight and 0.56 pounds of K2O…nearly four times more potassium than phosphorus.
Our challenge for the future is not just to produce high yields, but progressively higher yields. Understanding what such yields remove from the soil helps us manage fields for the long term. High yields can deplete soils of their fertility, but only if we allow it to happen. Understanding nutrient removal helps us take the long view in precision fertilizer management for today and tomorrow.