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From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Winter 2002, No. 4

MONTANA STUDY EXAMINES NUTRIENT UPTAKE BY SPRING WHEAT

A study conducted by Montana State University examined nutrient uptake and biomass production throughout the growing season. Only the above-ground portions of the wheat crop were examined. Plants were divided into leaves, stems, heads (without grain), and grain. No root measurements were taken. Nitrogen, which was being applied through the irrigation system, became limiting at the end of the season because of a limited water supply. This study addresses many recurring questions about nutrient uptake and removal by wheat.

How is wheat dry matter distributed among various plant portions? Total production of above-ground wheat dry matter was 6.32 tons per acre. At physiological maturity, approximately 38% was in the grain, 16% was in the heads, 37% was in the stems, and 9% was in the leaves. The grain yield was about 91 bushels per acre, assuming a test weight of 60 pounds per bushel at 12% moisture.

How much phosphorus does wheat take up during the season? Total phosphorus uptake in this study at physiological maturity was about 60 pounds of P2O5 per acre, or about 0.66 pounds of P2O5 per bushel. Approximately 80% of the total phosphorus was in the grain, 9% was in the heads, 8% was in the stems, and 3% was in the leaves. Phosphorus removed from the field by grain totaled about 48 pounds of P2O5 per acre, or about 0.53 pounds of P2O5 per bushel. Phosphorus was evenly distributed between the leaves and stems until the boot stage (Haun stage 9.2) when stems began to take up phosphorus more rapidly. At anthesis, the crop had taken up about 45% of the end-of-season total. Unlike nitrogen, phosphorus uptake occurred until physiological maturity. During grain fill, approximately 32 pounds of P2O5 per acre (53% of the total) was redistributed into developing kernels from the leaves, stems, and heads.

How much potassium does wheat take up during the season? Total potassium uptake at physiological maturity was about 111 pounds of K2O per acre, or about 1.22 pounds of K2O per bushel. Only about 20% of this total potassium was in the grain, 11% was in the heads, 58% was in the stems, and 11% was in the leaves. Removal of potassium from the field totaled about 22 pounds of K2O per acre, or 0.24 pounds of K2O per bushel. During stem elongation until just after anthesis, potassium uptake was most rapid and peaked at 167 pounds of K2O per acre, or about 1.84 pounds of K2O per bushel. From this point to the soft dough stage, the crop lost about 56 pounds of K2O per acre for a final uptake at maturity of 111 pounds of K2O per acre. Losses were attributed to potassium being leached from the leaves and culms by precipitation.

The uptake and removal values from this study differ to various degrees from estimates widely distributed in standard tables. Such variance demonstrates that published uptake and removal estimates provide a good starting point for such estimations, but that whenever possible, grain should be analyzed for nutrient content to better understand how much of the various nutrients are being removed under local growing conditions.


—TSM—

For more information, contact Dr. T. Scott Murrell, Northcentral Director, PPI, 3579 Commonwealth Road, Woodbury, MN 55125. Phone: (651) 264-1936. E-mail: smurrell@ppi-far.org
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