ARS | Publication request: FORAGES SHOW LITTLE RESPONSE TO HIGH SOIL TEST P

Submitted to: American Society of Agronomy Meetings
Publication Type: Abstract Only
Publication Acceptance Date: November 9, 2000
Publication Date: N/A

Technical Abstract: Soils on many dairy farms have increasing levels of soil test phosphorus (STP), caused by repeated cumulative fertilizer and manure applications rates in excess of crop P removal. We determined STP levels in several areas within dairy farm fields in MN and WI that have received manure, and measured apparent alfalfa and corn response to STP during the 1999 growing season. STP levels varied widely both among and within fields. Variation increased as the mean STP level increased. Site-specific manure applications may be acceptable even in high-STP fields, because they contain many low-testing areas. Corn yield, corn P uptake, and first-cut alfalfa yield were not related to STP, whereas second- and third-cut alfalfa yields increased slightly as STP rose from low to optimum. Tissue P concentrations in first-cut alfalfa increased with STP, but not in second- nor third-cut alfalfa. Relatively little P was removed in forage, indicating that STP reductions will be slow. Soil test potassium (STK) was highly correlated with STP, and alfalfa K concentration increased with greater STP. Therefore, in addition to potential adverse environmental effects, high P-testing soils on dairy farms may also produce forage that contains too much K to be fed safely to dry or close-up cows.