Crop Yield and Soil Phosphorus as Affected by Liquid Swine Manure Phosphorus Application Using Variable‐Rate Technology

Abstract

Soil‐test P (STP) and crop P needs vary within fields and variable‐rate technology can be used to apply P more effectively. The objective of this study was to evaluate within a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation grain yield and STP response to fixed‐rate (FR) and variable‐rate (VR) liquid swine (Sus scrofa domesticus) manure P application using strip‐trials, global positioning systems (GPS), yield monitors, and geographical information systems (GIS). Treatments replicated four or five times were a control and manure P applied with FR or VR once before soybean of two rotation cycles in one field and three cycles in another. Strip width and length were 18.3 m and 605 to 660 m, respectively. Initial soil samples were taken from 0.3‐ha cells and post‐harvest samples were taken each year by treatment (0.1‐ha cells). High N (for corn) and K rates were applied uniformly across all strips. Manure P increased strip‐average grain yield (P ≤ 0.05) in eight of nine site‐years but yield for FR and VR did not differ. The study of yield responses for field areas with different STP showed that manure P always increased yield with STP very low or low (<16 mg Bray‐P1 kg−1) and only in three site‐years with STP optimum (16–20 mg P kg−1). Yield responses differed between FR and VR in very low areas of one site‐year, where VR increased corn yield more than FR. The VR method reduced within‐field STP variability compared with FR with the exception of only one site‐year. Although VR did not increase yield compared with FR, less manure P was applied to high‐testing areas and it reduced STP variability.