Precision agriculture has evolved from a concept into an accepted management practice. The challenge now is how to best utilize these technologies for the benefit of agriculture. Nutrient management could be improved and spatial variability reduced by variable-rate (VR) application. The objectives of this dissertation were to assess the value of VR P fertilization and P-based liquid swine manure application for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production compared with the traditional uniform-rate (UR) application. On-farm research methods based on replicated, long narrow strips were adapted to precision technologies. These technologies included grain yield monitors, differential global positioning systems, and geographical information systems. Treatments consisted of a non-fertilized control, a UR method based on average soil-test P (STP) for the entire field and a VR method based on STP from 0.2 to 1.7 ha grid cells. Grain yield and temporal changes in STP were used to assess treatment differences in both studies. In addition, plant dry weight, P concentration, and P uptake (V5 growth stage) were used in the fertilizer study.;Phosphorus fertilization and manure often increased whole-field crop grain yield in fields in which average STP values were below the Optimum interpretation class for corn and soybean production. Analyses of yield for field areas with contrasting STP values often showed significant responses to fertilizer or manure P in field areas testing Optimum or less, but rarely in high-testing areas. Phosphorus fertilization increased early plant growth and P uptake more frequently than grain yield. Although the method of P application did not influence whole-field or within-field plant responses, VR reduced soil P variability compared with UR by increasing STP in low-testing areas and decreasing or not affecting STP of high-testing areas.;This research showed precision farming technologies are useful tools for improving nutrient management. Fertilization or manure application programs that vary the nutrient application rate may not result in increased yield compared with UR application methods. However, use of VR technology will result in better nutrient management and perhaps better water quality because of more efficient distribution of fertilizer or manure.