Field-scale variability of phosphorus and potassium uptake by no-till corn and soybean

Use of precision farming technologies requires better understanding of nutrient variability in soils and plants. This study assessed patterns of spatial variability of plant P and K content of no-till corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] on eight fields that had varied histories of fertilization. One hundred composite whole-plant samples (five corn plants and 10 soybean plants at V5 to V6 Growth stages) were collected in late spring from 2.2-m(2) areas spared 3.05 m along two intersecting transects. Dry weight (DW) and total P and K concentrations were measured in all samples. Plots of observed values, variography, and fast Fourier transform (FFT) analyses revealed clustering of values and periodic patterns of DW and nutrient content in most fields. The patterns of variability varied greatly among fields and directions within a field. The diversity of patterns probably results from nonuniform distribution of fertilizers and animal manures. Patterns of plant K uptake (KU) followed patterns of plant K concentration (KC) in some fields and those of DW in others. Patterns of plant P uptake (PU), however, usually followed those of DW. Implementation of plant analysis for no-till fields should address situations with high spatially structured variability of cyclic or clustered structure. The results show that the structure of the variability in P and K uptake is site specific and that both the sampling scheme and the optimal separation distance between sampling positions would vary greatly among fields. The results suggest that plant analysis would have similar sampling problems to those of soil testing for representing the P and K supplies of corn and soybean fields.