FIELD-SCALE VARIABILITY OF PHOSPHORUS AND POTASSIUM UPTAKE BY NO-TILLCORN AND SOYBEAN

Use of precision farming technologies requires better understanding of nutrient variability in soils and plants. This study assessed pattern s 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 ha d varied histories of fertilization. One hundred composite whole-plant samples (five corn plants and 10 soybean plants at V5 to V6 Growth st ages) 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 co ncentrations were measured in all samples. Plots of observed values, v ariography, and fast Fourier transform (FFT) analyses revealed cluster ing of values and periodic patterns of DW and nutrient content in most fields. The patterns of variability varied greatly among fields and d irections within a field. The diversity of patterns probably results f rom nonuniform distribution of fertilizers and animal manures. Pattern s of plant K uptake (KU) followed patterns of plant K concentration (K C) in some fields and those of DW in others. Patterns of plant P uptak e (PU), however, usually followed those of DW. Implementation of plant analysis for no-till fields should address situations with high spati ally structured variability of cyclic or clustered structure. The resu lts show that the structure of the variability in P and K uptake is si te specific and that both the sampling scheme and the optimal separati on 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 supplie s of corn and soybean fields.