Spatial Variability Patterns of Phosphorus and Potassium in No-Tilled Soils for Two Sampling Scales
The increasing use of grid soil sampling methods and variable-rate fertilization requires better understanding of patterns and causes of lateral variability of nutrients. This study assessed patterns of spatial variability of P and K for two scales of sampling on eight no-tilled corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields. The fields had received no P or K fertilization since the last harvest but had varied histories of fertilization. Fifty 10-core (0- to 15.2-cm depth) composite samples were collected from 2.2-m2 areas separated 3.05 m from each other along each of two 150-m intersecting transects laid out across and along old crop rows. Sixty-seven single-core samples spaced 7.6 or 15.2 cm were collected from 7.85-m segments of the transects across rows. Variography revealed high random and spatially structured variability for most fields. The structure of the variability differed among fields and directions within a field. Plots of soil-test P and soil-test K values, fast Fourier transform analyses, and semivariograms showed clusters and periodic patterns of variable regularity and size across fields. The cyclic patterns were more defined across crop rows. Small-scale cyclic patterns with a period of ≈ 1 m revealed for some fields probably resulted from repeated banded fertilizer applications. Large-scale cyclic trends with periods of 15 to 18 m or a multiple of this distance were probably the result of broadcast fertilization with commercial bulk spreaders. Sampling methods should be developed for fields with high spatial variability of cyclic structure at various scales. Producers should pay more attention to uniform application of fertilizers and manure.
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