Soil Sampling under No‐Till Banded Phosphorus

Abstract

AbstractUncertainty exists about the best sampling procedures for no‐till soils containing residual P‐fertilizer bands. This study was conducted to determine the distribution of residual P‐fertilizer bands and to define soil sampling procedures that minimize variability, while approximating the “true” P‐soil‐test value. A no‐till, banded‐P‐fertilization field study was conducted during 1986 and 1987 on three central Great Plains soils: Keith clay loam (a fine‐silty, mixed, mesic Aridic Argiustoll), Woodsen silt loam (a fine, montmorillonitic, thermic Abrubtic Argiaquoll), and Harney silt loam (a fine, montmorillonitic, mesic Typic Argiustoll). Soils were sampled laterally away from the band 1 to 2 yr after fertilization, and P was determined with NaHCO3 and acid‐fluoride extractions. Bands were accurately described by the exponential decay model (mostly r2 values of 0.98 or better). The derived exponential equations were used in computer evaluation of soil‐sampling procedures. When the location of P bands are known, sampling to include one in‐the‐band soil sample for every 20 between‐the‐band samples for 76‐cm band spacing, and for every eight between‐the‐band samples for 30‐cm band spacing, will result in a P soil test equal to the “true” P‐soil‐test mean. When the location of P bands is unknown and <20 subsamples are taken, paired sampling consisting of a first completely random sample and a second sample 50% of the band‐spacing distance from the first sample, perpendicular to the band direction, will reduce variability over completely random sampling. The greatest deviation from the “true” P soil test occurs when inadequate sampling includes rather than excludes the band, and will thus under‐estimate P‐fertilizer needs.