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Site-Specific Analysis & Management

Evaluation of Zone Soil Sampling Approaches for Phosphorus and Potassium Based on Corn and Soybean Response to Fertilization

^a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
^b Inst. Nacional de Investigaciones Agropecuarias (INIA), La Estanzuela, Colonia, Uruguay

^* Corresponding author (apmallar{at}iastate.edu)

Soil sampling approaches have been compared based on soil-test variation. This study evaluated sampling approaches for P and K based on yield response to fertilization. Strip trials were established on four fields for P and three fields for K managed with corn (Zea mays L.) and soybean (Glycine max L. Merr.) rotations and evaluated 3 or 4 yr (27 site-years). Treatments replicated three to four times were fertilizer and no fertilizer application. Soil test results from a dense grid-point sampling (DG) approach (0.08 to 0.27 ha) were used to simulate six approaches: (i) 1.0-ha grid cells (GC), and zones delineated based on (ii) soil series from digitized survey maps (SMZ); (iii) elevation (EZ); (iv) apparent soil electrical conductivity, EC_a (ECZ); (v) EZ and ECZ (EECZ); and (vi) EZ, ECZ, and slope (EECSZ). Grain yield monitors, global positioning systems (GPS), and geographical information systems (GIS) were used to describe crop responses. Estimates of soil-test variation were largest for DG, intermediate for GC, and less for other approaches. Crops responded (P 0.05) to fertilization in 20 site-years. Sampling approaches DG, GC, EZ, EECZ or EECSZ, ECZ, and SMZ identified a differential within-field yield response in 16, 8, 5, 3, 2, and 2 site-years, respectively. Differential yield responses seldom were explained by zone-mean soil-test values. Zone approaches often identified areas with different yield levels but were less effective than DG or GC at describing within-field variation of soil tests and yield response to fertilization. Zone approaches may be more effective in fields with shorter fertilization histories or soils with more contrast in properties.

Abbreviations: DG, dense grid-point sampling • EC_a, apparent soil electrical conductivity • ECZ, apparent soil electrical conductivity zone • EECSZ, combined EZ, ECa, and slope zones • EECZ, combined EZ and ECa zones • EZ, elevation zone sampling method • GC, grid-cell sampling • GIS, geographical information systems • GPS, global positioning systems • SMZ, soil series zone • STK, soil-test K • STP, soil-test P