Monolith soil core sampling to develop nitrate testing protocol for manure injection

Abstract

Injecting manure and commercial fertilizer beneath the soil surface is an important nutrient management practice that conserves ammonia-nitrogen (N) but creates distinct bands of N below the soil surface. To date, no widely accepted soil nitrate sampling protocol has been developed to account for the extreme heterogeneity created by injection. To develop sampling recommendations for Pre-Sidedress Nitrate Test (PSNT), we quantified patterns of NO₃⁻-N concentrations in soil of corn (Zea mays L.) plots injected with liquid dairy cattle (Bos taurus L.) manure at 76 cm spacing over 2 years. Soil monoliths were collected to allow precise sampling of 30 cm deep by 2.5 cm soil cores from which a mid-season PSNT was determined. Monte Carlo simulation was conducted to simulate the effects of alternative soil sampling protocols on bias and error. Results from the simulation support the following equispaced sampling protocol: five, 30-cm deep soil cores are spaced 15 cm apart and oriented in a line perpendicular to the injected manure bands, collected at four locations in the field, to produce a single composite of 20 samples for NO₃⁻ analysis. It is not necessary to know manure band location. As spatially discrete manure application patterns become more prevalent with the expansion of manure injection, we believe this PSNT sampling protocol balances risk of error with practical concerns needed to promote adoption.