The impact of soil sampling timing on soil test results.

Abstract

An accurate soil test provides the foundation for determining the optimal amount of fertilizer required for a sugarbeet crop. While some nutrients do not change much from fall to spring, others are more variable. It is well understood that available soil nitrogen (N) is dynamic and changes over time in response to natural processes such as mineralization, leaching, denitrification, immobilization, and human interventions such as tillage and residue management. For this reason, it is commonly recommended that soil sampling for N occur as close to planting as possible. Soils sampled too far ahead of planting are less able to accurately measure soil N available to sugarbeets at planting. This results in N fertilizer recommendations that may be sub-optimal for sugarbeet production and/or profitability. Despite these concerns, sugarbeet soils are routinely sampled in the fall, often as early as August. In the lead up to the 2020 crop year, Amalgamated Sugar sampled 1173 fields prior to January. This represented  43% of total contracted fields. With more than 2000 sugarbeet fields to sample each year, it would be logistically challenging to require that all fields be spring sampled. Instead, it is possible that improvements could be made to the fall soil sampling program if it was understand how much soil test N varies due to time of sampling. This report details the results of monthly soil sampling at 15 commercial sugarbeet fields in the fall of 2021 and spring of 2022. We found that soil samples taken in the fall significantly underestimated total available N relative to spring samples. As a result, fertility recommendations based on fall soil tests overestimated fertilizer N requirement resulting in unnecessary cost to the grower and the potential for negative impacts on sugar % and sugar quality. This work suggests that fertilizer recommendations based on fall soil tests could be improved by considering  their tendency to underestimate soil available N relative to spring soil tests.