Traditional soil tests sample the top 0-6 inches of soil. While this surface layer is important, it doesn’t tell the whole story. In many fields, significant amounts of residual nitrogen remain deeper in the profile. Deep soil testing, sampling down to 6-24 inches, helps farmers quantify this nitrogen and adjust fertiliser rates accordingly. With nitrogen prices high and supplies uncertain, accounting for existing nutrients can provide a substantial economic advantage.
An analysis by Texas A&M AgriLife Extension found that deep soil testing before planting corn, cotton or wheat could generate projected savings of $13-$189 per acre[1]. By recognising residual nitrogen, producers can reduce fertiliser inputs without compromising yield. These savings are especially attractive when nitrogen prices are elevated; from 2016 to 2020 anhydrous ammonia averaged around $500 per ton, but by May 2023 the price had climbed to about $700[2].
Deep testing is also a response to the current economic environment. Extension specialists DeDe Jones and Jourdan Bell note that with high input costs and low commodity prices, farmers should deep soil test to take advantage of any residual nitrogen and other nutrients[3]. Instead of fertilising blindly, growers can base decisions on measured nutrient levels, saving money and avoiding excessive vegetative growth that may require additional plant growth regulators[4].
How should deep soil testing be done? Agronomists recommend sampling to at least 24 inches (about 61 cm) whenever feasible[5]. Annual testing near planting time assesses the availability of nitrogen and other nutrients[6]. Residual nutrient levels vary widely from year to year, influenced by rainfall, irrigation and previous crop uptake[7]. Therefore, routine testing ensures that fertiliser decisions reflect current field conditions rather than outdated assumptions.
The return on investment for deep soil testing is compelling. Even at a sampling cost of roughly $1 per acre, the savings can exceed $30,000 on a 1,000-acre irrigated farm when 25 % of the budgeted nitrogen is found in the soil[8]. Larger residual nitrogen levels lead to even greater savings, more than $130,000 if the soil supplies the full nitrogen requirement[9]. At a time when fertiliser markets are volatile and global supply chains are stressed, deep soil testing is a powerful tool for protecting both profits and the environment.
References
- [1] [2] [3] [4] [5] [6] [7] [8] [9] Deep soil testing can help cut fertilizer costs https://www.farmprogress.com/soil-health/deep-soil-testing-can-help-cut-fertilizer-costs
