The Roots of Soil Fertility.

From early Greek, Roman and Chinese philosophers to modern climate and extraterrestrial scientists, the basic biological, physical and chemical principles of life’s essential elements have been the subject of inquiry.  Age of Enlightenment Era investigations into the essence of plant life provided insights into modern chemistry and established early examples of the scientific method, while applied soil fertility field trials at great agricultural research stations forged nutrient management principles, all leading to a flourishing global agriculture in supporting an ever growing human population.  The Laws of the Minimum, Diminishing Returns, and Mass Conservation, still guide our conceptual understanding of soil-plant- atmospheric nutrient cycling, providing foundations for mechanistic models, life-cycle analyses, and fertility decision-support smart-phone apps. 

Nitrogen fertilizers were adopted over the blink of our generation, following the discovery of industrialized mimicry of biological N fixation.  By the 1980’s fertilizer’s phenomenal boost to agricultural productivity was tempered by questions about where the applied N was going, with primary concerns at the time for water quality.  Those concerns quickly extended to soil quality, resource conservation and use efficiency, energy balances of biofuels, and climate change.  Nitrogen use efficiency (NUE) started as a trending topic and mushroomed into thousands of research papers and no less than 17 definitions. The NUEs calculated from a series of Mitscherlich-diminishing responses, when turned on its head, become the UNRs (unit N requirements) that appear in countless fertilizer guides for predicting yield-based N fertilizer levels that temporally and spatially fluctuate.  Single season estimates of 30% to 50% fertilizer N recovery quickly lead to the quick conclusion that all else is lost to the environment, but is it? Or are we still lacking better accounting of mass conservation in our mechanistic soil-plant models and in our fertility recommendations?  Nutrient balance analysis, an IPNI application of Lavoisier’s Mass Conservation, and our rotational NUE analysis, an accounting for N carryover and cycling, tell a different story of nutrient cycling and greater N recovery than is sometimes portrayed.  Nevertheless, Mitscherlich advises us to back off of maximum yield to further improve NUE, whether motivated by economics or environmental policy. 

Diminishing returns at the cellular level between substrate and reaction rate were also being modeled by Michaelis-Menten, and later applied to root nutrient uptake at the soil-root interface dynamics that underpin field-based nutrient responses.  Ionic interactions and environmental variability modify Mitscherlich responses and add complexity to field applications of the Law of Minimum.

Concomitant goals of greater food, fuel and soil fauna production while reducing reactive N appear counter-opposing but achievable.  We should continue to ground our research, models and policies in these century old laws of soil fertility at micro to macro scales as we develop and assess novel cropping systems.