Fertilizer Use Recommendations: Sample, Analyze and Interpret

This Extension Spotlight focuses on fertilizer use recommendations and how they are developed.

First, farmers sample soil and submit it for analysis. Then, labs and consulting firms return reports usually containing two types of information. We will get to what happens in between but let’s first examine the lab report.

The report usually contains two major sections. One section describes the primarily chemical characteristics of the soil such as its cation exchange capacity (CEC), pH, and nutrient levels. The other section tends to offer recommendations on fertilizer inputs based on results of the soil analysis.

Soil analysis by accredited labs is a relatively routine, clear, and standard process. In fact, the same soil submitted to different labs for analysis should produce very similar data. Recommendations on fertilizer use, however, can vary with who makes the recommendation. Why? How are recommendations developed?

In short, recommendations boil down to chemistry, math, plant biology, testing, and experience. Let’s explain.

Data from soil analysis are entered into formulae built from extensive work in correlation and calibration. Correlation describes the relationship between the amount of a nutrient that can be chemically extracted from soil in the lab and the amount of the same nutrient that can be extracted by plant roots in the field. Extraction with chemicals in the lab and by plant roots in the field is different. So, correlation is needed to help predict the relationship between the two approaches. Second, calibration clarifies the difference between a measured soil test value and the yield that may result when a nutrient is added as a fertilizer. Calibration should clarify yield at X lb/A versus at 2X lb/A and so on. Finally, the knowledge base of the recommender comes into play. They must interpret multiple types of information – soil analysis, soil types, cropping patters, etc – to develop a fertilizer “prescription” that fits the crop. This prescription may be specific to the variety, planting date, and other factors. Reliable recommendations are based on insights gained through many cycles of correlation and calibration and evaluating on-farm experiences.

The development of fertilizer recommendations is explained very well in a number of references. One of our favorites is free and available online from the University of Nebraska-Lincoln (thanks in part to federal funding). The UNL reference stands out partly because it explains three major philosophies used by folks who make fertilizer recommendations. Other references such as Simonne et al. and HortTechnology (21:656-685) further explain how fertilizer recommendations are being modernized.