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Soil health: What the soil test doesn’t tell you

Eric Young Published on 01 May 2014

A simple definition of soil fertility is a soil’s capacity to produce crops. For practical reasons, traditional fertility emphasizes soil chemical aspects that affect yield and nutrient availability.

In reality, crop yield and nutrient dynamics are the sum total of soil conditions in a given year, including the effects of tillage and other practices. In short, fertility in the field is a process.

The idea that crop growth potential is a function of multiple interacting factors is the basis of soil quality or soil health (two terms commonly used interchangeably).

The concept of soil health attempts to better integrate biological, chemical and physical factors and their interactions with the growing crop as the basis for soil management.

While soil health might seem like a buzzword, a concerted research effort by USDA scientists and others has occurred over the last several decades to better understand biophysical and chemical constraints on crop growth. Soil health can be thought of as having a “native” aspect and a more dynamic “management” aspect.

The native aspect of soil health reflects inherent soil properties such as texture/soil parent material, drainage class, organic matter content, N mineralization potential and water-holding capacity.

Management aspects focus on cultural practices that can lead to compaction and degraded soil, such as excessive tillage or erosion. Native and management aspects of soil health often interact with management.

For example, reducing compaction increases air and water infiltration into the soil; this in turn impacts oxygen status, root growth, nutrient uptake and soil N reactions, all of which impact crop growth and final yield.

While traditional agronomic soil testing remains the mainstay of land-grant university fertility guidelines, this approach can miss important physical and biological constraints on yield and nutrient dynamics.

For example, soil physical properties, compaction, seasonal nitrogen (N) availability and weather are all important variables explaining yield variation, yet not accounted for by routine fertility guidelines.

Assume a field has a silt loam texture and is derived from limestone. The soil test reported high P and K levels, an optimal pH at 6.8 and 4 percent organic matter content.

Does this alone imply high fertility? What about soil type and drainage impacts? What about compaction? How much N will be mineralized? All of these factors can have a strong impact on how fertile a given field is in a given year.

Like soil testing, soil health recognizes the importance of “the law of the minimum,” or the idea that crop yield is constrained by the most limiting nutrient or factor present.

If nutrient availability is high but soil compaction is limiting root growth, the high nutrient status is irrelevant from the crop’s perspective until compaction is relieved. No amount of any other factor will compensate for the compaction.

Soil health-based approaches also embrace a more “adaptive” approach to management, whereby in-season practices can be modified based on new information.

As an example, if moderate to severe soil compaction is found in a field, a farmer might then decide to reduce economic risk by choosing to seed it down to hay instead that season.

Identifying compacted fields and areas also determines where other practices might be needed to help alleviate compaction (e.g., deep tillage, improved drainage, crop rotation).

Soil health also attempts to better account for soil biology and its impact on N availability. Recall that most soil N resides in organic matter.

The conversion of organic N to plant-available forms (nitrate-N and ammonium-N) over the season is governed by bacterial activity, which is highly dependent on soil temperature and moisture (e.g., weather).

Many land-grant university agronomic recommendations use “book values” for assigning N credits (e.g., amount of N released by soils, manure and previous crops over the growing season) to simplify N recommendations.

While this approach is justifiable, it misses important dynamic processes, namely, weather-related N transformations.

The Cornell Soil Health program has developed a process-based simulation model that attempts to predict sidedress N needs for corn (Adapt-N).

Adapt-N is based on many years of controlled field research and modeling studies. It models corn growth/yield, N, N uptake, mineralization, denitrification, volatilization and N leaching losses using site-specific weather, soil and management information.

While more fieldwork is needed to further validate and calibrate the model, it is a good example of a soil health-based approach for making N recommendations (See more about this tool at this Cornell University).

The USDA-NRCS and many land-grant universities have active soil health research programs. Soil health will likely continue to grow in scope and application because it can help lower risk.

Reducing crop production risk is a must as farmers are expected to produce greater and more efficient crop yields on every acre. In the future, using more complex models for crop nutrition that incorporate soil health may indeed become the norm, not unlike the way nutrition models are used in dairy ration formulation.

Soil health assessment can be thought of as another tool to help farmers manage and reduce crop production risk. As greater amounts of soil health information becomes available in the future, producers will need a practical way to utilize the information and develop an understanding of how it fits in with more traditional fertility guidelines.  FG

Eric Young

Eric Young
William H. Miner Institute