Woods End Laboratories

Healthy Soil

Healthy Science

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Overview of Soil Health Tool Box

Soil Health Tool

A healthy soil consists of a matrix of biology that directly links with the supply of nutrients, soil structural integrity and in plant disease defense

Precepts for Soil Health Characterization:

  1. Focus is on active and not passive soil fertility;
    Example:
    Passive View: soils as ”Tabula rasa”. Fertility simply means replacing what plants remove.
    Active View: soils can replenish nutrients from “store-houses” that are biologically mediated.
  2. Plants interact with soil;
    Example:
    Cover crops and rhizospheres feed soil microbes which in turn enhance plant growth and pathogen defense.
  3. Soil biology is more involved in supply of crop nutrients than believed;
    Example:
    Tracer studies show less than half of the nitrogen in harvested parts came from added fertilizers. The other sources of the nutrients: biological N-fixation, microbial oxidation and natural weathering. Microbes mediate all the important nutrient-plant transactions.
  4. Economics of soil fertilization is being under-appreciated
    Example:
    Failure to measure potential nutrient pools means farmers may be encouraged to over-fertilize, which drives farm profits downwards.
  5. New test methods are essential in order to better mimic soil-plant processes.
    Example:
    Aggressive, strongly buffered soil extractants used to measure nutrients into ionic-states that are non-existent in nature. Using such powerful extracts to “calibrate” with crop yield

The Soil Health Tool is a set of soil test procedures recently developed through a cooperative research project of  Woods End Laboratories and Dr. Rick Haney (USDA-ARS). The project was launched after the 9th International Symposium on Soil and Plant Analysis held 2005 in Mexico.  It is an open-source, non-proprietary set of methods which comprehensively integrate soil nutrient recommendations with soil biological traits.

THE CONCEPT: Soil Health relates to the active concept of soil fertility, as opposed to a passive one. Soil testing and fertilization is based on modifications of the 19th century German Liebig & Mitscherlich models of  plant growth controlled by the least available nutrient.  In that view soils are managed to remove nutrient limitations and replace elements taken out due to harvesting crops.  The fact that soil and plants interact in the search for and supply of nutrients and that soils actively sustain and defend plants, has been treated as an interesting theory, and largely ignored (if not actively discredited) until recently.

Soil Health Tool

Two new tests from Woods End – CO2-respiration and Labile Amino-N, help review soil biological potentials.

NEW VIEW:  Soils participate intrinsically in providing a wholesome environment for plant growth, even as plants actively participate in building and sustaining soils. This is known from many long term plot studies. The best way to describe the new view is a circular matrix of fertility building, recognized by modern ecologists as a form of niche construction.

SOIL TESTING: To become more environmentally relevant (and to remain truly scientific) soil testing must take account of the underlying biological properties of the functioning plant-soil system. The present harsh economic realities for growers means we can no longer ignore these factors. If soil nutrient supply systems can be better harnessed  the economic upside to growers is huge. It is increasingly inappropriate to continue pouring phosphorus into soils that have received more than a ton/acre of P in the last decade, most of which is still present but “hidden” in soil.

WOODS END ROLE:  Since 1974 Will Brinton has been interested in means to express soil quality.  Woods End Laboratory was founded largely due to the dilemma of nutrient calculations for farmers choosing not to use inorganic fertilizers. Early biological (organic) farmers were forced to translate NPK tables into manure or compost nutrients even though the manner of biological nutrient release and its calibration to conventional soil tests were unknown. Woods End Laboratory performed early research on the ways “green manure” crops interact with soil. More recently Brinton developed a rapid assay for soil biological CO2 respiration (named Solvita after Solum vita),- with the hope of bringing the biological issue more to attention as in “seeing is believing”.

Soil Health Tool

TekmarTorch Analyzer for Soil Water Soluble Carbon and Soluble Organic Nitrogen an essential component of the soil health measurement.

USDA ARS ROLE: The additional linkages helping create the Soil Health Tool package were refinements by USDA ARS of the soluble carbon test and a 24hr CO2-burst protocol. This work was underway by 2000 if not earlier. The new soluble carbon test relates to the biologically active soil carbon pool. The CO2 burst innovation helped make 24hr testing realistic and reproducible in a lab environment.  Basal respiration – the original Solvita test – remains a foundation stone for investigating soil properties as they exist naturally.

NEW SOIL EXTRACTANT: The issue of harsh or highly buffered soil extractants was  a further step. The concern is the modern practice to fertilize to high reserve nutrient levels as determined from aggressive soil reagents, such as Mehlich3. The practice obscures the gentler organic-P cycle. Under some circumstances this source may supply ample P to plants on top of background weakly soluble pools.

A European soil chemist where Brinton trained noted that the normal reagent concentration that soil and plants effectively experience “should be about 0.01 molar”. Instead, labs especially in the USA use either high molar concentrations or highly buffered reagents – in some cases 50 times more powerful than anything present in soil. The new “H3A extract” developed in Dr. Haney’s lab contains plant-relevant organic acids at low concentrations. It is 60-times lower in ionic strength than a standard soil lab extract (compared to Modified Morgan). Importantly, being weakly buffered, its seeks more the pH of soil rather than forcing that soil to a fixed pH which may be irrelevant to circumstance.

CORNERSTONES OF SOIL HEALTH TESTING:  Putting all the new tests together – active respiration, water soluble carbon, a new extractant – results in  what USDA calls the SOIL HEALTH TOOL. Woods End labs is presently one of 2 -3 commercial labs now offering this approach, building on Woods End’s efforts over 30 years to integrate biological tests into chemical procedures. As a work in progress, it needs a period of time where it may be compared to past approaches.

SHORT-TERM HOPES? With new excitement for soil health evident across wide sectors of farming, a caution is needed for those who seek to prove (or disprove) quickly the theme of soil improvement via new soil tests. Soil improvement is slow and takes place over decades. So too, declines are also slow and therefore not noticed to the extent that they should have been.  A new example concerns sensitivity of soil tests to show relevant gains in a short time frame. It has been determined in the EU that it is not possible to prove gains/losses in soil carbon in less than 2 years without a statistical sample of 100 samples per field examined. For routine sampling, a resolution more precise that  5 tons/carbon/acre is barely possible. By drawing on some of the best long term and organic-plot studies conducted especially in Europe over the past 3 decades, we can learn to appreciate the required “imprecision” in short-time of soil quality measurements.

As a summary, the Soil Health Toolbox:

  • Accounts for soil microbial biomass, with a view that this is very nutrient-relevant;
  • Evaluates N + P mineralization potential, the core of the nutrient supply mechanism for plants
  • Represents nutrients likely to be “plant relevant” from lighter-weight extractants
  • Evaluates quality of the pool of soil organic carbon.this page updated March 3 2014

- for instructions on sampling soil, contact: lab@woodsend.org
- for a list of future events about the Soil Health Tool click here


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