Approximating Solubility/Availability & Communicating Information

Transcription

1 Approximating Solubility/Availability & Communicating Information Wayne P. Robarge Professor (Emeritus) Soil Physical Chemistry Department of Crop and Soil Sciences NC State University, Raleigh, NC Presented at: Fertilizer Method Development, Validation & Use AAPFCO Methods Forum XVII, Savannah, GA Feb , 2018

2 Outline What does water solubility mean to users (actually plants)? Coated fertilizers/soil amendments release curves? Why soil matters (solubility is in the eye of the beholder)? Soils contain many chemically-reactive surfaces Why plants matter (plants are biological systems/time sensitive)? Plant roots/bacteria/fungi (rhizosphere) know soil chemistry Can/should we try to assess availability (the bleeding edge)? We have techniques that work (greenhouse/field trials) Updating our current soil test plant correlations knowledge base?

3 What does water solubility mean? (A Traditional Perspective?) Soil Organic Matter Plant Roots Soil Flora and Fauna Ions in Soil Solution Leaching Fertilizer Granule Intermediate Reaction Products Clay Minerals Fe, Al, Mn Oxides and Hydrousoxides

4 What does water solubility mean? (Mass Flow versus Diffusion/Active Uptake) (Schematic Courtesy of Dr. John Havlin, NCSU)

5 Why Soil Matters (You cannot fight thermodynamics) Increasing Binding Energy SOM Complexes Inner/ Outer Sphere Complexes Solid Solutions 3-D Solid Matrix Co-Precipitates

6 Why Soil Matters (How long will it stay soluble?)

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8 Soil Solution Extraction/Measurement What is the soil solution - obvious batch solid/solution ratios > 1:2 probably not soil solution. Various methods that all yield results : Saturated paste Suction lysimeters (large and small - e.g. the Rhizon ) Tension lysimeters and hanging columns Zero-tension lysimeters Pressure extraction (gas or centrifugation) Immiscible displacement (centrifugation) Diffusion gradient/gradient techniques (bleeding edge) Is there a right answer?

9 Quick Word About Filtration What is soluble and what is not? There is the ideal and then there is reality. The act of filtration is defining what is soluble and is what is not soluble. Historically - soluble has meant what passes through a 0.45 micron filter. Why? Because in 1970 s EPA needed to define soluble and the most common filter available was Whatman cellulose acetate membrane filter with an effective pore diameter of 0.45 microns. Thus, soluble became < 0.45 microns. Today, have wide range of filters with either sharp size cutoffs (etched membrane filters) or effective pore diameters (cross-linked).

10 What Is Soluble? ( Dissolved Reactive P vs Dissolved Fe in Stream Water) Data provided by Mark River and Curt Richardson, Duke University, Durham, NC.

11 Why Plants Matter (Biological Systems/Roots Change in Space & Time/Rhizosphere)

12 What Controls Cd Bioavailability in Tobacco? (Known Accumulator/NC Soils Low in Cadmium - Cd) (2013 Mass Balance Study At Single Field Near Bath, NC)

13 Working With Biological Systems

14 Examples of Plant Uptake Throughout Season (Cadmium - Cd and Calcium Ca)

15 Examples of Plant Uptake Throughout Season (Copper - Cu & Zinc Zn; Nickel Ni & Selenium - Se)

16 Plants Are Not Passive (Root exudates/mycorrhizal Associations/Nutrient Sensitivity) (Schematic courtesy of Dr. John Havlin, NCSU)

17 Can/should we try to assess availability? Solubility is important (coded into law; e.g. liming products) Solubility vs. potential reactivity in soil environment? Plant is best indicator of bio-availability Variety of techniques each with their limitations Resurgence in Rhizosphere Research Better more affordable analytical instrumentation Genetic profiles now combined with classic investigations Please don t forget chemical thermodynamics (computer models) We need to update our current soil test knowledge base! Genetic changes in crop varieties/higher yield potentials Changes in fertilizers, amendments and other additives