Tales From A Poopsmith

Orthophosphate Structure in simple form

Phosphorus is a macronutrient needed for life to exist.  Phosphorus is primarily made available to plants through the mineralization of organic material (biomass, organic matter, plant residue or agricultural by-products) and the dissolution of primary and secondary phosphorus minerals.  The fate and rate of the phosphorus availability in a system depends on multiple factors which are ultimately site specific.  These factors include soil chemistry, temperature, management practice, pH and the physical soil make up including texture, density and structure.  Since phosphorus is becoming the new standard element to be tracked for pollution on dairies and agricultural land it, is important we understand the inputs and outputs intimately.  When attempting to account for the phosphorus in the system it is vital to use the appropriate testing methodology.   Failure to use the best practices for testing could skew regulation to one side or the other.  (Barrow, 1974; Fuhrman et al. 2005; Sims ed. 2005; Rowell, 2003)

When breaking down the testing soils for phosphorus there are three primary approaches to testing: (Toor et al. 2006; NRCS 2008; Davenport, 2007; Arie, 2007; Kuo, 1996)

1)      Immediately available soil phosphorus which is considered available to the plant immediately. 

Methods include Water Extractable Phosphorus and Ion-Exchange Membranes (Anion Exchange Resin).

2)      The labile phosphorus which could be available to the plant throughout the growing season.

Methodsinclude Olsen, Bray, and Mehlich 1 or 3.

3)      The non labile phosphorus which is contained in the clays or secondary minerals which is only available to the plant as it is broken down over time, usually a very slow process.  

Methods include Sequential Phosphorus Fractionation, Nuclear Magnetic Resonance Spectroscopy and X-Ray Absorption Near Edge Structure Spectroscopy.  

For the purpose of this post I would like to explore the pros and cons of these methodologies and which are the most useful to science and agriculture respectively. 

Please take a look at these three approaches and give me some feed back on the usefulness of a method and some of the factors affecting your answer. Please indicate if you are referring to a specific cropping system.    

References available just ask.