A. Phosphorus form in solution is a function of pH. The most abundant phosphorus species in the usual range of soil pH
     are orthophosphates.
B. Quantity vs. Intensity - This is the concept of buffering capacity. The total quantity of P in soil is related to the amount of
     P in solution (Intensity)
C. Mass flow provides only 1 to 20% of required P to plant roots
D. Root interception and diffusion provide most P to plant roots
E. Equilibria and definitions
  1. Solution P ⇄ Labile P ⇄ Nonlabile P
  2. Solution P - phosphorus in soil solution
  3. Labile P - readily available portion of the quantity factor. It is not in solution, but is readily soluble to enter solution    
      quickly.
  4. Nonlabile P - structural phosphorus, in organic or mineral compounds, slowly available, mostly insoluble compounds
      and minerals

A. ≈ 2% nucleic acids: DNA, RNA
B. ≈ 1% phospholipids: fatty acid esters, many derivatives of glycerol, lecithin
C. ≈ 35% inositol phosphates; sugarlike compounds, C6H12O6; phytic acid (hexaphosphate) most common
D. ≈ 62% other esters and substances of microbial origin
E. Mineralization and immobilization occur: microbial phosphatases active in mineralization

A. Sorption-desorption, precipitation-dissolution reactions determine the amount of P in solution (intensity)
B. Definitions
    1. Sorption - removal of P from solution and retention at soil surfaces, since P is an anion, this is often through    
        association with cations adsorbed to cation exchange sites, or through anion exchange sites in soil organic matter
    2. Desorption - release of sorbed P into solution
    3. Fixation - both sorption and precipitation reactions of P, collectively
C. Phosphates exist in compounds of varying solubility in soil
D. Fertilizer P applications result in increased P intensity in solution, which may cause precipitate reactions that limit P
     availability and solubility
E. Sorption occurs with CEC sites through cations adsorbed to them and variable charge surfaces (pH dependent sites,
    AEC)

A. Nature and amount of soil components (parent materials rich or poor in P, organic matter content)
B. pH - determines which mechanisms are dominant
C. Presence of cations or anions on exchange sites or in solution
D. Kinetics: temperature, time
E. Saturation of sorption complexes makes P more available (as in long-term fertilizer P applications)
     but also increases environmental implications of P management
F. Organic matter
  1. Generally increases P availability
  2. May increase leaching losses due to presence of organic acids that increase P solubility