Phosphorus

Chapter 4 in Lecture Manual

Labile P

the portion of Orthophosphate-P loosely associated with clay mineral surfaces and is available to crops during the growing season

Non-labile P

the portion of orthophosphate-P tightly associated with clay mineral surfaces and is not available to crops during the growing season

Erosion

the movement of soil and nutrients from their placement position due to wind and water

What are the P forms in nature?

Use of orthophosphate-P and forms used by plants

Plants uptake P as Orthophosphate Ions

H2PO4-,H3PO4, HPO4²-

Form depends on pH of soil

Ortho P used for energy storage (ADP/ATP), nucleic acids (DNA/RNA), and enzymes in cell membranes (phospholipids)

Ortho-P movement in soil

Highly immobile, moves by diffusion

Fixation with Fe and Al in acidic soils or with Ca in calcareous (high-pH) soils reduces P movement.

High clay soils also have reduced P movement

P nutrient cycle differences from N cycle

N Cycle: main loss is leaching & denitrification, nitrate is very mobile, has a large gaseous phase

P cycle: main loss is erosion, very low mobility, no gaseous phase, no biological fixation, organic forms (crop residue, manure, soil O.M.) must mineralize to release plant-available ortho. P

Status of Ortho-P Ion in Soil

solution ortho-P is a small fraction of total soil ortho-P

Most of ortho-P is associated with soil components (labile and non-labile P)

What factors influence soil ortho.-P availability to crops?

Influence of clay content in soils (quantity and clay type)

• As soil clay content increase, labile and non-labile ortho-P increases (Soils with more clay hold P tighter)

• Clay types - age (degree of weathering) is more impactful than structures, as total surface area increases, more ortho-P can be sorbed and fixed by Al and Fe minerals

Soil pH - maximum ortho-P availability occurs in a pH range of 6-7

With time, ortho-P forms compounds that become less available to crops

Identify popular synthetic fertilizer materials and chemical formula

Crushed Apatite - chemical form is orthophosphate-P

N-P combinations (most common used in SD)

• MAP - Mono-Ammonium Phosphate (11-52-0)

• DAP - Di-Ammonium Phosphate (18-46-0)

• NH4-polyphosphates (10-34-0)

Effective management of fertilizer P applications

Always use soil test results to determine amount needed

Delay P application 3 weeks after liming (Ca in lime will bind to the P fertilizer)

Subsurface application is ideal, can apply surface for perennial crops

Use tillage, starter P bands and strip till to place P fertilizer in root zone

Environmental risks associated with soil ortho.-P

Erosion: causes eutrophication in lakes, lowers profitability, threatens the enviroment

erosion loss is the greatest risk associated with P use

What can minimize soil ortho.-P erosional losses?

Minimize runoff, reduce tillage, use cover crops, manage runoff and drainage areas

chemical changes for labile ortho.-P

ortho. P ions form a single ‘covalent’ O-P bonds with clay mineral surfaces

chemical changes for non-labile ortho.-P

ion forms two ‘covalent ‘O-P bonds with Al or Fe on clay mineral surfaces