Phosphorus Chemistry in Soil and Water Systems

1
Phosphorus Chemistry in Soil and Water Systems

• Neil Hansen
• WCROC, Morris
• Department of Soil, Water, and Climate

2
(No Transcript)
3
(No Transcript)
4
Phosphorus Essential For Plants

• Element Relative Amount
• Molybdenum 1
• Copper 100
• Zinc 300
• Manganese 1000
• Iron 2000
• Boron 2000
• Chlorine 3000
• Sulfur 30000
• Phosphorus 60000
• Magnesium 80000
• Calcium 125000
• Potassium 250000
• Nitrogen 1000000

• 0.1 - 0.4 of dry matter
• ATP (energy)
• DNA, RNA
• phospholipids

N Hansen, 2001
5
Phosphorus Deficiency

• Stunting
• Limited root growth and branching
• Thin stemmed
• Delayed maturity
• Poor seed quality

N Hansen, 2001
6
Eutrophication

• Nutrients stimulate algal productivity
• Decay of algae depletes oxygen
• Light penetration limited

N Hansen, 2001
7
(No Transcript)
8

• Most of the Earths P is in the form of very
  slowly soluble minerals for example
• Tricalcium phosphate Ca3(PO4)2
• Fluorapatite 3 Ca3(PO4)2CaF 2
• Variscite AlPO42H2O
• Phosphate PO43-
• Phosphoric acid H3 PO4
• In soil solution HPO42- and
  H2PO4-
• In agriculture, P is most often expressed as P2O5
• P x 2.29 P2O5

N Hansen, 2001
9
N Hansen, 2001
10
Soil Phosphorus
N Hansen, 2001
11
Soil Phosphorus

• Total P concentration for MN soils averages 1200
  lbs P/ac
• Soluble P concentration averages 6 lbs P/ac

N Hansen, 2001
12
Soil Test P

• Extractants used to estimate the amount of P that
  will be available to a crop.
• Soil Test P includes soil solution P and a
  portion of the labile P.

Labile P
(1)
P in the Soil Solution
Slowly available P
N Hansen, 2001
13
STP Extractants

• Bray (HCl and NH4F)
• Olson (NaHCO3)
• Mehlich (HCl and H2SO4)

14
N Hansen, 2001
15
Contrasting P Issues

• Phosphorus losses of environmental importance are
  very small relative to phosphorus inputs.
• Cost of excess P are downstream and cross many
  boundaries

16

• Lake water concentrations between 0.01 and 0.02
  ppm generally accelerate eutrophication
• Soil solution concentrations 0.2 to 0.3 ppm

N Hansen, 2001
17
Phosphorus In Water
N Hansen, 2001
18
Phosphorus Chemistry in Water

• Bio-availability
• Particulate P Variable (5-10)
• Dissolved P Completely

Total P Soluble (Y x Particulate)
N Hansen, 2001
19
Crop and Environmental responses to applied P are
management dependent

• Application Rate and frequency
• Tillage Practice
• Placement
• Source

20
P Management For A Corn-Soybean Rotation
Sam Evans, George Rehm, Neil Hansen, George
Nelson WCROC Morris

• Variables
• Tillage system
• Fertilizer Rate
• Annual vs. biennial application
• Fertilizer Placement

N Hansen, 2001
21
N Hansen, 2001
22
(No Transcript)
23
2000 Rain Simulations on 36 Plots
N Hansen, 2001
24

• Only tillage had a significant effect on Total P
  loss in runoff
• Neither fertilizer rate nor placement affected
  total P in runoff, despite large differences in
  STP
• Soluble P losses were the same for all treatments

N Hansen, 2001
25
N Hansen, 2001
26

• Eroded Particles are a major source of P in
  runoff
• P concentration of eroded particles is enriched
  relative to STP
• Particulate P is not all bio-available
• Can settle out

N Hansen, 2001
27

• Soluble P in runoff can originate from soil,
  plant tissue, or applied P
• All soluble P is bio-available
• Not settled out

N Hansen, 2001
28
Estimating Risk of P moving to surface waters
-The P Index-

• Source characteristics
• Soil test P
• P application rate and source
• P application method
• Transport characteristics
• Soil Erosion
• Runoff class
• Connectivity to surface water

N Hansen, 2001
29
Phosphorus Index - Transport Factor Table
N Hansen, 2001
30
Source Factor Table
31
Risk Level

• Risk Level Source X Transport
• The site is then classified for risk
• Management Practice Recommendations based on
  outcome

32
Development of MN P Index

• If not developed properly for MN conditions
• Fail to meet water quality goals
• Farmers may incur undue regulatory pressures
• Validation is critical
• Soil types and landscapes
• Management practices
• Climatic conditions

N Hansen, 2001
33
Process Overview

• Identify soil P categories
• Develop a field scale risk assessment index
• Evaluate the P index on a field scale
• Evaluate the P Index for water quality risk
  assessment at the watershed scale

N Hansen, 2001