Phosphorus and Potassium

CNMP Core CurriculumSection 5 – Nutrient Management

CNMP Development Core Training Curriculum

These course materials have been developed as a cooperative effort between five land-grant universities and The Natural Resources Conservation Service.

Ames, Iowa 50011, (515) 294-4111.

Copyright © 1995-2006, Iowa State University of Science and Technology. All rights reserved.

Copyright Information

Objectives

• Understand the P cycle

• Learn effects of P in the environment

• P management exercise

• Understand K+ cycle and management issues

Required and Supplemental Materials

• Crop nutrient removal charts specific for phosphorus.

• NRCS 590 Nutrient Management Standard. • Land Grant University fertilizer recommendations

for phosphorus and potassium for relevant crops.• Supplemental Resources

– A complete resource on phosphorus is: Understand Phosphorus, University of Wisconsin A-3771 found at http://ipcm.wisc.edu/pubs/nutrient/USP_2002.htm

Phosphorus

• P cycle

• Soil and plant characteristics

• Environmental resource concerns

• Phosphorus management

The Phosphorus Cycle

Solution PSolution P

CropHarvestCrop

Harvest

Manure PManure P

Fertilizer PFertilizer P

CropResidue

CropResidue

LabileLabileStableStable

Organic P

LabileLabile

Inorganic P

StableStable

From: Livestock and Poultry Environmental Stewardship 34-6LeachingLeaching

Soil Phosphorus

• Derived from P bearing rocks & minerals

• Different forms: organic/inorganic, soluble/insoluble

• Very reactive: Fe, Al, Ca• Limiting nutrient in many unfertilized,

acid soils

Soil Phosphorus

– Maintaining the soil solution concentration for plant uptake is important.

– Nutrient strategy

RangeMaintenance

Range

Range

Nu

trie

nt

Rat

e

Buildup Drawdown

Level

Soil Test Level

Critical MaintenanceLimit

Soil Test Phosphorus

• Agronomic soil tests are used to determine plant available P.

• The tests are a prediction of the amount of desorbed labile (readily available) P into soil solution from the soil mineral surface, over the growing season.

Environmental Concerns

• Phosphorus is adsorbed to soil particles and moves with soil

• Pathways that transport soil to surface water are the primary pathways for P movement to surface water

Pathways for P Loss from Soils

Environmental Resource Concerns

Phosphorus in Water

• Often limiting nutrient in fresh water systems

• Addition stimulates algal growth

• Too much P = less dissolved O2

• Excessive P = EUTROPHICATION

Managing Agricultural P

• Minimize P sources– Diet manipulation– Manure technologies - No reduction in P,

but may increase options for hauling greater distances or marketing

• Fertilizer Management• Conservation Practices• No direct application of P to water

Dietary Manipulation Manure Treatment

Less Available P in Manure

Agronomic Application Rates

Erosion Control

Buffer Strips

Improved Water Quality

CNMP Development

• Create a Farm P Balance– Current soil tests– Current manure tests

• Determine crop P needs

• Use manure and wastewater volumes and manure test to determine P2O5 to be applied

• Use risk assessment tools to determine the lowest risk application areas

P Based Application Rate

• Soil Test Phosphorus (STP)

– Application based upon soil test analysis, and crop P needs, based on university recommendations

– Fertility strategy• Buildup low P soils• Maintenance• Drawdown

Nutrient Removal

• When STP values are optimum, high, or very high, it is recommended to apply on a P removal basis

• The nutrient removal can be calculated by using a removal factor per unit of yield for the specific crop

Corn bu 0.375 0.30

Plants Database

http://www.plants.usda.gov/

Phosphorus Removal• Corn removes 0.375 lbs P2O5 per bushel

• 150 bu x 0.375 = 56 lbs

• This corn crop would remove approximately 56 lbs of P2O5 per acre

Photo: University of Illinois

CNMP Problem Fields

• Historic P loading (often near production site)– Drawdown mode desirable– Mining results are widely variable

• Few years to few decades, function of soil mineralogy and chemistry

– Utilize risk assessment tool• Low risk: load, but poor use of plant nutrient• Medium: emergency load• High: restrictions

CNMP Problem Fields

• Low STP – Buildup mode desirable

• High risk site– PI high (Tennessee 201-300)– Transport factors too high

(slope, surface water, coarse textured soil)

• Siting problems– Floodplain, neighbors,

subsurface drainage

Example

If the phosphorus index is in the high-risk category, and manure can be applied only on a P removal basis, determine the amount of manure that can be applied to a field for a silage crop that yields 23 tons per acre?

Required Information

Corn silage yield 23 tons per acre

Manure P2O5, based on manure test

3 lbs/1000 gal

Total Manure N, based on manure test

10 lbs/1000 gal

P removed

USDA Plants Database

30 lbs of P per acre

Answer•To convert P to P2O5, multiply P x 2.29.

•30*2.29= 68.7 lbs P2O5 removed per acre

•Apply 68.7 or 69 lbs. of P2O5 per acre to meet the P removal rate.

•Manure analysis: 3 lbs P2O5 /1000 gallons

•69 lbs. of P2O5 ÷ 3 lbs. of P2O5 per 1000 gallons = 23 gallons

•23 gallons x 1000= 23,000 gallons of manure

•We can apply 23,000 gallons to meet the P removal rate

Other Considerations

• The method of application should be considered, in this instance, can the soil accept and hold 23,000 gallons if it is irrigated? If it were applied in one pass via injection, or via multiple applications that add up to 23,000 over time?

• What rate of other nutrients will be applied (specifically N and K) at this application rate, and are they agronomically acceptable?

Potassium

Potassium

• Potassium Cycle

• Ruminant considerations– Dairy herds, primarily lactating cows

• Potassium management

The Potassium Cycle

Potassium characteristics

• Soils can have > 40,000 lbs. Total K+ ac-1

• Almost all fixed in clay bridges and unavailable for plant uptake

• Exchangeable K+ is bound to soil particle surfaces, and is desorbed into soil solution at 1-10 ppm

• Not easily leached in soils, but can be leached

Excess K+ Concerns• Grass Tetany

– High levels of K+ and N in the plant can cause reduced amounts of plant magnesium (Mg)

– This can cause a Mg deficiency in grazing animals, and cause a condition called grass tetany or hypocalcaemia

– Avoid over-application of K+ to pastures

• Environmental Concerns– Generally, excess K+ in surface water has not been a

concern

• Agronomic considerations– Soil Test– Provide adequate K for crop production

Summary

• Understand the source and transport characteristics of the landscape

• Develop a crop budget using nutrient strategies and risk assessment tools

• Evaluate performance using good records

• Check (often) laws, permit requirements, and regulations governing nutrient management

Questions?