Phosphorus, Potassium and Micronutrients. Importance of Phosphorous Phosphorus is the 2 nd most...

Phosphorus, Potassium and Micronutrients

Importance of Phosphorous

• Phosphorus is the 2nd most often deficient nutrient

• Importance for plant nutrition: energy transfer — ATP genetic material — DNA protein synthesis — RNA

Importance of Phosphorous (cont.)

• P balances N −N delays maturity; P hastens maturity−N promotes vegetative growth; P promotes

reproductive growth (flowers/fruiting).−P particularly important in crops in which

we use flowers, fruits, or seeds

• P spurs early and rapid root growth

• P helps plants resist cold and disease

Phosphorus Hastens Corn Maturity

Plants take up P as:

1. Primary orthophosphate ion: H2PO4

- (pH < 7.0)

2. Secondary orthophosphate ion: HPO4

2- (pH > 7.0)

The form most common is a function of soil pH – both equally present at neutral pH

Solution P

P Deficiency Symptoms

Purplish tint or a purplish margin on older leaves

Slow growth; stunted plantsDelayed maturity; poor fruit, grain,

or seed development Plant uptake of P is slow in cool soils

(e.g. corn on cold days in April)

P Deficiency in Corn

Seedlings

P Deficiency in wheat

• Center plot no P was applied. •Stunted growth and poor tillering are most visible early indications of P deficiency in wheat.

Phosphorous Cycle

ADDITIONS: fertilizers/manure WITHDRAWALS: harvest/eroding

sediments

Phosphorus is immobile in the soil (therefore, need to inject or incorporate into root zone)

There is no loss of phosphorus as gas or leaching

Phosphorous Cycle (cont.)

Soil P containing minerals release P in plant available forms VERY slowly

─ amount of P in ‘available’ form at any one time <1%

Phosphorus in fertilizers react with minerals in soil and changed to unavailable forms.

Remember plants absorb P as:

1. primary orthophosphate H2PO4-

2. secondary orthophosphate HPO42-

Impact of P in EcosystemImpact of P in Ecosystem

EutrophicationEutrophication

P erosion into streams accelerates growth of algae and water plants. Subsequent plant decomposition uses up oxygen in the water.

The Phosphorus Problem1. Soil P level usually very low

2. P in rocks and minerals only very slowly available

3. When P is added in fertilizers it becomes “fixed” and unavailable for plant uptake.

− < 30% of fertilizer-applied P taken up by crop during the year it is applied.

− However, P level can be built up and eventually reach a ‘saturation’ point, at which point previous applied P will become available for plants. (Fig 13.17)

Figure 13.17 No off-site movement in drainage water until soil saturation

(lb

./acr

e)

The Phosphorus Problem (cont.)4. Availability regulated by soil pH.

− Maximum P availability occurs at pH of 5.5 to 7.2.

(optimal 6.5)

• pH > 7.2 then Ca and Mg react with P to form insoluble compounds

• pH < 5.5 then Fe, Al, and Mn react to form insoluble compounds

Management for Phosphorus

1. Lime acid soils to pH near 6.5 where P most available

2. Increase organic matter content. -- OM is a source of P as it decomposes

3. Enhance mycorrhizae fungi symbiosis with crop roots.-- mycorrhizae significantly improve P

acquisition.-- mycorrhizae enhanced by crop rotation,

OM addition, and conservation tillage.

Management for Phosphorus (cont.)4. ‘Banding’ vs ‘Broadcasting’ of P fertilizers

− P is immobile in soil. If broadcast stays near the surface

− Place P down in the root zone if possible

− Banding also minimizes P contact with the soil and slows down P fixation

Banded

Broadcast

Grain Drill set up to band liquid P fertilizer

Strip Till Injection of P in root zone

Strip TillStrip Till

CommonCommon Phosphorus Fertilizers Phosphorus Fertilizers

• Monoammonium phosphate (MAP) 11-52-0

• Diammmonium phosphate (DAP) 18-46-0

• Ammonium polyphosphate 10-34-0 (liquid)

• Triple superphoshate 0-45-0

−Ordinary superphosphate 0-20-0-12S

−Phosphoric acid 0-52-0

−Superphosphoric acid 0-68-0

Plants absorb mobile nutrients from large volume of soil

Crop absorption of Crop absorption of mobilemobile nutrients nutrients (Nitrogen) (Nitrogen)

Competition among plants for NCompetition among plants for N

Increasing plant population to obtain Increasing plant population to obtain higher yield creates competition for Nhigher yield creates competition for N

Adding N fertilizer eliminates competition for N among plantsAllows normal plant development and increased yields.

Absorption is from just a thin cylinder of soil around each root.

Crop absorption of immobile nutrientsCrop absorption of immobile nutrients (Phosphorus) (Phosphorus)

If supply of nutrient is adequate for 30 bushel/acre wheat yield, then supply will also be adequate for 60 bushel yield.

No Competition Among Plants For No Competition Among Plants For Immobile Nutrients Even At High Yield Immobile Nutrients Even At High Yield

GoalsGoals

Mobile vs. Immobile Nutrients

Mobile and immobile nutrients like fuel and oil

FuelFuel (the mobile nutrient); required in relation to the amount of work expected from the pickup

• Similarly, N required in relation to the yield expected.- 30 bushel wheat = 60 lbs N/acre- 60 bushel wheat = 125 lbs N/acre- 100 bushel wheat = 240 lbs N/acre

OilOil (the immobile nutrient); required more in relation to the level in the crankcase than by how much work is expected from the pickup.

• Similarly, P requirement determined from soil test and does not depend on yield goal.

Mobility Concept of Nutrients in the Soil

Soil nutrients that are immobile in the soil usually more effective if they are incorporated or injected below ground to where the roots can access.

Soil nutrients that are mobile can be broadcast on soil surface and rain/irrigation will leach them into the soil

Importance of Potassium

Potassium (often called potash) is taken up by plants as the K+ ion.

K+ remains in ionic form in cells and plant tissue (not synthesized into compounds).

K+ activates enzymes for the formation of protein, starch, cellulose, lignin, etc. (> 60 enzymes known to need K)

Importance of Potassium (cont.)

Potassium promotes “tough” growth (vs. N promotes soft growth). Plants well stocked with K have strong stems.

Promotes winter-hardiness.

The Potassium Cycle

Western Oklahoma soils are naturally very high in potassium… K fertilizers seldom needed.

OM not an important source of K+ .

No gaseous loss or microbiological process affecting K

K+ does leach (especially sandy soils)… but leaching less than NO3

-

Potassium DeficiencyCommon in crops grown in weathered

soils developed under high rainfall.

K Usually adequate

K Usually deficient

Deficiency Symptoms

1. Leaf tips and margins “burn” starting on old leaves.

2. Weak stalks, plants lodge easily.

3. Reduced drought tolerance

4. Legumes with small, white spots along leaflet margins.

K deficiency symptom in corn

K deficiency symptom in Alfalfa

K deficiency symptom in Soybean

Common Potassium Fertilizers

Potassium chloride 0-0-60- KCl - muriate of potash

Potassium sulfate 0-0-50-17S- K2SO4 - Sulfate of potash

MICRONUTRIENTS

Micronutrients (Trace Elements)

Iron (Fe) Chlorine (Cl)Manganese (Mn) Cobalt (Co) Boron (B) Molybdenum (Mo) Zinc (Zn) Nickel (Ni)Copper (Cu)

Concern for Micronutrients

1. Increased crop yields have resulted in greater removal of micronutrients from soils.

2. Modern, high-analysis fertilizers have little impurities. The older, less-pure fertilizers contained some micronutrients.

Concern for Micronutrients (cont.)

3. Increased knowledge of plant nutrition, and improved methods of analysis in the laboratory, are helping diagnose micronutrient deficiencies that might formerly have gone unnoticed.

4. Food grown on soils with low levels of micronutrients may provide insufficient human dietary levels, even though crop plants themselves show no signs of deficiency.

Micronutrients

Dominant role in plants is to activate enzymes.

Very minute amounts used; difference between sufficient and toxic level small

Apply only if soil and/or plant tissue tests indicate need.

Micronutrients Most Often Deficient In:

1. Sandy, leached soils

2. Soils with high pH -- Fe, Zn, Mn, Cu deficiency at high pH

3. Soils that have been intensively cropped and heavily fertilized with macronutrients only

Iron (Fe)Important part of many enzymes

(including formation of chlorophyll)

Taken up mainly as Fe2+ ion

Availability decreases at high pH (acid= available; alkaline=unavailable)

Iron (Fe)Deficiency symptom: interveinal

chlorosis on new leaves.

Deficiencies common in sorghum, soybeans, OWB’s

Variety differences?• EX: WW Ironmaster, soybeans

Interveinal Chlorosis (sorghum)

Iron Chlorosis (sorghum)Iron Chlorosis (sorghum)

Iron Chlorosis (soybean)

Iron Chlorosis (rose bush)

Iron (Fe)Oklahoma soils contain large amounts of

iron (>50,000 lb/A). However, most of the iron is in a form like rust that is not available to crops.

Correction of deficiency difficult!Soil application of iron containing

fertilizers quickly converted to unavailable forms.

Zinc

Plants absorb as Zn2+ ion Required for chloroplast and starch synthesis Zn deficiency occurs on high pH soils

[unavailable form at high pH] Deficiency common where topsoil cut off in leveling

land. Deficiency dependent upon plant species.

− Pecan, corn, cotton and soybeans more susceptible. − Zinc deficiency has yet to be found in wheat in Oklahoma.

Usual deficiency symptom is yellow to white areas between veins of leaves and shortened internodes.

Soil and foliar applications to correct deficiencies. Soil application of 4-6 lb/acre of zinc… lasts 3-4 years.

Zinc Deficiency (corn)

Characterized by a broad band of bleached tissue on each side of the midrib beginning at the base of the leaf. The midribs and leaf edges remain green.

Zinc Deficiency (cotton)

BoronRequired for cell division, water relations

Peanuts, alfalfa, cotton, sugar beets, sunflower sometimes deficient.

Most available in acid soils.

Usual deficiency symptoms: −Necrosis of terminal growth, causing lateral buds to

develop and producing a “witches broom effect”.−“Hollow heart” of peanut

Boron Deficiency Symptom: Hollow Heart of Peanut

Boron Deficiency Symptom:

Death of Growing

Point (alfalfa)

BoronStored mainly in OM; Can leach.

Addition of boron fertilizer as “insurance” is a dangerous practice; very little difference between ample boron and toxic level of boron.

Boron fertilizer recommendations usually < 1 lb/Acre

OTHER MICRONUTRIENTS AND DEFICIENCY SYMPTOMS

Expected occurrences of these deficiencies in Oklahoma are rather remote

ChlorineFunction not well understood Appears to inhibit some diseases (rusts and

take-all in wheat).− Some crops very sensitive (tobacco, some fruit

trees). Needed in very minute quantities.

− Researchers found touching plant leaves with their fingers transferred enough chlorine from perspiration on their skin to meet the plant’s requirement.

Deficiency symptom is yellow blotches on mature leaves

Limited to areas where potassium fertilizer (KCl) not used

Chlorine Deficiency Symptom