Phosphorus & Potassium

Roles of Phosphorus: 1. Essential for plant growth 2. Plays a role in photosynthesis, respiration, energy storage and transfer, cell division and other processes. 3. Promotes early root formation and growth 4. Vital in seed formation 5. Strengthens structural tissue – prevents lodging 6. Improves winter hardiness 7. Improves forage crop quality 8. Increases water use efficiency- reduces water stress 9. Can boost yield levels and reduce grain moisture levels 10. Increases uptake of Mg – prevents grass tetany

Plant Utilization: CropYieldP 2 O 5 taken up Alfalfa8 ton120 lb Bermuda8 ton96 lb Corn160 bu91 lb Cotton1000 lb51 lb Wheat60 bu41 lb Soybean60 bu58 lb

Phosphorus Deficiency: Symptoms: – Overall stunted appearance – Leaf shape is distorted – Dead areas arise on leaves – Purple or reddish color on leaves

A grower can expect to get 20-30% efficiency from water soluble Phosphorus fertilizer the first year after application.

Fertilizer Sources of P Available P Unavailable P

Plant Absorption of Phosphorus: Plants absorb most of their P in the orthophosphate ion (H 2 PO 4 ) dissolved in soil water. It may also be absorbed in the HPO 4 - ion but in smaller quantities.

 Primary orthophosphate ion: H 2 PO 4 - (pH < 7.0)  Secondary orthophosphate ion: HPO 4 = (pH > 7.0)  The form most common is a function of soil pH – both equally present at neutral pH Plants Take Up P As: Solution P

Where does P come from: P comes from the chemical weathering of apatite rock. The higher the ph of the soil the more Hydrogen ions are given up to form the 2 orthophosphate ions. Decomposing organic matter, humus and microbes. – 20-50% of soil P is in O.M.

How Does P Move In The Soil? There is no gas form of P P does not leach (unless it is in extremely high amounts) It is not subject to oxidation-reduction It does not move- except through diffusion

P Movement: It has been estimated that P greater than ¼ in away from a plant root will never move close enough to be taken up by the root.

Phosphorus mobility in plants: P is mobile in plants and moves readily from older tissue to newer tissue.

Other forms of P: Soluble forms of P will form compounds with: – Calcium in alkaline soils – Iron in acid soils – These forms are not available to plants

Factors Affecting Soil P Availability: 1. Amount of clay – clays fix P 2. Type of clay – Kaolinitic clays fix more P than other clays 3. Time of application – The longer the soil and added P are in contact the greater the chances for fixation. 4. Aeration – for microbial breakdown and plant growth. 5. Compaction – influences aeration

Factors Affecting Soil P Availability: 6. Moisture – inc. soil moisture to optimum levels make P more available 7. Temperature – influence plant growth and O.M. decomposition. 8. Other nutrients – Calcium increases P uptake 9. Soil pH 10. Crop root type

Plant root Higher P concentration Lower P concentration Diffusion

Key to P Fertility: P does not move in the soil P does not change forms So Soil: pH Microbial activity Weathering Having crop residues Using Manures Commercial fertilizers

Soil pH effect on P optimum P availability – pH 6.0 to 7.5 acid soil (pH < 6.0) – lime to bring pH up alkaline soils (pH >7.5) – too costly to acidify S can be used

Ammonium phosphates  most commonly used P-fertilizers –ammonium orthophosphates  nearly 100% water soluble

Other sources of P  monoammonium phosphate (MAP) –NH 4 H 2 PO 4 –  diammonium phosphate (DAP) –(NH 4 ) 2 HPO 4 –

Other sources of P  ammonium polyphosphate (APP) –(NH 4 ) 3 HP 2 O 7 + NH 4 H 2 PO 4 orthophosphate polyphosphate – –mainly used in fluid fertilizers

Calcium phosphates  triple superphosphate (TSP) –monocalcium phosphate Ca(H 2 PO 4 ) 2 –40-46% P 2 O 5 –water soluble

 ordinary superphosphate (OSP) –monocalcium phosphate and gypsum Ca(H 2 PO 4 ) 2 + CaSO 4 2H 2 O –18-20% P 2 O 5 –10% S –high water solubility

 phosphate rock –fluoroapatite [Ca 3 (PO 4 ) 2 ] 3 CaF 2 –25-40% P 2 O 5 –insoluble more soluble in acid soils

Other P sources  nitric phosphate  phosphoric acid  potassium phosphate  basic slag  farmyard manure –esp. chicken

Fertilizer Production  strip mining of P-rock –deposits N. Africa N. America –Florida, N. Carolina, Idaho, Utah Russia

Methods of Applying P Fertilizer: 1. Banding – applying P in the row with the seed. – Plants utilize the P more – Co-banding with N increases uptake of both nutrients 2. Broadcast- applying to soil surface, – Less efficient more becomes fixed 3. Plow down – mixed deeper into soil – Good for deep rooted plants

Potassium

Roles of Potassium: Essential in photosynthesis, respiration and protein synthesis. Important in translocation of heavy metals (Fe) Helps plant overcome effects of disease Important in fruit formation Improves winter hardiness Important in osmotic regulation in plants Important in carbohydrate breakdown Important in ionic balance

Helps Plant Fight Disease: Trials have shown: – Leaf blight and stalk rot in corn – Wilt and damping off in cotton – Mold and mildew in soybeans – Black spot and stem end rot in potatoes – Leaf spot and dollar spot in turf

K Strengthens stems Thickens cuticle Reduces the number of shrunken moldy discolored soybeans or seed.

N to K balance in Forages NutrientsYield lb/a 2 nd cutting , , , , , ,999 – Bermuda grass in Texas

Plant Deficiency Symptoms: Increased drought stress Burned or scorched appearance on leaf margins. Older leaves show deficiency symptoms first in grasses – Newer leaves on cotton Plants grow slowly and have poor root development Weak stems Plants are more subject to diseases

Forms of Potassium in Soil: 1. Unavailable K – found in rock minerals – Released as rock weathers – takes hundreds of years 2. Slowly available K – fixed or trapped between layers of certain soil clays – Available as clays swell and shrink. 3. Available soil K – K found in soil solution plus K held in exchangeable form by soil organic matter and clays. About 2% or less of total K in soils

How K moves in the soil: K does not move readily in soils except sandy or organic soils. When it does move it does so by diffusion

How do plants take up K? Plants can take up K in two ways: – 1. Soil solution – 2. Exchangeable K – loosely held by soil clays and organic matter.

Factors Affecting Plant Uptake of K: 1. Any factor affecting/limiting root growth 2. Soil aeration 3. Level of K in soil 4. Fixation of K 5. CEC level 6. Soil temperature – low temps reduce K availability 7. Soil moisture – drought reduces K uptake

Methods of Applying K Fertilizer: 1. Broadcast 2. Broadcast, plow down 3. Direct seed placement 4. Fertigation 5. Deep placement or knifed

Sources of K Fertilizer: K deposits occur as beds of solid salts beneath the earths surface, and brines in dying lakes and seas Mined in 4 ways: 1. Conventional underground coal mining method 2. Continuous mining method 3. Solution mining – sodium chloride brine dissolves KCl pumps to surface for refining 4. Surface Brine Recovery- K and other minerals harvested with solar evaporation from natural brines – Great Salt Lake, Dead Sea

Minerals K is Mined From: 1. Sylvinite – KCl and NaCl (20-30% K2O) North American KCl (0-0-60) 90% of K sold in U.S. and Canada 2. Sylvite – KCl (63% K2O) 3. Langbeinite – K2SO4 and MgSO4 (23% K2O)

Fertilizer Grade Particle Size: 1. White Soluble – ideal for liquids 2. Special Standard 3. Standard 4. Coarse 5. Granular – suitable for bulk blending

Other Sources: K 2 SO 4 – Sulfate of potash (SOP) 50% K 2 O and 18% S – Used in Cl- sensitive crops or where Cl buildup is a problem K 2 SO 4 -2MgSO 4 – Potassium-magnesium sulfate “K- Mag” or “Sul-Po-Mag” (22% K2O, 11% Mg, 22% S) – Good source of water soluble K and Mg KNO3 – Potassium nitrate ( ) – Little or no Cl or S