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Phosphorus Section J Soil Fertility and Plant Nutrition.

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Presentation on theme: "Phosphorus Section J Soil Fertility and Plant Nutrition."— Presentation transcript:

1 Phosphorus Section J Soil Fertility and Plant Nutrition

2 Phosphorus as a Plant Nutrient After N, it is the nutrient most likely to be deficient to plant growth. Plants use about _______ as much P as N or K Functions: –Component of amino acids, proteins, DNA, RNA –Energy transfer reactions ( ATP ) –Cell membranes (phospholipids) %

3 Phosphorus Deficiency CornTomato

4 Phosphorus Deficiencies P is a __________ nutrient, so deficiencies are first seen in ___________ leaves. Deficiency symptoms: –stunted plants –dark green color –purple streaks or spots on leaves mobile old

5 Nutrient Removal (kg/ha/yr) Source: Plant Nutrient Use in North American Agri., PPI, 2002 NPKN/P Ratio Broccoli (100 lb yield) Celery (100 lb yield) Corn (bushel of grain – 56 lb) Alfalfa (ton) Oranges (ton)

6 Composition of organic fertilizers

7 Phosphorus vs. Nitrogen Similarities: –Mineralization and immobilization of both N and P can be important for supplying available P to plants –Both occur as oxyanions: nitrate (NO 3 - ) and phosphate and (HPO 4 -, H 2 PO 4 2- ) –Both can contribute to pollution

8 Phosphorus vs. Nitrogen Differences: –Most (>95%) of soil N is organic in nature, usually 50% or less of soil P is organic –Plants use about 4-10X more N than P –Phosphate does not leach through soils as readily as nitrate –No gaseous forms of P; therefore no gaseous losses –There is no P counterpart to N fixation

9 The Phosphorus Cycle Adsorbed P Solid Inorganic P compounds Dissolved Inorganic P Organic P Plant uptake Microbial immobilization Mineralization

10 Phosphorus in Soils Soils may contain from 0.1 to 0.02% P N:P ratio in soils is about 8:1 There is little relationship between total soil P and available P; only a tiny fraction of total P is available to plants Forms of soil P: –Organic - various P forms associated with humus –Inorganic - mineral P, adsorbed P –P in soil solution (ionic forms)

11 Mineralization-Immobilization of P Organic PInorganic P Immobilization and Mineralization of soil P are similar to that of N: If added organic materials have a C:P ratio of >300, there will be net immobilization P if <200 there will be net mineralization of P

12 Mineral Forms of P In neutral to alkaline soils, most mineral P will be as Ca-phosphates. Most of these are quite insoluble. In acid soils, most mineral P will be as Fe and Al-phosphates. Most of these are quite insoluble. The insolubility of most P minerals is one important reason that P availability to plants is usually low.

13 Adsorbed P Phosphate ions (HPO 4 -, H 2 PO 4 2- ) are strongly adsorbed to the surfaces of: –Iron oxides, especially in acid soils –CaCO 3, especially in alkaline soils –Adsorption is at a minimum in neutral (6-7) pH Adsorption reactions are another reason that P availability in soils is limited.

14 Brady and Weil, Figure Phosphorus availability and pH

15 from Foth and Ellis, 1997

16 P Reactions with Soil Minerals

17 Many tropical soils are depleted of P without phosphate, even weeds barely grow Courtesy Potash and Phosphate Institute

18 P Availability Governed by: –Mineralization-Immobilization of humus P –But primarily by: Adsorption-desorption reactions of ionic P with Al and Fe oxides or CaCO 3 and Solubility of various P minerals - Fe and Al phosphates in acid soils, and Ca phosphates in alkaline soils

19 Soil pH and Phosphorus Availability

20 H 3 PO 4 H 2 PO 4 - HPO 4 2- PO Mole fraction of total P pH

21 Phosphorus Availability in Soils Only H 2 PO 4 - and HPO 4 2- in solution can be utilized by plants

22 Phosphorus “Fixation” Like N, much of the P applied in fertilizers is not recovered by plants in the first year. The reason is different: –P reversion is the process wherein available, soluble P forms applied in fertilizers naturally transform back into less soluble forms over time. –This is a non-biological process

23 Phosphorus Fixation Phosphorus “fixation” (sometimes called “reversion”) refers to reactions of P in soils that cause P added in fertilizer to become less available with time: –Reactions with Ca in calcareous soils –Reactions with Al/Fe in acid soils

24 Soil Likely to “fix” P

25 Factors Causing P fixation in Neutral or Calcareous Soils P forms relatively insoluble Ca phosphates in neutral to alkaline soils –hydroxyapatite –octacalcium phosphate Phosphate ions may be adsorbed to CaCO 3 particles and on Ca-saturated clays

26 Phosphorus Reactions in Desert Soils H 2 PO 4 - HPO 4 = Ca 8 H 2 (PO 4 ) 6 octocalciumphosphate Na 2 HPO 4 sodiumphosphate Calcareoussoils Sodicsoils Inorganic P

27 Phosphorus Reversion Alkaline soils –MCP (fertilizer) over time transforms: –MCP → DCP → TCP → OCP → Apatite –A similar process happens (with different forms) in acid soils –This lowers the availability of P –The reversion process usually takes several months to years to be complete

28 Ca Phosphates Most solubleMCP DCP TCP OCP Least solubleApatite Form added in fertilizer Chemical transformation with time in a calcareous soil

29 Factors Causing P Fixation in Acid Soils Precipitation from soil solution with Al or Fe: vivianite –Fe 3 (PO 4 ) 2. 8H 2 O strengite –FePO 4. H 2 O variscite –AlPO 4. 2H 2 O –Adsorbed on surface of Fe and Al oxides –Adsorbed on clay particles (i.e. kaolinite)

30 Griffin is a highly-weathered clay soil

31 Consumption of N, P 2 O 5, and K 2 O in the U.S. N P 2 O 5 K 2 O Current P consumption is similar to the late 1960s From PPI

32 U.S. phosphate fertilizer consumption by crop in 2001 USDA-ERS, USDA-NASS, AAPFCO, PPI Corn grain 38.4% Other crops 17.6% Alfalfa 7.5% Soybeans 7.7% Wheat 16.5% Corn silage, 3.7% Cotton, 3.6% Potatoes, 2.5% Sorghum, 2.5% Total P 2 O 5 consumption 4.3 million short tons

33 Average P use on corn and soybeans relative to crop removal Use Removal Gap is growing Potash and Phosphate Institute, 2001


35 Ratio of P removal by crops to fertilizer applied. Potash and Phosphate Institute, 2001

36 Ratio of P removal by crops to fertilizer applied plus recoverable manure. Potash and Phosphate Institute, 2001

37 Increasing concerns about P from fertilizers and animal manures entering surface water.

38 Canadian lake fertilized with P Unfertilized lake

39 P Availability P availability to plants is limited because: –soils often contain low amounts of P –mineral forms of P are insoluble –adsorption of ionic P –P does not move to the roots by mass flow because it is so insoluble –P must move to roots by diffusion

40 P Availability P Availability is most likely to be limited in: –Weathered soils: ___________________________ –Acid soils: ________________________________ –Alkaline soils: ______________________________ –Cold soils: ________________________________ –Soils high in Fe oxides: ______________________ high Fe oxide content binds P ions P precipitates with Ca, lower solubility P ions move slowly P ions bind to Fe oxides

41 Sanchez 1980

42 Sanchez 1982

43 Improving P Availability Soil and tissue testing Control soil pH if possible Use organic sources, i.e. manure Placement - critical!!

44 Measuring P Availability Soil tests –Neutral to alkaline soils - extraction of soil with 0.5 M NaHCO 3, measure P in the extract –Acid soils - extraction of soil with HCl and NH 4 F, measure P in the extract Tissue tests –Not as many P tissue tests as for N, fewer standards exist


46 Sample of P Soil Test Guidelines




50 Phosphorus Fertilizers Manufactured from mined apatite minerals –Apatite is treated with H 2 SO 4 or H 3 PO 4 to form various inorganic P fertilizers: superphosphate (0-20-0) solid triple superphosphate (0-45-0)solid mono ammonium phosphate ( )solid di ammonium phosphate ( )solid ammonium polyphosphate ( )liquid Phosphoric Acid (0-52-0)liquid –Organic: manures contain 0.5 to 2.0% P P analysis in commercial fertilizers is expressed as %P 2 O 5

51 Managing Soil P Managing soil P for maximum availability –If possible, assure an optimum pH (6-7) –Keep in mind that P is especially unavailable in cold soils. –Apply P in bands in soil –Use soil testing before planting each season, use appropriate guidelines. –Band-apply NH 4 + and P together--this usually increases P availability, particularly in alkaline soils. Why??


53 Nutrient Mobility in Soils Mobility in soils refers to the relative rate of movement of soluble nutrient forms in soils. Mobility is a function of soil texture and mineralogy (generally slower in clay soils) Usually, N (NO 3 - ), S (SO 4 2- ), and Cl (Cl - ) are considered mobile in soils Most other elements are less mobile in soils.

54 Nutrient Mobility in Soil Soil volume exploited for mobile nutrients: N, S, Cl Soil volume exploited for immobile nutrients: Most others

55 Because P is immobile, we cannot rely on movement of irrigation water to transport P. Apply immobile nutrients here (close to roots)

56 Take-Home Message for P Management P is less exciting, but no less important than N. Plants take up ______% as much P as N Manures contain about ____% as much P as N. P is less subject to losses in soils compared to N, is usually immobile in soils. Timing of P applications to crops is less critical than for N

57 Band Broadcast Sanchez, Swanson, and Porter 1990

58 Response of Celery to P Rate and Placement Espinoza, Sanchez, and Schueneman, 1993


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