Lecture 32: Soil Phosphorus and Cation Nutrients

Transcription

1 Lecture 32: Soil Phosphorus and Cation Nutrients

2 Transformation of Soil P

3 Mineralization of Organic P in Soil P in soil OM can be mineralized and immobilized by the same processes as S and N Immobilization P in Soil OM PO 4 3- (aq) microbes Mineralization Mineralization depends on temperature, moisture, and tillage (plowing) Same dependence as for the degradation of soil OM Mineralized P will be fixed in a mineral form if not taken up quickly by plants

4 Fixation of Inorganic Phosphorus Varies with ph Adsorption to minerals Spodosols Oxisols Ultisols Alfisols Mollisols Aridisols

5 P Adsorption in Acid Soils I = Salt Concentration Most P fixation in acidic soils occurs by adsorption onto: Fe, Al, Mn oxides Kaolinite These minerals are common in many soils Most soils can fix a large amount of P!! P adsorption most favorable below ph 7

6 Phosphorus Fixation Increases with Time in Acidic Soils P in Soil Solution P Adsorbed on Iron Oxide Surfaces Increasing Time After P Addition Decreasing P Uptake by Plants P Precipitated as Iron Phosphate

7 OM Inhibits Fixation by Oxide Minerals OM binds to surface of oxide minerals, preventing phosphate from adsorbing

8 Inorganic P in Alkaline Soils In alkaline soils, phosphate reacts with Ca 2+ to form poorly soluble solid phases Progresses through a sequence of mineral forms with decreasing solubility: CaHPO 4 2H 2 O Ca 3 (PO 4 ) 2 Ca 5 (PO 4 ) 3 (OH,F,Cl) Brushite Whitlockite Apatite This is a major limitation on P availability in calcareous soils in arid and semiarid regions Low P availability in Aridisols, Inceptisols and Mollisols of arid regions

9 Forms of P in Soil Change during Soil Development Occluded = Fixed: Precipitated and strongly adsorbed Non-Occluded = Not Fixed: In soil solution and weakly adsorbed Mineral = Calcium phosphate (apatite) in parent material Net loss of P from leaching from: Filippelli (2008) Elements Entisol Inceptisol Alfisol Ultisol Oxisol

10 Mycorrhizal Fungi Important for Accessing Available and Fixed P No Fungi: Limited volume of soil from which to draw phosphorus With Fungi: Access to greater volume of soil, and fungi can dissolve phosphate minerals

11 Mycorrhizal Fungi Important for Accessing Fixed P

12 Key Concepts in Soil P Cycling Soil P primarily cycles between biota, soil biomass and organic matter, and the soil solution This represents a small fraction of the P in soil Mineralized P will be fixed if not taken up rapidly Inorganic reactions often fix phosphorus in forms that are poorly soluble P is fixed by adsorption to Fe and Al oxides and by forming Fe and Al phosphates under acidic conditions P is fixed as calcium phosphates under alkaline conditions P becomes progressively incorporated into more insoluble forms the longer it is in soil The amount, forms, and availability of P in soils changes as a soil develops

13 Soil Potassium (K)

14 Potassium (K) is an Essential Nutrient Many biochemical functions Influences osmotic balance in cells This reduces water loss in plants Important in neuron function and muscle contraction Activates enzymes responsible for energy metabolism, starch synthesis, nitrate reduction, and sugar degradation Essential for N fixation and photosynthesis A good supply of K is needed for crop production Plants leaves contain 1 to 4% K Comparable to N, 10 times P and S contents

15 Potassium Deficiency

16 General Properties of Soil K Potassium is the third most likely element to limit plant growth after N and P Potassium exists almost exclusively as K + Potassium does not form any gases The behavior of potassium in soil is controlled by cation exchange and mineral weathering Potassium causes no problems such as eutrophication and is non-toxic High amounts have same effects on soil as sodium, but this is exceptionally rare

17 Potassium and Soil Fertility High levels of K are present in most mineral soils More potassium is present than any other nutrient Soils made of quartz sand are low in potassium However, most potassium is unavailable to plants Contained in primary silicate minerals (feldspar) or in clays like illite, where it is non-exchangeable The exchangeable pool of K is fairly small Potassium may readily leach from soils Leaches faster than phosphorus Even greater leaching would occur if not for cation exchange

18 Forms of Potassium in Soil K in primary minerals, such as feldspars (90-98%) Unavailable for plants Nonexchangeable K in secondary minerals, such as illite and vermiculite (1-10%) Slowly available to plants Exchangeable K on soil colloids, such as smectites and OM (1-2%) Readily available for plants K + in the soil solution ( %) Readily available for plants

19 Potassium Hosts in Soils Orthoclase = Potassium feldspar (KAlSi 3 O 8 ): K + released by weathering Clays: Exchangeable and nonexchangeable K

20 Nonexchangeable K is an Important Long-Term K Reservoir in Soil Ultisol from South Carolina Weathering slowly frees K from nonexchangeable pool, making exchangeable K

21 Potassium Fixation by Clays Weathering = K-Release Fixation = K-Binding Mica or Illite Weather Illite/Partially Collapsed Vermiculite Vermiculite Nonexchangeable Potassium Exchangeable Potassium

22 Leaching Losses of Potassium Reduced in Limed and Alkaline Soils In acid soils, substantial Al 3+ on exchange sites Difficult for 1+ cation to replace 3+ cation High K leaching losses At higher ph: Al 3+ hydrolyzes and precipitates Exchange sites filled with Ca 2+ and Mg 2+ Easier for K + to exchange onto clays K is better retained

23 Key Concepts in Soil K K is an essential nutrient comparable in abundance in plants to N; Third most common limiting nutrient Soils contain large amounts of K, but most is unavailable to organisms Unavailable K occurs in silicate minerals (feldspar) and in non-exchangeable forms in clays like illite The exchangeable pool of K is small and K readily leaches from soil; Leaching losses are larger in acid soils K can be fixed by vermiculite and illite Will only be released if the clays are weathered Fixed and mineral forms serve as long-term buffers of soil K

24 Calcium and Magnesium

25 Calcium Calcium is required for bones and proper functioning of physiological processes in animals Essential macronutrient for all plants Plants use calcium in amounts similar to nitrogen and potassium Plant foliage contains between 0.1 and 5% calcium Some plants require 1-3% Ca in their leaves Trees store large amounts of Ca in their woody tissues Net Ca uptake in trees is comparable to N uptake

26 Physiological Role of Ca in Plants Major component of middle lamella of cell wall Ca-pectin molecules provide stiffness Involved in cell growth and division, membrane permeability, enzyme activation Protect cells against toxicity from other elements from: Smith (2001) Nature Reviews Molecular Cell Biology 2, 33-39

27 Forms of Ca in Soils Three main pools of Calcium in soil: Ca-containing minerals (calcite and feldspar) Calcium-humus complexes Exchangeable calcium on clays and humus Most calcium taken up by plants comes from exchangeable Ca and Ca in easily weathered minerals like calcite (CaCO 3 ) Calcite precipitation in arid soils may limit Ca availability Substantial amounts of calcium are deposited from the atmosphere in dust (natural and anthropogenic)

28 Calcium Hosts in Soils Plagioclase feldspar (CaAl 2 Si 2 O 8 NaAlSi 3 O 8 ): Ca 2+ is released by weathering Clays: Exchangeable Ca 2+

29 Calcic Horizons

30 Magnesium Plants take up less magnesium than calcium Constitute about 0.15 to 0.75% of foliage One-fifth of magnesium in plants in found in the central component of chlorophyll Required for photosynthesis Plays a role in the synthesis of oils and proteins Activates enzymes involved in energy metabolism Connects ATP to enzymes that catalyze many physiological processes involving phosphorylation Chlorophyll a

31 Forms of Mg in Soils Main source of magnesium in soil is exchangeable Mg on clays and humus Plant uptake and leaching reduce this pool This is replenished by mineral weathering Dolomite, biotite, hornblende, serpentine Also from dissolution of some 2:1 clays Some Mg is released through OM mineralization It has been suggested that in unpolluted forests much of the Mg taken up by trees may come from atmospheric deposition

32 Processes Affecting Ca and Mg Availability in Soil

33 Losses of Ca and Mg from Soil Loss from Missouri Cropland Ca Loss (kg/ha/yr) Mg Loss (kg/ha/yr) Erosion by Water Crop Removal Leaching Substantial amounts of the Ca and Mg are lost from soils through leaching In natural ecosystems these are replaced by mineral weathering and dust deposition Also partially offset by liming in agricultural soils Highly weathered soils (e.g., oxisols) can be nearly completely leached of Ca and Mg

34 Difference in Base Cation Cycling of Vegetation Affects Ca and Mg in Soil Spodosol Alfisol Mollisol From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology

35 Soil Calcium-Magnesium Ratio Ca 2+ is more tightly held on exchange sites than Mg 2+ In typical soils the CEC is commonly 5-20% Mgsaturated and 60-90% Ca-saturated This gives a typical Ca:Mg ratio of about 6:1 1:1 to 15:1 are all adequate for plants and soil health Plants can meet their nutrient needs Soil aggregation and biological activity unaffected Low ratios (below 1:1) can cause problems for grazing animals and, at 1:6 and below, plants

36 Key Concepts in Soil Ca and Mg Calcium has many key physiologic roles in organisms, including helping to stiffen the cell walls of plants Plants take up substantial quantities of Ca from soil; trees store excess calcium in their trunks Ca deficiencies are rare but when present make plants susceptible to metal toxicity The most important role of Mg in plants is in chlorophyll

37 Key Concepts in Soil Ca and Mg Available Ca and Mg in soil are primarily in exchangeable forms on clays or humus Ca and Mg are slowly released by mineral weathering; Ca also may occur as carbonates Substantial amounts of Ca and Mg are added to soil from atmospheric dust Ca and Mg are lost from soils primarily through leaching The Ca:Mg ratio must be in a certain range in order to not cause deleterious effects on biota 6:1 is typical, 1:1 to 15:1 has no adverse effects; Mg>Ca causes substantial problems