Chapter 2

1. Chapter 2 NUTRIENT GENERAL CHEMISTRY AND PLANT FUNCTION

2. What nutrients do plants need? • Plants require 16 nutrients; each is a chemical element • Plants do not require organic matter, enzymes or hormones as nutrients taken up from the soil. • Plant requirements for these substances is met by the plant’s own manufacture of them. • Except for carbon (C), hydrogen (H), oxygen (O), and boron (B) the nutrients are absorbed primarily as chemical ions from the soil solution.

4. CHOPKNS CaFe MgB Mn ClCuZn Mo

5. What makes these nutrients essential? • Must satisfy three specific criteria: • 1. Plants cannot complete their life cycle without the element. • 2. Deficiency symptoms for the element can be corrected only by supplying the element in question. • 3. The element is directly involved in the nutrition of the plant, apart from its effect on chemical or physical properties of the soil.

6. What affects the soil availability of these nutrients? • Most of the nutrients are absorbed as ions from the soil solution or the soil cation exchange complex. • Understanding the general chemistry of the nutrient ions, as it relates to their concentration in the soil solution, is critical to developing an understanding of how to manage their availability to plants. • What affects nutrient ion solubility? • Solubility is strongly influenced by the charge of the ion. • The first step to understanding solubility of nutrient ions and molecules is to know ionic and molecular charges. Help comes from identifying common ions, from group I, II and VII of the periodic table, that have only one standard valance in the soil environment. • To know these “standard” ions is as important to basic chemistry as knowing ‘multiplication tables’ is to basic mathematics.

7. www.webelements.com

8. Elements that have only one valance state in the soil environment. WEB ELEMENTS Oxidation number or oxidation state: charge of an atom that results when the electrons in a covalent bond are assigned to the more elctronegative atom Ionic Bond: electrostatic forces that exist between ions of opposite charge (left side metals combined with right side NM) Covalent Bond: sharing of electrons between two atoms Metallic Bond: each metal atom is bonded to several neighboring atoms (give rise to electrical conductivity and luster)

9. A positively-charged ion, which has fewer electrons than protons, is known as a cation • A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion

10. Ion/molecule Name Oxidation State • NH3 ammonia -3 • NH4+ ammonium -3 • N2 diatomic N 0 • N2O nitrous oxide +1 • NO nitric oxide +2 • NO2- nitrite +3 • NO3- nitrate +5 • H2S hydrogen sulfide -2 • SO4= sulfate +6 • N: 5 electrons in the outer shell • loses 5 electrons (+5 oxidation state NO3) • gains 3 electrons (-3 oxidation state NH3) • O: 6 electrons in the outer shell • is always being reduced (gains 2 electrons to fill the outer shell) • H: 1 electron in the outer shell • N is losing electrons to O because O is more electronegative • N gains electrons from H because H wants to give up electrons

11. oxidation state - the degree of oxidation of an atom or ion or molecule; for simple atoms or ions the oxidation number is equal to the ionic charge; "the oxidation number of hydrogen is +1 and of oxygen is -2" • The oxidation state or oxidation number is defined as the sum of negative and positive charges  in an atom , which indirectly indicates the number of electrons  it has accepted or donated. Oxygen: oxidation number = -2Hydrogen: oxidation number = +1Nitrogen: oxidation number = 0 N Oxidation StateH or ONH3 Charge = 0 3(+1) = 33-(0))= +3 -3 N gains 3 NO3 Charge = -1 3(-2) = -6-6- (-1)) = -5 +5 N loses 5NH4 Charge =+1 4(+1) = 44 – (+1)) = 3 -3N gains 3 N is losing electrons to O because O is more electronegative N gains electrons from H because H wants to give up electrons

12. Oxidation State • Cr(OH)3, O has an oxidation number of −2 H has a state of +1 • So, the triple hydroxide  group has a charge of 3×(−2 + 1) = −3. As the compound is neutral, Cr has to have a charge of +3.

13. Using this information, we can determine the charge of molecules or the oxidation state of elements in a charged molecule Ex: CO3=, we should be able to determine, by difference, that the oxidation state of C is 4+ (3*-2=-6)-2 showing = +4 CaCl2 is an uncharged calcium chloride molecule The chemical formula, name, and charge of each molecule should be carefully studied (memorized). Significance of each to soil fertility is presented and discussed in later chapters.

14. General effect of ion charge on solubility • Availability of nutrient ions to plants and the solubility of compounds they come from, or may react to form, can be discussed from the perspective of the general reaction: An+ + Bm-  AmBn

15. Reactant ions An+ and Bm- combine to form a compound (usually a solid) predicted by their electrical charges. • The higher the charge of either the cation or anion, the greater is the tendency for the compound or solid to be formed. • When the solid is easily formed, only small concentrations of the reactants are necessary for the reaction to take place. Because of this, the compound or solid that forms is also relatively insoluble (it will not easily dissolve in water), or it does not easily break apart (reaction to the left). Conversely, if the cation and anion are both single-charged, then the compound (solid) is not as easily formed, and if it does form, it is relatively soluble. An+ + Bm-  AmBn

16. “Real life” examples of charge-influenced solubility • A common compound that represents single charged ions is sodium chloride (NaCl, table salt), whose solubility is given by the equilibrium reaction, An+ + Bm-  AmBn Na+ + Cl-  NaCl

17. Examples • Common table salt is very soluble and easily dissolves in water. Once dissolved, the solid NaCl does not reform until the ions, Na+ + Cl-, are present in high concentration. • When water is lost from the solution by evaporation the solid finally reforms as NaCl precipitate. • Iron oxide or rust, represents multiple charged ions forming a relatively insoluble material. When iron reacts with oxygen and water (a humid atmosphere), a very insoluble solid, rust or iron oxide, is formed

18. How does all this relate to nutrient availability? • With regard to solubility of inorganic compounds, we may expect that when both the cation and anion are single charged, the resulting compound is usually very soluble. • Examples are compounds formed from the cations H+, NH4+, Na+, K+ and the anions OH-, Cl-, NO3-, H2PO4-, and HCO3- (bicarbonate). Note that NH4+, K+, Cl-, NO3-, and H2PO4- are nutrient ions. • Because monovalent ions are very soluble, when a monovalent cation reacts with OH- to form a base, the base is very strong (e.g. NaOH, KOH). • Strong Acid Strong ElectrolyteStrong Base Weak Electrolyte • Similarly, when a monovalent anion reacts with H+ to form an acid, the acid is a strong acid (e.g. HCl, HNO3). The monovalent molecules H2PO4-, and HCO3-, which are products of multi-charged ions that have already reacted with H+, are exceptions. • Except for H+ and OH-, whenever either the cation or anion is single charged and reacts with a multiple charged ion, the resulting compound is usually very soluble. Another exception to this rule is for F-, which reacts with Al+++ to form insoluble AlF3, a reaction important to soil test extractants of P in acid soils

19. Multiple charged ions • Divalent cations Mg 2+ , Ca2+ , Mn2+ , Fe2+ , Cu2+ , Zn2+ • Divalent anions SO42-, CO32- (carbonate), HPO42-, and MoO42- • Trivalent cations Fe 3+ and Al 3+ • Trivalent anion PO43- • When monovalent anions Cl- or NO3- react with any of the multi-charged cations Mg 2+ , Ca2+ , Mn2+ , Fe2+ , Cu2+ , Zn2+ Fe 3+ and Al 3+ the solid compounds are all quite soluble. • Similarly, when any of the monovalent cations NH4+, Na+, or K+ reacts with any of the multi-charged anions SO42-, CO32-, HPO42-, MoO42-, or PO43-, the solids are all quite soluble. • If both the cation and anion are divalent, the resulting compound will be only sparingly soluble. An example is gypsum (CaSO4*2H2O). • If one of the ions is divalent and the other is trivalent, the compound will be moderately insoluble. An example is tricalcium phosphate, Ca3(PO4)2. • If both the anion and cation are trivalent, the compound is very insoluble. An example is iron (ferric) phosphate, FePO4 AlPO4 ?

20. How can these general rules be simplified? • Once charges of the ions in a compound are known, we can get some idea of the compound solubility by simply adding the charges. • For example, if the sum of the anion and cation charges is 2, then the compound is very soluble (e.g. NaCl). • As the sum of the charges increases, the solubility of the compound decreases. • Whenever one of the ions is monovalent the compound is usually very soluble (e.g. KCl, CaCl2, and FeCl3 are all soluble even though the sum of charges is 2, 3, and 4, respectively). • Many examples where these simple rules are a good predictor of solubility. The sum of charges in CaSO4 is 4, and it is less soluble than CaCl2. • The sum of charges in Ca(H2PO4)2 is 3 (Ca 2+ and H2PO4-) and it is more soluble than CaHPO4 where the sum of charges is 4 (Ca 2+ and HPO42-). Similarly, Ca3(PO4)2 has a charge sum of 5 (Ca 2+ and PO43-) and is less soluble than CaHPO4.

21. Relative solubility of compounds formed from the reaction of anions (An-) and cations (Mn+) of different charges.

22. Why are some nutrients mobile and some immobile in the soil? • With a general understanding of nutrient ion solubility, it is now easier to examine the relative nutrient mobility in soils. • Bray: Nutrient management is closely linked to how mobile the nutrients are in the soil. • Relative mobility of nutrients in soils is governed primarily by • inorganic solubility • ionic charge • ionic adsorption (e.g., cations on the soil cation exchange sites), and • biological immobilization

23. Are all highly soluble nutrients mobile in the soil? • Monovalent nutrient ions have a good chance of being mobile in soils. • Monovalent anions, Cl- and NO3-, are mobile in the soil because they are not adsorbed on ion exchange sites. • They have the wrong charge (-) for adsorption on cation exchange sites and they are too weakly charged, compared to SO4 -- for example, to be adsorbed on anion exchange sites. • Furthermore, most soils have limited anion exchange capacity (tropical soils are an exception). • Monovalent cation nutrients, K+ and NH4+, are highly water soluble, but relatively immobile in soils because they are adsorbed on cation exchange sites. These nutrient ions become more mobile in sandy, low organic matter soils that have extremely low cation exchange capacity. • Plants absorb B as the uncharged, undissociated, boric acid molecule (H3BO3). Since this form of B is highly water-soluble and has no charge, it is mobile in soils.

24. Are all divalent and trivalent nutrient ions immobile in the soil? • Divalent and trivalent nutrient ions are immobile in soils (exception SO42-) • In tropical soils, are enough anion exchange sites to provide significant adsorption of SO42- and cause it to be somewhat immobile. Although sulfate compounds, such as CaSO4 and MgSO4 are relatively insoluble, the equilibrium concentration of SO42- with these solid compounds is far greater than that needed for plant growth. Phosphate is immobile in soils because it tends to form insoluble compounds with Ca in neutral and calcareous soils and Al and Fe in acidic soils (described in more detail in the chapter on P). Molybdate (MoO4 --) reacts to from insoluble solids similar to the solid-forming reactions described for phosphate. • The divalent cation nutrients, Ca 2+ , Mg2+ , Cu2+ , Mn2+ , and Zn2+ are adsorbed on cation exchange sites in soils, which prevents them from being mobile. In addition, when divalent and trivalent anions are present, these cations will react to form sparingly soluble and insoluble solids (e.g. Ca3(PO4)2). • Iron absorption by plants involves both Fe2+ and Fe+3 . • Both are immobile in soils. • Reduced form is usually not present in significant amounts, but could be absorbed on cation exchange sites. • Trivalent iron forms insoluble solid oxides (rust) that prevent the ion from being mobile. • Reduced (Fe++) Oxidized (Fe+++)gains electrons loss of electrons • Ferrous Ferric

25. Mortar http://www.swarthmore.edu/NatSci/prablen1/Geology_Pictures/geologypictures.html • The dry mix of hydrated lime and sand is mixed to form mortar. The initial mix is plastic yet stiff. Slowly, the hydrated lime reacts with the CO2 in air to form a hard and almost impervious mass of calcium carbonate: • Ca(OH)2 + CO2 -> CaCO3 + H2O • One could consider this as reforming the limestone from which the lime was originally obtained. • CaCO3 + heat -> CaO + CO2 • CaCO3 (calcite or aragonite) is relatively stable and durable under ambient conditions; witness limestone cliffs, marble, crustaceans, coral, etc. • Acidic precipitation is the enemy of mortar and all other CaCO3-based materials

26. Cement • Concrete is a mixture of cement, sand, water and stone (aggregate). • Ordinary Portland Cement is a mixture of five compounds in these approximate amounts: • 55% Ca3SiO5 20% Ca2SiO4 8% Ca4Fe2Al2O10 • 12% Ca3Al2O6 5% CaSO4*2H2O • Ordinary Portland Cement can be modeled as its most abundant and important component tricalcium silicate, Ca3SiO5, and tricalcium dialuminum oxide, Ca3Al2O6. • When formed into a paste with water, these compounds hydrate to form a rigid gel and matte of crystals.

27. Manufacture of Cement • 1. Fire limestone + shale at 1400-1500oC. Shale is a sedimentary rock made up mostly of quartz and clay minerals such as kaolinite, Al2Si2O5(OH)4. 3 CaCO3 + SiO2 + heat -> Ca3SiO5 + 3 CO2 9 CaCO3 + Al2Si2O5(OH)4 +heat -> Ca3Al2O6 + 2 Ca3SiO5 + 9 CO2 + 2 H2O • The product is called clinker because it is a partially fused mass of material • 2. The clinker is ground with the additional gypsum to the consistency of flour. The gypsum is needed to control the rate of hardening.

28. How does cement work? • Mix water plus cement to make a paste in the ratio of about 2.5 parts of cement to 1 part of water (by weight). At first the paste can flow because the water acts as a lubricant for the cement grains. • After a few hours (the induction period) the cement grains start reacting with the water (hydrating) and filling the spaces between grains with a hard mass of calcium-silicate-hydrate gel and crystals of a complex calcium aluminum sulfate hydroxide hydrate known as ettringite.

29. How does cement work? (Reactions) • Ca3SiO5 + 3 H2O -> Ca2SiO4*2H2O(gel) + Ca(OH)2 rigid gel • or • 2 Ca3SiO5 + 7 H2O -> Ca3Si2O7*4H2O(gel) + 3 Ca(OH)2 • and • 2 Ca3Al2O6 + 3 CaSO4*2H2O + 24 H2O -> Ca6Al2(SO4)3(OH)12*26H2O Ettringite

30. What are the plant concentrations, functions and deficiency symptoms of the essential nutrients? • Plant concentration of nutrients is helpful in managing nutrients that are mobile in the soil. • For these nutrients, the crop requirement can be estimated by multiplying yield times plant concentration. • Nitrogen • Nitrogen component of all amino acids • part of all proteins and enzymes • Plants contain from 1 to 5 %N • Wheat (2.35%) Corn (1.18%N) Soybeans (5.2%N) • Young legumes contain about 4 % N (25 % crude protein) and recently fertilized turf may contain 5 % N. • Nitrogen is a structural component of many plant compounds including chlorophyll and DNA. • Deficiencies of N are the most common, worldwide, of any of the nutrients.

31. Nitrogen • Wherever non-legumes are grown in a high-yielding monoculture system, and the crop is removed in harvest as a part of the farming enterprise, N deficiencies occur within a few years. (Straw, Residues) • Deficient plants are stunted • Low protein content • develop chlorosis (yellowing) at the tip, progressing along the mid-rib toward the base of the oldest leaf. http://www.nue.okstate.edu/Spatial_N_Variability.htm

32. Nitrogen • If the deficiency persists, the oldest leaf becomes completely chlorotic, eventually dying, while the pattern of chlorosis begins developing in the next to oldest leaf. The pattern of chlorosis develops as N is translocated to newly developing tissue (N is mobile in plants). Nitrogen deficiency reduces yield and hastens maturity in many plants.

33. Nitrogen Deficiency. • Shows up as chlorosis (yellowing) at the tip of the oldest leaf. • Progresses toward the base of the leaf along the midrib (corn). • Chlorosis continues to the next oldest leaf, after the oldest leaf becomes almost completely chlorotic, if deficiency continues.

34. Nitrogen Deficiency in Corn. chlorosis (yellowing) at the tip of the oldest leaf.

35. Nitrogen Deficiency in Corn. Chlorosis continues to the next oldest leaf

36. Phosphorus • The P content of plants ranges from about 0.1 to 0.4 %, and is thus about 1/10th the concentration of N in plants. • Storage and transfer of energy as ADP (adenosine di-phosphate) and ATP (adenosine tri-phosphate). High-energy phosphate bonds (ester linkage of phosphate groups) are involved

37. ATP • Biochemical reaction illustrating the release of energy and primary orthophosphate when ATP is converted to ADP (R denotes adenosine).

38. Phosphorus • Plant symptoms of P deficiency include poor root and seed development, and a purple discoloration of oldest (lower) leaves. Purple discoloration at the base of plant stalks (corn) and leaf petioles (cotton) is sometimes a genetic trait that may be incorrectly diagnosed as P deficiency.

39. CORN Phosphorus Deficiency. purple coloring and sometimes yellow on lower (oldest) leaves.

40. Phosphorus Deficiency. • Deficiency in Oklahoma cultivated soils is related to historical use of P-fertilizers. • P builds up in soils when high-P, low-N fertilizers are the only input. • 10-20-10 and 18-46-0.

41. Rate of P Applied • So, we know that plants have 1/10 the amount of P as N • If both N and P were deficient, would we apply • 1/10 the amount of N • 1/5 the amount of N • 1/2 the amount of N • Doesn’t matter, Bray said so

42. Potassium. • The content of K in plants is almost as high as for N, ranging from about 1 to 5 %. Potassium functions as a co-factor (stimulator) for several enzyme reactions and is involved in the regulation of water in plants by influencing turgor pressure of stomatal guard cells. • Potassium is mobile in plants and the deficiency symptoms are similar to those for N, except the chlorosis progresses from the tip, along the leaf margins (instead of the midrib), toward the base of the oldest leaf. • Leaf margins usually die soon after chlorosis develops, resulting in a condition referred to as “firing” or leaf burn. Deficiencies are related to soil parent material, fertilizer use and cropping histories.

43. Potassium Deficiency. • Common in crops grown in weathered soils developed under high rainfall. • Symptoms are chlorosis at the tip of the oldest leaf (like N), that progresses toward the base along the leaf margins.

44. Potassium Deficiency. • Common in crops grown in weathered soils developed under high rainfall. K Usually adequate K Usually deficient

45. Potassium Deficiency. • Chlorosis at the tip of the oldest leaf progressing toward the base along the leaf margins (corn, alfalfa).

46. Calcium and Magnesium • Calcium deficiencies are rare, although the concentration of Ca is relatively high (0.5 %) in plants. The primary function is in the formation and differentiation of cells. Deficiency results in development of a gelatinous mass in the region of the apical meristem where new cells would normally form. Chlorophyll, showing the importance of N (apex of four pyrrole rings) and Mg (centrally coordinated atom in the porphyrin type structure). R indicates carbon-chain groups.

47. Mg cont. • Magnesium is present at about 0.2%, or one-half the concentration of Ca in plant tissue. • Soil Mg levels are considerably lower than for Ca, and Mg deficiency does occasionally occur. • Magnesium functions as a co-factor for several enzyme reactions and is the centrally coordinated metal atom in the chlorophyll molecule • Intermediately mobile in plants, leading to deficiency symptoms of interveinal chlorosis in lower leaves. • Deficiencies may be expected in deep, well-drained soils developed under high rainfall that are managed to produce and remove high forage yields (Example?)

48. Magnesium and Sulfur additions. • Lime, especially dolomitic, adds Mg. • Rainfall adds 6 lb/acre/yr of S. • Like 120 lb of N (crop needs 1 lb S for every 20 lb N).

49. Magnesium Deficiency in Alfalfa.

50. Magnesium and Sulfur deficiencies. • Occur on deep, sandy, low organic matter soils in high rainfall regions with high yielding forage production. • Storage capacity for Mg and S is low. • Large annual removal of nutrients.