Chapter 12 (2): Soil Phosphorus, Potassium, Magnesium,

Calcium, and Micronutrients

From: Singer & Munns 2006 Soils: An Introduction, pages 193

Main Objectives 1. Understand main mechanisms controlling P-fixation, and

main factors influencing P-availability. 2. Know the double role of organic matter in P nutrition. 3. Know the main forms of P in soil. 4. Know the fact that K does not change chemical form in its

cycle. 5. Know the importance of pH control on the availability of

micronutrients

Key Terms and Concepts:

Eutrophication P-fixation K-fixation Chelate Siderophores

PHOSPHORUS (P) 1. Why do plants need it? All organisms need P to make cell membranes and nucleic acids, etc. 2. Is it mobile once assimilated into plant tissues? Yes. Plants do move P around. 3. What is the relative quantity of plant need (macro, secondary, & micro)? Approximately 0.3% of plant dry matter. It is a MACRO nutrient. 4. Where does the element come from (or what are the sources globally and locally?) It is believed that all original P came from rock weathering. 5. How does it cycle through time and space? Phosphorus cycles with rock cycling and leaching. There is no gaseous form of P, so it is called imperfect cycle. Through geologic time (hundreds of millions of years), P-containing sediments are buried, uplifted and subject to rock weathering, completing the global cycle.

From Brady & Weil 2008 14th Edition

Figure 13.15

From: Schlesinger 1997

6. Does the element go through chemical and/or biological transformations in plant-soil systems? If it does, do you understand all the key processes that regulate such transformations? Although it always in phosphate form, but it does change chemical existence in terms of organic vs. inorganic, different chemical associations (e.g., H2PO4

- and HPO42- in solution,

insoluble P, or organic P). Microorganisms play a major role in the transformation from organic P to inorganic P, and in mobilizing chemically fixed P (mycorrhizae and rhizosphere activities). Phosphorus is easily fixed by chemical reactions with Ca2+, Mg2+, Fe2+ etc. P-fixation is pH-dependent, and is the process that removes P from available pools. Both too high and too low of pH values can resulted in P-fixation, for example: PO4

3+ + Ca2+ (or Mg2+, Fe2+, Fe3+, Al3+,etc) → ↓Ca3(PO4)2 Although the total P content of most soils is large, only a small fraction of the total P is available to biota, primarily because of chemical P-fixation. Mining the P from apatite [ 3Ca3(PO4)2], which is primarily biological origin, is the main source of P fertilizer (about 14 million metric tons per year).

Figure 12.21; Soil pH controls P-fixation which reduces P availability.

The effect of pH on the relative concentrations of the three species of phosphate ions. At lower pH values, more H+ ions are available in the solution, and thus the phosphate ion species containing more hydrogen predominates. In near-neutral soils, HPO422 and H2PO42 are found in nearly equal amounts. Both of these species are readily available for plant uptake.

Roles of diffusion and mycorrhizal hyphae in the movement of phosphate ions to plant roots. In soils with low solution phosphorus concentration and high phosphorus fixation, slow diffusion may seriously limit the ability of roots to obtain sufficient phosphorus. The hyphae of symbiotic mycorrhizal fungi help overcome this problem. They penetrate the soil, absorb the phosphorus, and by cytoplasmic streaming inside the hyphae, transport phosphorus to the plant roots. This makes the plant much less dependent on the diffusion of phosphate ions through the soil.

7. What factors and processes influence or control the availability to plants? Chemical fixation (pH, Oxisols, etc), microbial mobilization and/or immobilization, etc. 8. Can you relate the above questions to management decisions? Banding application of P fertilizer vs. broadcasting? Liming/pH alteration/P availability/P fixation 9. Can you relate the above questions to relevant environmental issues? Over fertilization leads to eutrophication of aquatic ecosystems.

From: Tilman et al. 2001. Science 292:281-284

Hypoxia and Eutrophication

Increased nutrient input to aquatic ecosystems may cause eutrophication. Eutrophication leads to excessive growth of algae and cyanobacteria. Later after death of these excessive biomass, much increased decomposition by bacteria depletes oxygen in the water, which causes fish kills and other detrimental effects –the Dead Zone.

POTASSIUM (K) 1. Why do plants need it? Essential metal ion for >60 enzymes. 2. Is it mobile once assimilated into plant tissues? Yes. It only exists in mobile form. 3. What is the relative quantity of plant need (macro, secondary, & micro)? Approximately 0.8% of plant dry matter. It is a MACRO-nutrient, 4. Where does the element come from (or what are the sources globally and locally?) Rocks, rock weathering, clay minerals (such as mica), and ash produced from burning plant materials. 5. How does it cycle through time and space? Rock cycle, weathering, leaching, and ash after fire.

Potassium Deficiency is characterized with leaves showing tip-burn (marginal necrosis), or at severe deficiency status, showing necrosis in the interveinal spaces between the main veins along with interveinal chlorosis.

Figure 12.33

6. Does the element go through chemical and/or biological transformations in plant-soil systems? If it does, do you understand all the key processes that regulate such transformations? It always stays as K+ in solution or associated with clay/rock minerals. 7. What factors and processes influence or control the availability to plants? It can get entrapped between layers of illite and similar clays during drying-wetting cycles (See Figure 12.32 in the textbook). Entrapping K+ is called Potassium fixation. Soil cation exchange is the main mechanism for K stabilization/storage, in addition to rock weathering and clay entrapping 8. Can you relate the above questions to management decisions? What should a farmer do if his or her sandy soil is poor in K? 9. Can you relate the above questions to relevant environmental issues? Assuming that there will be a mass production of switchgrass (Panicum virgatum) in the Midwest region for bio-energy for many years and in a central location, what may happen to the soil K status?

Diagram illustrating how liming an acid soil can reduce leaching losses of potassium. The fact that the K+ ions can more easily replace Ca2+ ions than they could replace Al3+ ions allows more of the K+ ions to be removed from solution by cation exchange in the limed (high-calcium) soil. The removal of K+ ions from solution by adsorption on the colloids will reduce their loss by leaching, but they will still be at least moderately available for plant uptake.

Calcium (Ca) and Magnesium (Mg) 1. Why do plants need it? Ca: A cementing agent for dividing cells (calcium pectate). It is critical for the integrity and functioning of cell membranes. Mg: is the most critical metal ion for chlorophyll. Many enzymes need Mg2+. 2. Is it mobile once assimilated into plant tissues? Calcium is NOT mobile. It is "fixed once being utilized in tissues. Magnesium is mobile. Magnesium acts more like K+ in term of mobility in plant tissues. 3. What is the relative quantity of plant need (macro, secondary, & micro)? Approximately 1.0 to 2.5 % of plant dry matter is calcium. Approximately 0.5 % of plant dry matter is magnesium. Both belong to the "secondary" group in term of quantity. 4. Where does the element come from (or what are the sources globally and locally?) Rocks, rock weathering, and minerals. 5. How does it cycle through time and space? Rock cycle, weathering, and leaching.

Symptoms of Ca deficiency in cauliflower: tip burns of young leaves.

6. Does the element go through chemical and/or biological transformations in plant-soil systems? If it does, do you understand all the key processes that regulate such transformations? Both Ca2+ and Mg2+ may be in four forms: (1) soluble ions in soil solution; (2) absorbed onto cation exchange sites; (3) insoluble salts (e.g., CaCO3/MgCO3, CaSO4/MgSO4); (4) organic Ca/Mg in biomass. The first two forms are available to plants. Most agricultural soils have abundant supply, except for highly weathered and oxidized soils or very old sandy soils where all calcium and magnesium have been leached out. Calcium and magnesium like many other metal ions get leached out quickly during the initial stage of litter decomposition, therefore, organic forms only exists in very small quantity. In addition to parent materials, leaching is the single most important factor that can change soil calcium or magnesium status (precipitation/irrigation, acid deposition, acidic litter materials, etc.).

7. What factors and processes influence or control the availability of Ca & Mg to plants? pH matters. How much calcium and magnesium in the soil as a whole matters the most. 8. Can you relate the above questions to management decisions? What will be your advice to a farmer whose soils do not have enough Ca or Mg? 9. Can you relate the above questions to relevant environmental issues? A known consequence of long-lasting acid rain is accelerated loss of Ca and Mg associated with acid leaching. What should be our strategies to deal with this issue?

MICRONUTRIENTS: Boron (B), Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Chlorine (Cl) and Molybdenum (Mo) 1. Why do plants need them? Mostly enzyme activators. Please see Table 12.7 in the textbook for more information. 2. Are they mobile once assimilated into plant tissues? NO for most (H3BO3, Fe3+, Zn2+, Mn2+) But YES for Mo and Cl. 3. What is the relative quantity of plant need (macro, secondary, & micro)? Trace amount (MICRO-nutrients) 4. Where does the element come from (or what are the sources globally and locally?) Rocks, parent minerals, SOM. 5. How does it cycle through time and space? Rock cycle, weathering, and leaching.

6. Does the element go through chemical and/or biological transformations in plant-soil systems? If it does, do you understand all the key processes that regulate such transformations? Boron is a non-metal nutrient, exists in either H3BO3 or B(OH)4

- when pH >8.5. Zn acts similarly as Mg or Ca. Mn exists either as Mn2+ or MnO2 (insoluble) Fe3+ mostly insoluble, so it needs chelates and siderophores Cu exists as Cu2+ or Cu(OH)+ in solution under low pH, as Cu(OH)2 at higher pH ( >7) Mo exists in the form of MoO4

2- like PO43-, has problem of chemical fixation.

Cl- is very soluble. Too much Cl- is often the problem than not enough. Definitions: A chelate is a coordination complex with a ligand and a metal ion in the center of the ligand. A ligand is any compound capable of forming a chelate. Siderophores are special chelates formed from ligands produced by micro-organisms or plant roots.

Figure 12.39 a: ferric ethylene-diamine-tetra-acetate

Two ways in which plants utilize micronutrients held in chelated form. (a) Dicotyledonous plants such as cucumber and peanuts produce strong reducing agents (NADPH) that reduce iron at the outer surface of the root membrane. They then take in only the reduced iron, leaving the organic chelate in the soil solution where it can complex another iron atom. (b) Grass plants such as wheat or corn apparently take the entire chelate–metal complex into their root cells. They then remove the iron, reduce it, and return the chelate to the soil solution.

7. What factors and processes influence or control the availability of micronutrients to plants? Soil pH is the most important factor. The type of parent materials is also important. Also micronutrients can have the problem of toxicity, as well as deficiency. 8. Can you relate the above questions to management decisions? Understanding that sandy, high pH soils with low SOM are more likely to have micronutrient deficiency, what would you do if you have found out that your plants are showing micronutrient deficiency symptoms? 9. Can you relate the above questions to relevant environmental issues? Under what circumstances would you most likely find toxic levels for some of the micronutrients?