(1)Fewer of Si 4+ positions are filled by Al3+ in the illite. (2)There is some randomness in the stacking of layers in illite. (3) There is less potassium in illite. Well-organized illite contains 9-10% K2O. (4) Illite particles are much smaller than mica particles. Ferric ion Fe3+
Vermiculite is similar to montmorillonite, a 2:1 mineral, but it has only two interlayers of water. After it is dried at high temperature, which removes the interlayer water, expanded” vermiculite makes an excellent insulation material.
Steady water cannot sustain the reaction.
At any given time, the vast majority of nutrients and trace elements in soil are adsorbed onto the surface of clays and organic matter. Some, however, remain in the soil solution (soil water). These nutrients in solution can move back and forth between the soil surface and the soil solution. This is called ion exchange. When plant roots penetrate into the soil, they begin to remove nutrients from the soil solution to meet their nutrient needs. As plants remove nutrients from the soil solution they often exude other elements into the soil solution. The plant uptake of nutrients disrupts the balance between nutrient ions in the solution and nutrient ions on the soil surface is. To get back into balance, nutrients move from the soil surface out into solution and are then available for root uptake. Adsorption of nutrients, trace elements and other chemicals onto soil surfaces keeps them in the soil, usually in available forms, and limits how much could be lost in drainage water or runoff from the surface.
Minerals are the source of K for plants
Stable soil organic matter is made up of large complex organic molecules that are resistant to further attack from soil microbes. Pieces of soil organic matter appear like coiled, twisted strands. This material coats particles of silt and clay and helps to hold clay and silt together in soil aggregates. The coiled structure also gives organic matter a very large surface area. Soil organic matter is also like a sponge. It can soak up large amounts of water and store it for plants to use. Soil organic matter has a very high cation exchange capacity. Unlike many layer clays, the cation exchage capacity of organic matter changes as soil pH changes. As soil pH decreases (becomes more acid) more and more hydrogen cations stick to organic matter. At low pH this hydrogen is held very tightly and will not exchange with nutrients or other elements. As soil pH increases the hydrogen is held less strongly and readily exchanges with other nutrient and trace element cations like calcium, magnesium, potassium, and sodium. These cations will also exchange with each other at near neutral pH.
Together, clays and organic matter account for most of the cation exchange capacity of the soil. Cation exchange capacity refers to the total amount of cations that a soil can retain. The higher the exchange capacity, the better the soil is able to retain plant nutrients and other elements. The exact cation exchange capacity of soil depends on: what type and how much clay it has, how much organic matter it has, and what its pH is.
When excessive amounts of nutrients are added to soil, the capacity of the soil to adsorb them may be exceeded. Nutrients then cannot move from the soil solution onto soil particles. Nutrient concentrations then build up in the soil solution. In this diagram the orange balls represent nitrate and the light green balls represent phosphate. With increased nutrients in the soil solution there is increased likelihood that nutrients could be lost in runoff water or drainage water.
Figure 37.6 Contour tillage.
Acquisition of K in Plants
1 Acquisition of K in Plants
2 Road Map General Introduction Sources weathering and K release Mechanisms of uptake Fates of the Solution K Path ways in & through the cells Factors affecting the K uptake Functions
3 Facts about Potassium• Potassium is a soft, highly reactive metallic element, it occurs only in nature as compounds.• Potassium was formerly called Kalium, which explains the symbol “K”
4 Soil Potassium • Second most in terms of plant use. • Taken up as cation (K+) • Mineral sources in soils are important. • Organic sources in soils are not important. •Often limiting and often added; third number on fertilizer bag • Roles in plants: stomatal control, cell division, translocation of sugars, enzymes
5 Potassium Reserves Feldspar Mineral • Potassium is found in minerals like feldspars and micas (90% of Soil K) • K is fixed inside of clay minerals ( 9% of soil K) • K is on the soil exchange sites ( 1%) • K is in the soil solution (0.1%)
6 Minerals-Illite (mica-like minerals) • Si8(Al,Mg, Fe)4~6O20(OH)4·(K,H2O)2. Flaky shape. • The basic structure is very similar to the mica, so it is sometimes referred to as hydrous mica. Illite is the chief constituent in many shales.potassium K • Some of the Si4+ in the tetrahedral sheet are replaced by the Al3+, and some of the Al3+ in the octahedral sheet are substituted by the Mg 2+ or Fe3+. Those are the origins of charge deficiencies. • The charge deficiency is balanced by the potassium ion between layers. Note that the potassium atom can exactly fit into the hexagonal hole in the tetrahedral sheet and form a strong interlayer bonding. • The basal spacing is fixed at 10 Å in the presence of polar liquids (no interlayer swelling). Trovey, 1971 ( from 7.5 µm Mitchell, 1993) • Width: 0.1~ several µm, Thickness: ~ 30 Å 6
7 Minerals-Vermiculite (mica like minerals) • The basal spacing is from 10 Å to 14 Å. • It contains exchangeable cations such as Ca2+ and Mg2+ and two layers of water within interlayers. • It can be an excellent insulation material after dehydrated. Illite VermiculiteMitchell, 1993 7
8 Exchangeable vs. Non-exchangeable KExchangeable KReadily bufferssoil solution K Non-Exchangeable K Slowly buffersSoil tests measure exchangeable K soil solution K
9 Potassium Vermiculite, illite trap K+ Montmorillinite releases K+ K k k k k k k k Vermiculite illite montmorillinite
10 K release during mineral weathering Havlin et al. (1999)
11 Weathering & Dissolution of Clay Minerals •The CO2 gas can dissolve in water and form carbonic acid, which will become hydrogen ions H+ and bicarbonate ions, and make water slightly acidic. • CO2+H2O → H2CO3 →H+ +HCO3- •The acidic water will react with the rock surfaces and tend to dissolve the K ion and silica from the feldspar. Finally, the feldspar is transformed into kaolinite. •Feldspar + hydrogen ions + water → clay (kaolinite) + cations, dissolved silica • 2KAlSi3O8+2H+ +H2O → Al2Si2O5(OH)4 + 2K+ +4SiO2 •Note that the hydrogen ion displaces the cations. 11
12 Hydrolysis Feldspar + carbonic acid +H 2 O = kaolinite (clay) + dissolved K (potassium) ion + dissolved bicarbonate ion + dissolved silica Clay is a soft, platy mineral, so the rock disintegrates
13 Moving nutrients from soil to plants Nutrients in soil solution Plant Root Exchangeable K ↔ Solution K Nutrients on soil clay and organic matter
15 Fates of potassium in the soil solution Applied K depends on the CEC and clay minerals Plant uptake lost to leaching retained by soil particles precipitated as secondary minerals Crop removal
18 Cation Exchange Capacity • Cation exchange capacity (CEC) is the total amount of cations that a soil can retain • The higher the soil CEC the greater ability it has to store plant nutrients • Soil CEC increases as – The amount of clay increases – The amount of organic matter increases – The soil pH increases
19 Excessive Nutrient Loading Nutrients in soil solution Plant Root Exchangeable K ↔ Solution KNutrients on soil clay Nutrient loss in and organic matter drainage water
20 Soil properties: pH Soil pH: Influences nutrient solubility. K, Ca, and Mg most available at pH > 6.0. P availability is usually greatest in the pH range of 5.5 to 6.8. At pH values less than 5.0, soluble Al, Fe, and Mn may be toxic to the growth of some plants. Most micronutrients (except Mo and B) are more available in acid than alkaline soils.
21 Environmental Factors Affecting K Availability to a Plant 78 % of K supplied to root via • Soil moisture diffusion – Low soil moisture results in more tortuous path for K diffusion – takes longer to get to root – Increasing K levels or soil moisture will increase K diffusion – Increase soil moisture from 10 to 28 % can increase toatl K transport by up to 175 % • Soil Aeration – High moisture results in restricted root growth, low O and slowed K absorption by the root
22 Environmental Factors Affecting K Availability to a Plant • Soil temperature – Low temperature restricts plant growth and rate of K uptake – Providing high K levels will increase K uptake at low temperatures • Reason for positive response to banded starter • Soil pH – At low pH, K has more competition for CEC sites – As soils are limed, greater amount of K can be held on CEC and K leaching reduced.
23 Environmental Factors Affecting K Availability to a Plant • Leaching related with Texture – K leaching can occur on course textured or muck soils particularly if irrigated – Large fall K applications to sandy or muck soils discouraged
24 Mechanism of K Acquisition Mass flow Diffusion Root interception (Richardson et al., 2009)
25 Mass flow Movement of plant nutrients in flowing soil solution
26 Diffusion Migration of nutrient from area of higher concentration to lower concentration 78% K uptake by Diffusion
27 Root interception Growth of plant roots into new soil areas where there are untapped supplies of nutrients
28 Apparent free space (AFS) is the cell wall and intercellular spaces of the epidermis and cortex of the roots (regions of the root that can be entered without crossing a membrane; apoplast space of the root epidermal and cortical cells). AFS occupied about 10%~25% of root volume. AFS includes space accessible to free diffusion and ions restrained electrostatically due to charges that line the space.
31 Uptake of K+ into cells • Using 86Rb (radioactive K+ analog) found K+ absorption is biphasic, i.e. there are two types of K+ transport systems. (1) high affinity uptake system. This system is active at low [K+] (≦200µm). It is probably a H+- ATPase-linked K+-H+ symporter. (2) low affinity uptake system. This system is bidirectional.