1. Let s quickly review some of the phosphorus fixation reactions in soils. 2. At low ph (acidic conditons below 6.0), phosphorus fixation occurs

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Richard Johnson
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2 1. Let s quickly review some of the phosphorus fixation reactions in soils. 2. At low ph (acidic conditons below 6.0), phosphorus fixation occurs between phosphates and iron or aluminum in the soil solution and on the soil particle surface. 3. Under alkaline soils, phosphorus fixation occurs between Calcium (and Calcium carbonate) and phosphates. 2

3 1. Under acid conditions the high concentration of hydrogen + ions dominate so iron or aluminum are free to enter in adsorption reatctions in the solution as well as on soil particle surfaces. 2. Iron and aluminum become attractive sites for adsorption reactions with phosphate anions. 3. Click to next slide. 3

4 1. In this illustration, the orthophosphate anion is shown forming an adsorption complex with iron and aluminum ions. 2. However, labile P formed by these bonds is only a very small fraction of total soil P. 3. Click to next slide. 4

5 1. More prevalent adsorption reactions involve the development of much stronger and harder to break bonds with metals as well as similar bonds with organic matter. 2. Click to next slide. 5

6 1. Read slide. 2. Click to next slide. 6

7 1. Read Text 2. Iron and aluminum hydroxides are also insoluble and precipitate out of the soil solution. 3. Although using calcium carbonate to adjust soil ph within the range of 6.0 to 7.0 is recommended for increased phosphate availability, it does not prevent phosphates from forming complexes with metals on the soil surface. These adsorption reactions by phosphates are not dependent on a charge usually via ligand exchange 4. Click to next slide. 7

8 1. Under alkaline soils, phosphorus fixation occurs between Calcium (and Calcium carbonate) and phosphates. 2. Click to next slide. 8

9 1. Calcium carbonate and phosphate anion undergo a fixation reaction to form Di- Calicum Phosphate complex. 2. Phosphate is fixed and takes longer to enter active pool of phosphates for plant uptake. 3. Click to next slide. 9

10 1. CrossOver does indeed, add a new dimension to phosphate availability in soils 2. Click to next slide. 10

11 1. Click and read slide 2. Let s look at how CrossOver tackles phosphate fixation and retention in soils. 3. Click to next slide. 11

12 1. Click and read slide 2. Let s look at how CrossOver tackles phosphate fixation and retention in soils. 3. Click to next slide. 12

13 1. Read Text 2. Click to next slide. 13

14 1. Read Text 2. Click to next slide. 14

15 1. Read Text 2. Click to next slide. 15

16 1. Read Text 2. Click to next slide 16

17 1. Read Text 2. Click to next slide. 17

18 1. Read Text 2. Ligands are ions or molecules that bind to a central metal atom in this case, phosphates are ligands that are shown attached to either iron (Fe) or aluminum (Al). 3. Ligand exchanges involve substitution reactions. By increasing the silicate concentration in the soil solution via CrossOver application, one establishes an environment where the silicates (which are ligands also) will replace phosphates in the adsorption complex. 4. Click to next slide. 18

19 1. Illustration of phosphorus desorbed from iron and aluminum complexes on soil surfaces by substitution of silicate anions. 2. Phosphorus is released and is available to be absorbed by plants or adsorbed onto other mineral surfaces. 3. Click to next slide. 19

20 1. Read Text 2. While release of phosphate ions by silicates is beneficial, the freed phosphate ions can quickly readsorb on soil particles, free metals or organic matter. 3. But silicates released by CrossOver also act as a preventative treatment, for fixation -- a second process Silicate Adsorption. 4. Click to next slide. 20

21 1. Read Text 2. Click to next slide. 21

22 1. Illustration of the formation of silicon-rich surfaces through polymerization and siloxane linkages. These can be initiated from direct adsorption of silicate anions onto mineral surfaces or the desorption of phosphorus from mineral surfaces by silicate anions. 2. As concentrations of monosilicic acid increase, polymerization reactions result in unique Si-rich compounds being adsorbed on soil surfaces. 3. In high ph soils, exchange reactions involving silicate anions are usually with calcium. 4. Click to next slide. 22

23 1. Read Text 2. Click to next slide. 23

24 1. Illustration showing adsorption of phosphate on silicon-rich surfaces. Phosphates attached to these silicates are usually held in a plant-available form. 2. This helps prevent phosphate leaching while keeping P available for plant uptake. 3. Click to next slide. 24

25 1. Read Text 2. Click to next slide. 25

26 1. Read Text 2. This concludes Phosphorus Fixation Training Module. 26

Lecture 15: Adsorption; Soil Acidity

Lecture 15: Adsorption; Soil Acidity Surface Complexation (Your textbook calls this adsorption ) Surface Complexation Both cations and anions can bind to sites on the external surfaces of soil minerals