Ionic Compounds

Formulas for Ionic Compounds

A chemical compound must have a net charge of zero.

In a chemical formula showing the combining of ions to make a compound, both cations and anions must be present.

The numbers of each will create net zero charge.

Na Cl → NaCl sodium chloride

1+ 1- 0 charge

Ionic Compounds

Mg2+ and Cl- → MgCl2Magnesium Chloride

2+ 2 x (1-) 0 charge

Ba2+ and O2- → BaOBarium Oxide

2+ 2- 0 charge

Try these:

Ca2+ and Cl-

Na+ and S2-

K+ and I

Naming (Type I) Ionic Compounds

1. cation goes first

2. anion goes second

3. the anion has -ide suffix

4. this is basic; the nomenclature gets more complex with more rules

Cation Exchange Capacity (CEC)

1. cations can be exchanged with other cations in a soil

2. the total number of cations that can be traded in a soil is cation exchange capacity (CEC)

3. the higher the CEC, the more cations that can be retained

4. all soils have a different CEC

5. the more organic matter and clay in a soil , the higher the CEC

6. this because organic matter and clay have a negative charge, so they attract positive ions

7. clay particles are smaller and have the greatest surface area per unit of volume of soil, therefore it attracts the most ions

Positively charged ions in soil:

Calcium

Magnesium

Potassium

Sodium

NH3 (ammonium)

Hydrogen (H is not a nutrient; measured by pH)

Expressing CEC

● CEC of soil is expressed in terms of milligram equivalents per 100 grams of

● soil, meq/110g

● the percent base saturation is the amount of that ion available for plant uptake

● the ideal base saturation percentages of these nutrients is approximately:

65% Ca (Calcium)

15% Mg (Magnesium)

4% K (Potassium)

1-3% Na (Sodium)

Sodic and Saline-Sodic Soils

● Sodic soils have excessive Na of >15% with >pH 8.5

● Saline-Sodic soils have >15% Na but <pH 8.4

● excessive sodium in the soil causes physical changes to the soil

● this makes it difficult for air and water movement in the soil

● this in turn affects plant health through the root system

● Remedy:

1. deep rip soil/add manure to root zone

2. this allows excess Na to leach out

3. add Calcium, gypsum, or Sulfur

Using CEC

● highly weathered soils with low organic matter = low CEC

● less weathered soils with high organic matter = high CEC

● clay soils can retain large amounts of cations against potential loss from leaching

● sandy soils with low CEC retain less cations

● this affects timing and application rates of fertilizer

EX: What might happen if fertilizer is applied to sandy soil in the fall before a really wet winter? How would this affect the fertility of the soil in the spring?

The Calcium to Magnesium ratio determines how tight or loose a soil is. The more Calcium a soil has, the looser it is; the more Magnesium, the tighter it is, up to a point. Other things being equal, a high Calcium soil will have more Oxygen, drain more freely, and support more aerobic breakdown of organic matter, while a high Magnesium soil will have less Oxygen, tend to drain slowly, and organic matter will break down poorly if at all. In a soil with Magnesium higher than Calcium, organic matter may ferment and produce alcohol and even formaldehyde, both of which are preservatives. If you till up last years corn stalks and they are still shiny and green, you may have a soil with an inverted Calcium/Magnesium ratio. On the other hand, if you get the Calcium level too high, the soil may lose its beneficial granulation and structure and the excessive Calcium will interfere with the availability of other nutrients.

Because Calcium tends to loosen soil and Magnesium tightens it, in a heavy clay soil you may want 70% or even 80% Calcium and 10% Magnesium; in a loose sandy soil 60% Ca and 20% Mg might be better because it will tighten up the soil and improve water retention. If together they add to 80%, with about 4% Potassium and 1-3% Sodium, that leaves 12-15% of the exchange capacity free for other elements, and an interesting thing happens. 4% or 5% of that CEC will be filled with other bases such as Copper and Zinc, Iron and Manganese, and the remainder will be occupied by exchangeable Hydrogen , H+. The pH of the soil will automatically stabilize at around 6.4 , which is the "perfect soil pH" not only for organic/biological agriculture, but is also the ideal pH of sap in a healthy plant, and the pH of saliva and urine in a healthy human.