# Potato Review Group

## Contents

Glossary

Anionic and cationic nutrients

A note on moles

Dynamic equilibrium

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• Nitrogen
• Phosphorus
• Sulphur

• Boron
• Calcium
• Magnesium
• Manganese
• Potassium
• Zinc

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## A note on moles

Moles are a way in which chemists account for numbers of atoms or ions or molecules, and this is important because chemical reactions involve interactions between pairs (or groups) of individual atoms/ions/molecules. For example water is H2O, so for each oxygen there are two hydrogens and to get the right amount of each component, the calculation would need to be done by number. It turns out that each atom weighs a different amount (due to the number of protons and neutrons in its nucleus) so in this example, providing 1 g of O and 2 g of H would not give the correct proportions.

### Consider this example which is relevant to ratios of cations in soil:

1. A ‘mole’ is a specific number (6.023 x 1023) of atoms, or ions, or molecules. Each atom/molecule has a ‘relative atomic/molecular mass’ (respectively; RAM or RMM) which is measured in grams per mole (g mol-1).
• There is no practical difference between the mass of an atom and of an ion of the same element, e.g. between Ca and Ca2+, because the missing two electrons have barely any mass.
• For example, the RAM of Ca is 40.1 g mol-1 and the RAM of Mg is 24.3 g mol-1. Therefore 1 mol Ca weighs (40.1/24.3 =) 1.65 times the amount as 1 mol of Mg.
2. Although Ca and Mg in soil are both measured in ‘parts per million’ extractable (equivalent to mg kg-1 or mg L-1), this does not give a ‘fair’ comparison of the amounts present, when considering their roles in complexation. Complexes form using certain numbers of atoms of Ca, Mg and P, not weights of each.
3. Therefore if your soil concentrations are 2000 mg Ca kg-1 and 400 mg Mg kg-1, the mass ratio is 2000/400 = 5.0, but the ratio of the number of atoms is (0.050/0.016 =) 3.1, because:
• 2000 mg = 2 g = 2/40.1 = 0.050 mol Ca (in each kg of soil)
• 400 mg = 0.4 g = 0.4/24.3 = 0.016 mol Mg (in each kg of soil)

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## Dynamic equilibrium

Dynamic equilibrium means that although individual atoms or ions move between chemical locations in soil, overall there is a steady concentration of the specified nutrient in each of those locations.

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To be completed.