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Agrochemicals - Fertilisers

Simply defined, agrochemicals are chemicals that are used in agriculture. They can be classed (incompletely) as such:

  • Fertilisers - provide essential nutrients to the plants
  • Pesticides - kill pests that would damage the crop
  • Plant growth regulators - allow for the control of the growth pattern of the plants, such as promoting fruiting at a particular time
  • Animal manure - can also be classed as a fertiliser, though it can be distinguished as a waste product
  • Others - such as spreaders, to distribute low concentration of other agrochemicals


All plants require carbon, hydrogen and oxygen, which come from \(\ce{CO2}\) and \(\ce{H2O}\) through photosynthesis

\[ \ce{CO2 + H2O -> O2 + carbohydrates} \]

Plants do also undergo cellular respiration though, to convert \(\ce{O2}\) into energy. These processes can be moderated by agrochemicals as well.

Other Nutrients

In addition to the CHO, plants also need a few other nutrients. The biggest of these are NPK, which are required in large amounts.

  • Nitrogen is important for green growth of the plant
  • Phosphorus is important for root, flower and fruiting growth
  • Potassium is important for plant health and disease resistance
Macronutrients Secondary nutrients Micronutrients
Nitrogen (green growth) Sulphur (proteins cysteine/methionene) Iron (co-factor, N and S metabolism)
Phosphorous (roots, flowers and fruit) Magnesium (co-factor e.g. chlorophyll) Manganese (N fixation)
Potassium (health and immunity) Calcium (cell wall rigidity) Boron (reproductive health, cell walls)
Chlorine (osmotic balance, resistance to disease)
Zinc (co-factor, hormone production)
Copper (chlorophyll production, general metabolism)
Molybdenum (N fixation)

NPK Ratio

Since they’re so important, the NPK ratio is a reported measure on fertilisers, however it measured as the mass ratio between the compounds \(\ce{N2}\), \(\ce{P2O5}\) and \(\ce{K2O}\). $$ \text{NPK}=\ce{N2:P2O5:K2O} $$ The phosphorus is in that form since it’s the anhydride form of phosphoric acid, which is the most concentrated form and is the form that plants require the most. Potassium oxide, aka potash (\(\ce{K2O}\))is the residue left over form the complete combustion of plant matter.

Measuring NPK Ratio

For a given ratio, we need to consider the conversion between the three model molecules and the elemental mass, in order to convert the NPK ratio from one molecular form to another. We can tabulate the values to make this more simple:

Given Ratio of \(\ce{N2}\):\(\ce{P2O5}\):\(\ce{K2O}\) 5 10 5
Molar Mass of \(\ce{N2}\):\(\ce{P2O5}\):\(\ce{K2O}\) 28 141.94 94.20
Molar Mass of \(\ce{N2}\):\(\ce{P2}\):\(\ce{K2}\) 28 61.94 78.2
Proportional mass \(\color{grey}28/28=\) 1 \(\color{grey}61.94/141.94=\) 0.44 \(\color{grey}78.2/94.2=\) 0.83
Ratio of \(\ce{N}\):\(\ce{P}\):\(\ce{K}\) \(\color{grey}5\times1=\) 5 \(\color{grey}10\times0.44=\) 4.4 \(\color{grey}5\times0.83=\) 4.2

This simplifies to: $$ \text{Atomic mass ratio}=(N:P:K)\times(1:0.44:0.83) $$

These ratios are the mass proportion of the NPK molecules that are comprised by N, P and K

Sources of NPK

As plants remove these vital nutrients from the soil, it’s the role of fertilisers to replenish them. The form that these fertilisers come in can make quite a large difference to how they are uptaken by the plant.



Cycling the growth and decomposition of legumes are a good source of nitrogen as fixated from atmospheric \(\ce{N2}\), however due to the low quantities of molybdenum in Australian soil, we can’t really use this method


Typically has a low nitrogen content (0.5%-2.5%) and can be processed by aerobic flotation to increase the nitrogen content (e.g. blood and bone), however manure is comparatively affordable and sustainable as it’s a by-product of the meat and dairy industries.


\(\ce{NH3}\) contains 82% nitrogen by weight, so is highly concentrated and is applied directly as a liquid or a gas, however it’s highly volatile, so it’s not going to remain in soil for too long.

Ammonium Nitrate

\(\ce{NH4NO3}\) contains 35% nitrogen by weight and is added in granular form, making it easier to handle, however it’s also highly explosive, so is not considered safe


\(\ce{(H2N)2C=O}\) contains 47% nitrogen by weight and is much easier to handle than ammonium nitrate and ammonia. It also has carbon in it, which may add extra nutrients to depleted soil.

Urea can also be modified to form biuret and triuret (urea bonded through a \(2^\circ\) amine), which have a much slower release

Ammonium Sulphate

\(\ce{(NH4)2SO4}\) only contains 21% nitrogen by weight, but it’s not explosive and is soluble in water. It also provides extra sulphur to depleted soil.

Other sources

Other sources of ammonia are typically in the form of ammonium salts and all tend to have issues in that they acidify the soil, promote over-cropping and don’t provide an organic replacement for the dissolved \(\ce{N2}\) that plants really want.


To fertilise with phosphorus, there is a competing process, called phosphorus fixation, in which the phosphorus will precipiate out of soil with iron and aluminium (and calcium, pH permitting), making them inaccessible to plants. While nitrogen fixation causes the nitrogen to become more available, due to its inclusion in proteins, phosphorus fixation makes in less available do to its conversion into an inorganic form.

Phosphorus has traditionally been added as bone meal, and was sourced from phosphate rock from the ocean floor.

Nowadays, Phosphorus comes in two forms, phosphoric acid and superphosphate, both of which produce HF as a byproduct, making it a dangerous process

\[ \begin{gather} \text{Phosphoric acid}\\ \ce{Ca10F2(PO4)6 + 10H2SO4 + 10H2O -> } \color{blue}\ce{6H3PO4} \color{black}\ce{ + 10CaSO4.2H2O + }\color{red}\ce{2HF}\\ \text{Superphosphate}\\ \ce{Ca10F2(PO4)6 + 7H2SO4 + 3H2O -> }\color{blue}\ce{3Ca(H2PO4).H2O}\color{black}\ce{ + 7CaSO4 + }\color{red}\ce{2HF} \end{gather} \]

Ammonium Phosphate

It’s possible to kill two birds with one stone and add ammonium phosphate, which can be present in multiple forms, depending on the initial concentrations of ammonia and phosphate:

  • \(\ce{NH4H2PO4}\)
  • \(\ce{(NH4)2HPO4}\)
  • \(\ce{(NH4)3PO4}\)


Potash in fertilisers is typically found as the carbonate form (\(\ce{K2CO3}\)) and is therefore very alkaline. This particular additive is always used as a naturally occuring substance, though can come from granite dust, greensand, manure, vegetable salt (\(\ce{KCl}\)), potassium sulphate and potassium nitrate.

Plant Hormones/Growth Regulators

These are used to control/trigger certain stages of growth of the plant.

  • Auxins - stimulate plant growth, particularly root and stem growth
    • Used for rooting cuttings
  • Cytokinins - generally growth stimulating
    • Used to increase crop yields
  • Gibberellins - promote germination and flowering
  • Abscistic Acid - inhibits growth and aids in stress tolerance
  • Ethylene - promotes fruit ripening