Chemistry only

NPK fertiliserTo stay healthy and grow well plants need to take in a variety of minerals through their roots from the soil, these include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S) ,iron (Fe), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), nickel (Ni) as well many other elements. Most of these elements are found as ionic compounds in the soil. This means that the plants are absorbing ions from the soil through their roots.

Perhaps the three most essential elements from the list above that a plant requires are nitrogen (N), phosphorus (P) and potassium (K). The image opposite shows why these three elemenets are essential. Basically nitrogen will make "everything" above the soil grow well, phosphorus will take care of "everything" below the soil and potassium helps tackle infection and helps the plant deal with the stresses due to poor weather and infection, potssium improves the vitality of the plant

You may think that plants could use the nitrogen from the air as a source of nitrogen, but unfortunately this is not the case. Nitrogen is a very unreactive gas and most plants, except some plants called leguminous plants are unable to use atmospheric nitrogen. Leguminous plants have nodules (swellings) on their roots that contain bacteria that can convert atmospheric nitrogen into nitrates which acts as a source of nitrogen for the plant. Most plants rely on a source of nitrate ions (NO3-) in the soil, which they take in through their roots, the nitrate acts as a source of nitrogen for the plant. The nitrogen is essential for the manufacture of proteins by the plant.

Farmers need to replace essentail minerals that are removed from the soil when crops are harvested. If the soil is not fertilised then any new crops which are planted will be in a poor condition and crop yields would be low. Farmers will add fertilisers to their fields to ensure there are sufficient minerals present for healthy plant grwoth. Most fertilisers are solids, normally powders or small granuales called prills. Fertilisers are mostly ionic compounds such as:

Most fertilisers are a mixture of many different compounds, most from the list above. These different mixes of compounds will contain different amounts of nitrogen, phosphorus and potassium. Depending on what crops are being grown and on the condition of the soil a farmer will chose a fertiliser with a specific N:P:K content that will maximise his yield of crops. In the image below you can see that the N:P:K content of each of the fertilisers is different, so each particular fertiliser maybe suitable for different crops or soil types. NPK fertiliser