0620 · 10.2Fertilisers

How fertilisers supply nutrients

0:00A plant can receive sunlight and water yet still grow poorly if its soil lacks essential mineral nutrients. Growing plants take up these nutrients, and harvesting crops removes some of them from the field. Fertilisers help replace what is missing. By the end of this video, you will understand what they supply and why their chemical composition matters. A fertiliser is a substance added to soil to supply nutrients needed for plant growth.

0:22Here, nutrients means substances that provide essential chemical elements. Plants need these elements to make new cells and maintain their normal processes. Adding an appropriate fertiliser can therefore improve growth when the soil does not supply enough. It helps to tell an element apart from a compound. An element contains only one type of atom, while a compound contains different elements chemically combined. Fertilisers usually provide nutrient elements in compounds, rather than as the uncombined elements.

0:49A plant needing nitrogen does not mean that we should put nitrogen gas into its soil. Many fertiliser compounds dissolve in the water within soil. This produces a solution containing ions, which are electrically charged particles. Plant roots can absorb suitable nutrient ions from that solution. The important connection is that a fertiliser supplies elements in forms the plant can use.

Ammonium, nitrate and NPK fertilisers

1:10Two important groups of substances used as fertilisers are ammonium salts and nitrates. Both can supply nitrogen because their characteristic ions contain that element. An ammonium salt contains ammonium ions, while a nitrate contains nitrate ions. These names tell us something useful about the compound before we consider any particular example. Ammonium nitrate is an example that belongs to both groups. It contains ammonium ions as well as nitrate ions.

1:35Both types contain nitrogen, making this compound useful as a nitrogen fertiliser. The word salt here means an ionic compound, not simply the sodium chloride used in cooking. Another example is potassium nitrate, which supplies nitrogen and potassium. Its nitrate ions contain nitrogen, while its potassium ions provide the other nutrient element. This illustrates why looking at the whole compound matters. A fertiliser can supply more than one useful element, depending on its composition.

2:04Nitrogen is not the only element needed for improved plant growth. Phosphorus and potassium are also important nutrients. A fertiliser supplying all three can address shortages that a nitrogen fertiliser alone would leave unchanged. However, these are not the only elements plants need, and fertiliser does not replace sunlight or water. This bag is labelled NPK, the chemical symbols for nitrogen, phosphorus and potassium, in that order. Notice that K, the symbol for potassium, does not match the first letter of its English name.

2:31NPK fertilisers provide all three elements for improved plant growth. The label describes the nutrients supplied, rather than naming one particular chemical compound. A mixture of suitable compounds can provide that combination of nutrients. One ingredient might supply nitrogen, while another provides phosphorus or potassium. The essential point is that the complete fertiliser supplies all three elements. It does not need to contain the same amount of each, because nutrient requirements can differ.

Choosing fertilisers for nutrient shortages

2:59Consider a worked example in which soil lacks nitrogen, phosphorus and potassium. A gardener has ammonium nitrate and potassium phosphate available. We need to decide whether either substance alone supplies the full combination. Start by identifying the nutrient elements provided by each compound, rather than guessing from the word fertiliser. Ammonium nitrate supplies nitrogen, but it contains neither phosphorus nor potassium. Potassium phosphate provides potassium, and its phosphate ions contain phosphorus.

3:27Combining these substances therefore supplies nitrogen, phosphorus and potassium. The mixture meets the stated nutrient need, although this information alone does not tell us the quantities to apply. A common mistake is to assume that any nitrogen fertiliser supplies the complete combination. Ammonium nitrate shows why that is incorrect, because being useful does not mean supplying every required nutrient. Another mistake is to describe these fertilisers as mixtures of uncombined elements.

3:52Instead, they normally contain compounds that provide the named elements in usable forms. Do not describe fertiliser as a substitute for photosynthesis. Plants make sugars through photosynthesis, while mineral nutrients support their growth and other processes. Supplying a missing nutrient can help, but adding fertiliser does not automatically solve every growing problem. The benefit depends on what the plant needs and what its surroundings already provide. An examination might ask you to state two groups of substances used as fertilisers.

4:21Give the group names directly: ammonium salts and nitrates. If asked to describe the use of NPK fertilisers, identify what they provide. They supply nitrogen, phosphorus and potassium for improved plant growth. The central idea is that fertilisers supply essential elements, usually through compounds that make nutrients available to plant roots. Ammonium salts and nitrates are used as fertilisers because they can provide nitrogen. Fertilisers supplying nitrogen, phosphorus and potassium provide a broader combination for improved plant growth.

4:50Understanding the composition lets you explain their purpose rather than simply remembering a label.