Why does a plant need nitrogen, phosphorus and potassium?

Answer

Dear Kristien,

Just like us, plants need nutrients to survive and grow. Nitrogen, phosphorus and potassium are the three “macronutrients”, in other words, these are the elements that plants need most. For example, nitrogen is needed for photosynthesis, potassium for water absorption and transport of substances in the plant and phosphorus for the energy supply in plant cells.

However, other elements such as sulphur, magnesium, calcium… are also important for many processes in plants (eg magnesium for photosynthesis) and there are also “micronutrients” or trace elements, these are substances that are needed in very low quantities by plants. A shortage of these micronutrients, for example manganese, zinc and iron, can cause your plant to not grow well and show deficiency symptoms (eg brown spots due to manganese deficiency in corn and potato).

Based on the color of the leaves of a plant or spots that appear on them, you can deduce whether a plant has a deficiency (or overdose!) of a certain nutrient. A clear concise publication about this is this: http://edepot.wur.nl/292716 (but much more can be found about this). It does require some prior knowledge to know which element is causing poor growth or discoloration. For example, a yellowish color of the leaf can indicate a nitrogen deficiency, but it can also be an iron deficiency (or even if you water your plant too much, it can also develop yellow leaves). That prior knowledge mainly comprises some information about the plant species (how does that particular plant species react to deficiencies, are there any sensitivities? Such as blueberry, which is very sensitive to a deficiency of iron. There are also nitrogen-fixing plant species, such as white clover, which, in collaboration with bacteria at the roots can extract nitrogen from the air, this species is more likely to suffer from a potassium deficiency than a nitrogen deficiency.), about the soil type (pH, texture, organic matter…) and the fertilization history. Plant diseases make the diagnosis even more complex because the symptoms sometimes resemble deficiency symptoms.

Kind regards,

Stephanie