NPK chart: crop fertilisation and maths

For the last few decades, plant fertilisation has been based on 3 components:

  • The analysis of the soil and its compounds, pH, levels of NPK nutrients
  • The crop’s physiological needs in terms of nitrogen, phosphorus and potassium for growth
  • The NPK nutrients to input in the form of fertiliser to bridge the gap between crop demands and the stock available in the soil.

As soil analyses are not always conducted systematically, plant fertilisation is primarily based on crop demands. Calculated using the NPK demand chart identified per crop, the input of major nutrients has long been a mathematical equation: 

  • producing 1 quintal of wheat requires 3 units of nitrogen. So for a target of 70 quintals/hectare, a farmer needs to provide 210 units, i.e. 630kg of ammonium nitrate (33.5%) over the wheat growth cycle.

This initial approach to nitrogen-based plant fertilisation was later supplemented by an analysis of nitrogen residues in order to optimise the fertiliser dose taking into account the mineralised nitrogen at the end of winter.  This complementary approach emphasises the importance of the soil’s biological activity, as well as the cycle of organic matter.


NPK chart: a minimalist approach to fertilisation


An understanding of the laws of soil and ecosystems makes it possible to see that ‘classic’ agronomy with NPK fertiliser inputs over-simplifies the plant fertilisation model as it only partly takes into account the NPK inputs of different types of organic matter.

Adding compost or manure provides nitrogen, phosphorus and potassium elements. The level of nutrients in this ‘external’ organic matter is analysed in a lab and can be estimated to a high degree of accuracy. 

But these organic fertilisers, partially decomposed through the input process, are then mineralised or humified depending on their carbon/nitrogen ratio, the microbiological populations present in the soil, the soil pH level, etc. 

Likewise, the decomposition of the root system, plant leaves and stems and all of the fauna in the soil are a source of mineral nutrients for successive crops. 

Although the road is long before the organic elements decompose into water-soluble minerals in the soil to be assimilated by plants, they must be considered when rationalising crop fertilisation. Of course, the more developed the biological activity, the more it provides an internal source of minerals.


From the NPK chart to a comprehensive fertilisation rationale


In the face of the major global challenges of population growth, urbanisation and decreased farming land, the goal of agricultural production must remain per-hectare yields. Only by considering all of the production factors can we guarantee the sustainability and quality of this yield. The future of farming practices will therefore involve:

  • plant fertilisation through the adoption of eco-fertilisation
  • soil fertilisation through the addition of soil enrichers, manure and organic matter designed to increase humus content
  • fertilisation of soil fauna like worms by limiting tillage and maintaining permanent soil cover
  • fertilisation of soil microflora through the addition and restoration of carbon-rich organic matter

By putting into practice this comprehensive rationale, fertilisation becomes a supplement to the natural cycle rather than the primary actor. The development of these practices around the world and on different soil types shows an improvement in the quantity and quality of agricultural products and an effect on water quality. 

Their large-scale development has thus become a key focus.