Inorganic Fertilisers – NPK Demystified
Plants require a number of non-mineral and mineral nutrients for their healthy growth. The non-mineral nutrients – Hydrogen, Oxygen and Carbon – are acquired from air and water through “photosynthesis”. The mineral nutrients are absorbed by the plants from the soil through its roots.
The mineral nutrients comprise macronutrients and micronutrients. Macronutrients can further be grouped into primary and secondary nutrients. The primary macronutrients are Nitrogen (N), Phosphorus (P) and Potassium (K), abbreviated as NPK. The secondary macronutrients are Sulphur, Calcium and Magnesium. The micronutrients (also called trace elements) that the plants need in small quantities are Boron, Chlorine, Cobalt, Copper, Iron, Manganese, Molybdenum, Nickel, Silicon, Sodium and Zinc.
Soil that is rich in organic matter usually contains adequate quantities of secondary macronutrients and micronutrients. As plants use large quantities of primary macronutrients (NPK) for their growth, these get depleted from the soil and need to be replenished through fertilization. Therefore during discussions about plant nutrients and fertilisation, the focus generally is on NPK only.
Among the NPK nutrients, Nitrogen is required for plant growth above the ground. Nitrogen is mainly responsible for leaf and stem growth, as well as for overall size and vigour and the green foliage. Too much supply of Nitrogen fertiliser produces quick growth, but weaker plants that are more susceptible to attacks by diseases and pests. Yellowing leaves is an indication of Nitrogen deficiency.
Phosphorus, the second major element in plant nutrition, stimulates root growth. Phosphorus helps the plant set buds and flowers and improves vitality. To absorb Phosphorus, plants require pH value of soil to be 6.5 – 6.8. An easy way to maintain the right pH value is to apply compost. With Phosphorus deficiency, the leaves turn to purplish/bluish colour.
Potassium improves overall vigour of the plant. It helps the plants to make carbohydrates and improves disease resistance. Potassium also increases the yield of fruit trees and vegetable plants. Curled and scorched leaves is an indication of Potassium deficiency.
A quick code to remember NPK functions can be: Nitrogen for top (promotes top growth), Phosphorus for bottom (promotes good roots) and Potassium for overall health (benefits the whole plant).
Nutrients get depleted from the soil over time due to absorption by plants. Fertilisers are added, generally to the soil, to replenish the depleted nutrients. Commercially available fertilisers fall into the category of inorganic (chemical) fertilisers and organic fertilisers. Inorganic fertilisers contain one or more of the NPK macronutrients in mineral form that are readily absorbable by the plants. An example is Urea, a common fertiliser, which has 46% Nitrogen by weight. Another example is 18:18:18, a general purpose fertiliser, which has NPK in equal proportion. Fertilisers with various NPK numbers (ratios), designed for different types of plants, are commercially available.
NPK number indicates the proportion, by weight, of each macronutrient that the fertiliser contains. The first number is the proportion of Nitrogen (N), the second of Phosphorus (P) and the third is of Potassium (K). When a 4th number is included (NPKS), it indicates the percentage Sulphur content.
Total of the percentages of the ingredients should ideally add up to 100%. However, it does not. This is because an inert material added as filler. The filler comprises chemicals and/or ingredients, such as fine sand or limestone granules, purpose of which is (1) to keep the NPK stable while in storage, (2) to dilute the strength of the fertiliser, and (3) to allow a more uniform application of the nutrients. For example, a 100 Kilograms (KGs) bag of fertiliser with an NPK ratio of 18:18:18 contains 18 KGs of Nitrogen, 18 KGs of Phosphorus, 18 KGs of Potassium and the rest 46 KGs is filler.
The N number in NPK reflects the actual amount of elemental Nitrogen. However, the numbers for P and K do not reflect the actual amount of elemental Phosphorus and Potassium present in the fertiliser; rather they represent the amount of oxide in the form of Phosphate (P2O5) and Potash (K2O) in the fertiliser. P2O5 has 43.6% elemental Phosphorus and K2O has 83% elemental Potassium. Therefore, as an example, 18:18:18 fertiliser will have 18% elemental Nitrogen, 7.85% (=18 x 43.6%) elemental Phosphorus and 14.94% (=18 x 83%) elemental Potassium.
NPK contents of some commercially available fertilisers are given in the table below.
| Sl. No. | Fertiliser | %N | %P | %K |
| 1 | Ammonium Super Phosphate | 3-6 | 48-53 | 0 |
| 2 | Di-ammonium Phosphate (DAP) | 18 | 46 | 0 |
| 3 | Factamphos (also contains Suphur 13%) | 20 | 20 | 0 |
| 4 | Mono-ammonium Phosphate | 12 | 61 | 0 |
| 5 | Potassium Chloride | 0 | 0 | 60 |
| 6 | Potassium Nitrate | 13 | 0 | 44 |
| 7 | Super Phosphate | 0 | 18-50 | 0 |
| 8 | Urea | 46 | 0 | 0 |