NPK Fertiliser Explained, and the Element Your NPK Program Is Missing

What NPK means on the bag

NPK stands for nitrogen, phosphorus and potassium, the three nutrients crops need in the largest amounts. Every fertiliser bag (or fertilizer bag, if you prefer the US spelling) carries an NPK ratio that tells you the percentage of each by weight. A bag labelled 10:4:6 contains 10% nitrogen, 4% phosphorus and 6% potassium.

Reading the ratio lets you compare products on what they actually deliver, not the marketing on the front. Two bags at different prices can supply the same nutrients at very different costs per kilogram of N, P or K.

What each element does, and what shortage looks like

Nitrogen (N) drives leaf growth and protein. It is what makes crops green and vigorous. Shortage shows as pale, yellowing older leaves and stunted growth. Too much shows as lush, soft growth that lodges and cops disease.

Phosphorus (P) drives root development, early vigour and energy transfer inside the plant. Shortage shows as poor establishment, purpling leaves and delayed maturity. The catch with phosphorus is that it locks onto iron and aluminium minerals in the soil, so a soil test can show plenty of total P while the crop goes hungry.

Potassium (K) regulates water, disease resistance and grain fill. Shortage shows as leaf edge scorch, poor grain fill and crops that wilt fast in the dry. Hay and silage paddocks get mined of K because every bale carries it away.

The element your NPK program is missing

Silicon is not part of NPK, and it is not classed as essential. It is a beneficial plant nutrient, and the research case for it keeps growing. Plants take up silicon in quantities comparable to the major nutrients, deposit it in cell walls, and use it to cope with stress.

The most interesting interaction is with phosphorus. Research published in Scientific Reports shows silicon competes with phosphate for binding sites on iron minerals in soil. In plain terms, silicon can help release phosphorus that is already in your soil but locked up.

MaxSil's red ferrosol work shows why this matters. Soils with high total phosphorus on a soil test, for example 2,600 ppm, can still be short of available P. The phosphorus is there; the plant just cannot get it. The way to think about silicon is that it can help unlock the phosphorus you have already paid for.

For nitrogen, the evidence is at farm scale: Tony Donovan at Midfield Meats reported cutting urea use by more than 40% across 14,000 hectares after adding MaxSil, while maintaining or increasing production (Stock & Land, June 2026).

How to add silicon to an NPK program

MaxSil is not a replacement for N, P and K. It works alongside them. MaxSil Powder supplies 40,000 ppm soluble silicon for broadacre and blending, at a typical starting rate of 50 kg/ha confirmed by a MaxSil agronomist after a soil test. MaxSil Granules supply 10,000 ppm soluble silicon in a certified organic compost blend for cereals and pasture.

MaxSil blends with urea, DAP and MAP and goes out through standard spreading gear, so adding silicon to your NPK program costs no extra passes. Start with a strip trial, measure the difference, and scale what works.