Nutrient Removal by a Crop

Nutrient removal is the floor under any fertiliser programme: whatever leaves the field in the harvested crop must eventually be replaced, or the soil reserve is being mined. It is yield multiplied by a removal coefficient specific to the crop and the part harvested.

Typical figures for cereals run about 20 kg of nitrogen, 4 kg of phosphorus and 5 kg of potassium per tonne of grain. Oilseeds and pulses remove considerably more nitrogen per tonne; legumes fix a large share of theirs, so their removal figure overstates what must be bought.

What is harvested matters more than any other variable. A cereal crop cut for grain leaves the straw, which holds most of the potassium the crop took up; bale that straw and potassium removal can triple. Silage and whole-crop harvests remove everything, which is why a maize silage field runs down its potassium reserve far faster than a grain maize field at identical yield.

Removal is not the same as fertiliser requirement, and treating them as equal is a common shortcut with two errors in it. Removal ignores losses — leaching, denitrification, volatilisation and fixation all take nutrient that the crop never sees — and it ignores what the soil supplies from its own reserves and from mineralising organic matter. Replacement-based programmes work well for phosphorus and potassium, which are relatively immobile and accumulate predictably. They work badly for nitrogen, whose fate depends on weather more than on arithmetic.