Free farm tool

Nutrient removal calculator

How much N, P and K last season's harvest took off the land, and what it would cost to put back.

Crop

Coefficients are per ton of grain at harvest moisture.

Area harvested

Yield

t/ha

Tons of grain at harvest moisture per hectare that left the field.

Removal per ton harvested

Typical values from published crop nutrient removal tables. If you have a lab analysis of your own crop, use those figures instead. P and K are elemental, not P₂O₅ or K₂O.

N, kg/t

P, kg/t

K, kg/t

Fertiliser prices (optional)

Rand per kg of elemental nutrient, to put a value on what was removed. Leave a price blank to skip it.

N, R/kg

P, R/kg

K, R/kg

Work out R/kg from a product price

R/kg nutrient = product price per ton ÷ (nutrient % × 10). Use the elemental % on the bag; if the label gives P₂O₅ or K₂O, multiply by 0.436 for P or 0.830 for K first. For a product with two nutrients, such as MAP, this puts the whole price on one of them, so it overstates that one.

Product price

R/t

Nutrient content

Enter a price and a nutrient % between 0 and 100.

Enter the area harvested (more than 0 ha) and the yield to see how much N, P and K the crop removed.

How this is calculated

Every ton of grain, seed, tubers or hay that leaves the field takes nitrogen, phosphorus and potassium with it. The calculator multiplies your yield by a removal figure for each nutrient. P and K are given as elemental P and K, as on South African fertiliser bags, not as P₂O₅ or K₂O (P = P₂O₅ × 0.436; K = K₂O × 0.830).

Removed per hectare (kg/ha) = yield (t/ha) × removal (kg per ton). 6 t/ha of maize at 15 kg N, 3 kg P and 4 kg K per ton removes 90 kg N, 18 kg P and 24 kg K per hectare.
Removed on the whole area (kg) = kg/ha × area (ha). On 100 ha that is 9 000 kg N, 1 800 kg P and 2 400 kg K.
Value per hectare (R/ha) = the sum of kg/ha × price (R per kg of nutrient). At R20/kg N, R45/kg P and R25/kg K: 90 × 20 + 18 × 45 + 24 × 25 = 1 800 + 810 + 600 = R3 210/ha, or R321 000 on 100 ha.
Price per kg of nutrient = product price (R/t) ÷ (nutrient % × 10), because a ton of product at 46% N holds 460 kg N. Urea at R9 200/t works out at 9 200 ÷ 460 = R20 per kg N.

Default removal, kg per ton of harvested product (N, P, K): maize (grain) 15, 3, 4; wheat (grain) 20, 3.5, 4.5; sorghum (grain) 15, 3, 3.5; soybean 60, 5.5, 18; sunflower 28, 5, 7; dry beans 35, 4.5, 14; groundnuts 40, 3.5, 8; potatoes 3.5, 0.6, 5.5; maize silage 4, 0.7, 4; lucerne hay 28, 2.5, 22. These are typical values from published crop nutrient removal tables. Actual removal changes with variety, yield level, soil fertility and season, so edit them, and use a lab analysis of your own crop where you have one. Silage figures are per ton as fed, not per ton of dry matter.

Soybean, dry beans, groundnuts and lucerne are legumes. A well-nodulated legume fixes most of its nitrogen from the air, so the N it carries off is not what you need to replace; use the P and K figures for that.

This is the nutrient that left in the harvest, not a fertiliser recommendation. Soil reserves, losses, residues left on the land and what the next crop needs all matter, so check a recent soil analysis before deciding what to apply.

Read the guide: Replacing the nutrients your harvest removed

Results are estimates for planning. Check product labels and local recommendations before you apply anything.