Fertilisers: Types and Plant Nutrients – Fertilisers (Senior 4 Chemistry) | Ganzaa

Fertilisers

Lessons

Senior 4 · Chemistry

Fertilisers

4 lessons

Lesson 1 of 4

Fertilisers: Types and Plant Nutrients

25 min

What the numbers on the bag mean

In Season A of 2024, according to the National Institute of Statistics of Rwanda (NISR), 64.5% of Rwandan farmers used inorganic (mineral) fertilisers and 89.1% used organic fertilisers such as manure and compost. The bags in a cooperative store are printed "Urea 46-0-0", "DAP 18-46-0" or "NPK 17-17-17". Each harvest carries nutrients out of the field in grain, tubers and leaves; a fertiliser puts them back. This lesson explains which elements a crop needs, in what form it takes them up, and how to read, and calculate from, a fertiliser label.

Definition

Fertiliser: any substance added to soil (or sprayed on leaves) to supply one or more elements that plants need for growth. Example: urea, $\mathrm{CO(NH_2)_2}$ , supplies nitrogen.

Macronutrient: an element needed in large amounts (kilograms per hectare): the primary nutrients N, P and K, and the secondary nutrients Ca, Mg and S. Micronutrient (trace element): an element needed in very small amounts (grams per hectare), such as Fe, Mn, Zn, Cu, B, Mo, Cl and Ni.

Straight fertiliser: supplies one primary nutrient (urea: N only). Compound fertiliser: supplies two or more (DAP: N and P; NPK: all three).

Organic fertiliser: of plant or animal origin, such as manure or compost. Inorganic (mineral) fertiliser: a manufactured or mined salt, such as ammonium sulfate.

What plants need, and in what form

Plants take up nutrients as ions dissolved in soil water, through their roots. Carbon, hydrogen and oxygen come from air and water; everything else must come from the soil.

Table 15.1: The main plant nutrients

Element	Taken up as	Main roles in the plant	Signs of deficiency
Nitrogen	$\mathrm{NO_3^-}$ , $\mathrm{NH_4^+}$	amino acids and proteins, chlorophyll, DNA; leafy growth	pale yellow older leaves; stunted growth
Phosphorus	$\mathrm{H_2PO_4^-}$ , $\mathrm{HPO_4^{2-}}$	ATP (energy transfer), DNA, cell membranes; roots, flowers and seeds	poor roots; dark or purplish leaves; late ripening
Potassium	$\mathrm{K^+}$	water balance and opening of stomata; activates enzymes; fruit quality and disease resistance	scorched, brown leaf edges; weak stems
Calcium	$\mathrm{Ca^{2+}}$	cell walls	damaged growing tips
Magnesium	$\mathrm{Mg^{2+}}$	the central atom of chlorophyll	yellowing between the veins
Sulfur	$\mathrm{SO_4^{2-}}$	some amino acids (cysteine, methionine)	yellowing of young leaves
Micronutrients	e.g. $\mathrm{Fe^{2+}}$ , $\mathrm{Zn^{2+}}$ , $\mathrm{MoO_4^{2-}}$	parts of enzymes	varied; for example, iron shortage gives pale young leaves

Types of fertiliser

Type	Examples (formula)	Nutrients
Straight nitrogen	urea $\mathrm{CO(NH_2)_2}$ ; ammonium nitrate $\mathrm{NH_4NO_3}$ ; ammonium sulfate $\mathrm{(NH_4)_2SO_4}$	N (and S in ammonium sulfate)
Straight phosphate	single superphosphate $\mathrm{Ca(H_2PO_4)_2 + CaSO_4}$ ; triple superphosphate $\mathrm{Ca(H_2PO_4)_2}$	P (and Ca, S)
Straight potash	potassium chloride (muriate of potash) KCl; potassium sulfate $\mathrm{K_2SO_4}$	K
Compound	diammonium phosphate (DAP) $\mathrm{(NH_4)_2HPO_4}$ ; NPK blends such as 17-17-17	N and P; N, P and K
Organic	farmyard manure, compost, green manure, crop residues	small amounts of many nutrients, plus organic matter

Reading an N-P-K label

The three numbers on a bag are percentages by mass. By convention: the first is the % of nitrogen (as N); the second is the % of phosphorus expressed as $\mathrm{P_2O_5}$ ; the third is the % of potassium expressed as $\mathrm{K_2O}$ . To convert to the elements: $\%\,\mathrm{P} = 0.436 \times \%\,\mathrm{P_2O_5} \qquad \%\,\mathrm{K} = 0.830 \times \%\,\mathrm{K_2O}$ because $\frac{2 \times 30.97}{141.94} = 0.436$ and $\frac{2 \times 39.10}{94.20} = 0.830$ .

Figure 15.1: Types of inorganic fertiliser on Rwandan plots that received one, 2024 Season A (national figures, NISR Seasonal Agricultural Survey 2024, Table 52). DAP and urea together account for 84% of plots.

Worked example 1: which nitrogen fertiliser is richest?

Calculate the percentage by mass of nitrogen in urea, ammonium nitrate and ammonium sulfate. (H = 1.008, C = 12.01, N = 14.01, O = 16.00, S = 32.06)

Solution. $M(\mathrm{CO(NH_2)_2}) = 60.06 \qquad \%\,\mathrm{N} = \frac{2 \times 14.01}{60.06} \times 100 = 46.7\%$ $M(\mathrm{NH_4NO_3}) = 80.05 \qquad \%\,\mathrm{N} = \frac{2 \times 14.01}{80.05} \times 100 = 35.0\%$ $M(\mathrm{(NH_4)_2SO_4}) = 132.14 \qquad \%\,\mathrm{N} = \frac{2 \times 14.01}{132.14} \times 100 = 21.2\%$ Urea has the highest nitrogen content, so less of it has to be bought, transported and spread for the same mass of nitrogen. This is why commercial urea is labelled 46-0-0.

Worked example 2: what is in a 50 kg bag of NPK 17-17-17?

Calculate the masses of nitrogen, phosphorus and potassium in a 50.0 kg bag of NPK 17-17-17.

Solution. Each number is 17% of the bag: $0.17 \times 50.0 = 8.5\ \mathrm{kg}$ of N, of $\mathrm{P_2O_5}$ and of $\mathrm{K_2O}$ . Converting: $m(\mathrm{P}) = 0.436 \times 8.5 = 3.7\ \mathrm{kg} \qquad m(\mathrm{K}) = 0.830 \times 8.5 = 7.1\ \mathrm{kg}$ So the bag holds 8.5 kg N, 3.7 kg P and 7.1 kg K: 19.3 kg of nutrient elements. The rest is the oxygen counted in the oxide convention, the other ions in the salts (such as $\mathrm{Cl^-}$ and $\mathrm{SO_4^{2-}}$ ) and filler.

Warning

Common mistakes. (1) Reading the second number as % of phosphorus: it is % of $\mathrm{P_2O_5}$ ; multiply by 0.436. (2) Thinking a 17-17-17 bag is 51% nutrients: the elements N + P + K make up only about 39% of it. (3) Saying plants take in "nitrogen gas" from fertiliser: they absorb nitrate and ammonium ions dissolved in soil water. (4) Calling lime a fertiliser: it mainly corrects soil acidity, although it does supply calcium.

Safety. Store fertilisers dry, off the floor, in their labelled bags and away from food, animal feed and children. Wash your hands after handling them and never use a fertiliser bag to carry food. Ammonium nitrate is an oxidising agent: keep it away from fuels, oils and sources of heat.

Tip

N for leaves, P for roots and seeds, K for fruit and strength. On a label, only N is the element itself; P and K are given as oxides, so multiply P by 0.436 and K by 0.830.

KayScience: NPK Fertilisers Explained (7 min). Watch for what nitrogen, phosphorus and potassium each do in a plant, and how NPK fertilisers raise crop yields.

✦

Practice

Name the three primary nutrients and give the ion in which each is taken up.
Distinguish between a straight and a compound fertiliser, with an example of each.
A maize crop has pale yellow older leaves and is stunted. Which nutrient is probably lacking, and which fertiliser would you suggest?
Calculate the percentage by mass of nitrogen in diammonium phosphate, $\mathrm{(NH_4)_2HPO_4}$ (P = 30.97).
Calculate the percentage of K in pure KCl (K = 39.10, Cl = 35.45) and express it as % $\mathrm{K_2O}$ .
A field needs 50 kg of nitrogen per hectare. What mass of urea (46.7% N) supplies this?
Calculate the mass of phosphorus in a 50.0 kg bag of DAP labelled 18-46-0.
A farmer compares two bags of the same price: ammonium sulfate (21-0-0) and urea (46-0-0). Which gives more nitrogen for the money, and by what factor? Give one reason the farmer might still choose ammonium sulfate.

Show answersHide answers

N as $\mathrm{NO_3^-}$ or $\mathrm{NH_4^+}$ ; P as $\mathrm{H_2PO_4^-}$ or $\mathrm{HPO_4^{2-}}$ ; K as $\mathrm{K^+}$ .
A straight fertiliser supplies one primary nutrient (urea, N); a compound fertiliser supplies two or more (DAP: N and P; NPK 17-17-17).
Nitrogen; a nitrogen fertiliser such as urea or ammonium sulfate (or an NPK with high N).
$M = 132.06$ ; $\%\,\mathrm{N} = \frac{28.02}{132.06} \times 100 = 21.2\%$ .
% K $= \frac{39.10}{74.55} \times 100 = 52.4\%$ ; % $\mathrm{K_2O} = \frac{52.4}{0.830} = 63.2\%$ .
$\frac{50}{0.467} = 107\ \mathrm{kg}$ of urea.
$\mathrm{P_2O_5} = 0.46 \times 50.0 = 23\ \mathrm{kg}$ ; P $= 0.436 \times 23 = 10\ \mathrm{kg}$ .
Urea: $\frac{46}{21} = 2.2$ times as much nitrogen per bag. Ammonium sulfate also supplies sulfur, which a sulfur-deficient soil may need.

Quick check

Match each nutrient to its main role in a plant.

Tap one on the left, then its partner on the right.

Key points

Plants take up nutrients as ions: $\mathrm{NO_3^-}$ / $\mathrm{NH_4^+}$ (N), $\mathrm{H_2PO_4^-}$ (P), $\mathrm{K^+}$ (K), plus secondary nutrients (Ca, Mg, S) and micronutrients.
Fertilisers are straight (one primary nutrient) or compound (two or more), organic or inorganic.
An N-P-K label gives % N, % $\mathrm{P_2O_5}$ and % $\mathrm{K_2O}$ by mass; % P = 0.436 × % $\mathrm{P_2O_5}$ , % K = 0.830 × % $\mathrm{K_2O}$ .
Urea (46.7% N) is the richest common nitrogen fertiliser. In Rwanda (2024 Season A) DAP and urea were used on 84% of fertilised plots (NISR).

Sources and credits

Image: Tripelsuperfosfaat (triple phosphate).jpg by Rasbak, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Image: Ammonium sulfate sample.jpg by Adam Rędzikowski, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Rwandan data: Seasonal Agricultural Survey, Annual Report 2024, National Institute of Statistics of Rwanda (NISR), 2025 (Table 0 and Table 52).
N-P-K convention and conversion factors adapted from Labeling of fertilizer, Wikipedia contributors, licensed under CC BY-SA 4.0, adapted by Ganzaa.
Video: GCSE Chemistry – NPK Fertilisers Explained (Nitrogen, Phosphorus & Potassium), KayScience, embedded from YouTube.
The text, tables, chart, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1What the numbers on the bag mean

2What plants need, and in what form

3Types of fertiliser

4Reading an N-P-K label

What the numbers on the bag mean

What plants need, and in what form

Types of fertiliser

Reading an N-P-K label