Qualities of a Good Fertiliser

25 min

When the soil itself is the problem

The Ministry of Agriculture and Animal Resources (MINAGRI) reports that soil acidity affects nearly 50% of Rwanda's cropland, especially in the highland districts. In its 2024–2025 annual report it describes field trials on very acid soils (pH 4.2 to 5.0) in Burera District. Beans grown with no lime and no phosphate gave 0.82 tonnes per hectare. Adding 40 kg of phosphorus per hectare alone raised it only to 1.16 t/ha. Adding 2 tonnes of lime per hectare raised the yield to 2.13 t/ha, and lime plus 40 kg of phosphorus per hectare gave 4.01 t/ha, almost five times as much. The right fertiliser in the wrong soil is money wasted. This lesson sets out what makes a fertiliser good, and the chemistry behind each quality.

Definition

Soil pH: a measure of the acidity of the soil solution. Most crops grow best between about pH 6 and 7. In acid soils (pH below about 5.5) phosphate is locked up and $\mathrm{Al^{3+}}$ ions become soluble and poison roots.

Liming: adding a base such as calcium carbonate (agricultural lime, ishwagara) or calcium oxide to raise the pH of acid soil: $\mathrm{CaCO_3 + 2H^+ \rightarrow Ca^{2+} + CO_2 + H_2O}$ .

Leaching: the washing of dissolved nutrients down through the soil, below the roots, by rain or irrigation water. Nitrate is easily leached.

Hygroscopic: absorbing water vapour from the air. Example: ammonium nitrate, which therefore becomes sticky and cakes.

Seven qualities, and the chemistry behind them

A high percentage of nutrient. Every kilogram of filler or of non-nutrient ions has to be paid for, carried up the hill and spread. Urea (46.7% N) beats ammonium sulfate (21.2% N) on this count.
Soluble enough, but not too soluble. Roots absorb only dissolved ions, so the fertiliser must dissolve. But a very soluble nitrate can be leached by heavy rain before the roots reach it. Splitting the dose (some at planting, some later) or using slow-release forms matches the supply to the crop's needs.
A suitable effect on soil pH. Ammonium ions in the soil are oxidised by bacteria to nitrate (nitrification), and this releases acid: $\mathrm{NH_4^+ + 2O_2 \rightarrow NO_3^- + 2H^+ + H_2O}$ Repeated use of ammonium fertilisers therefore makes soil more acidic; on already acid soils they must be used with lime.
Easy to store and spread. It should not absorb water and cake into lumps. Granules or prills flow freely and spread evenly; powder blows away and cakes.
Safe. It should not be poisonous, flammable or explosive in normal handling, and should be free of harmful impurities such as biuret in urea or cadmium in phosphates.
Cheap per kilogram of nutrient, not per kilogram of bag.
Matched to the crop and the soil. It should supply the nutrients that are actually short, in a form the plant can absorb, at the time it needs them. Soil testing tells the farmer which.

Table 15.2: Some common fertilisers compared

Fertiliser	Nutrient content	Solubility in water	Effect on soil pH	Handling
Urea	46.7% N	120 g per 100 cm³ (25 °C)	acidifying (through nitrification)	granules; ammonia can be lost if left on the surface of warm, moist soil
Ammonium nitrate	35.0% N	192 g per 100 cm³ (20 °C)	acidifying	hygroscopic, cakes; an oxidiser that can explode in a fire
Ammonium sulfate	21.2% N, 24% S	76.4 g per 100 g	strongly acidifying	stores well
DAP	18% N, 46% $\mathrm{P_2O_5}$	57.5 g per 100 cm³ (10 °C)	acidifying in the long run	granules, stores well
Potassium chloride	60–63% $\mathrm{K_2O}$	34.0 g per 100 cm³ (20 °C)	little effect	stores well

Figure 15.2: Bean yields in MINAGRI's field trials on acid soils in Burera District (MINAGRI Annual Report 2024–2025, Table 31). Phosphate gave its full benefit only once lime had raised the soil pH.

A tractor with a twin-disc spreader applies granular urea to a crop of oilseed rape in England, March 2009. Granules flow freely and can be spread evenly, one of the qualities of a good fertiliser. Photo: Michael Trolove, CC BY-SA 2.0, via Wikimedia Commons

Worked example 1: how much lime does ammonium sulfate cost the soil?

A farmer applies ammonium sulfate containing 100 kg of nitrogen. Assuming all the ammonium is nitrified, calculate the mass of calcium carbonate needed to neutralise the acid produced. (N = 14.01; $\mathrm{CaCO_3}$ = 100.09)

Solution. $n(\mathrm{N}) = n(\mathrm{NH_4^+}) = \frac{100 \times 10^{3}\ \mathrm{g}}{14.01\ \mathrm{g\,mol^{-1}}} = 7.14 \times 10^{3}\ \mathrm{mol}$ Each $\mathrm{NH_4^+}$ releases 2 $\mathrm{H^+}$ on nitrification: $n(\mathrm{H^+}) = 1.43 \times 10^{4}\ \mathrm{mol}$ . Each $\mathrm{CaCO_3}$ neutralises 2 $\mathrm{H^+}$ : $n(\mathrm{CaCO_3}) = 7.14 \times 10^{3}\ \mathrm{mol}$ . $m(\mathrm{CaCO_3}) = 7.14 \times 10^{3} \times 100.09 = 7.14 \times 10^{5}\ \mathrm{g} = 714\ \mathrm{kg}$ Over seven times the mass of nitrogen applied: a strong reason to test soil pH and lime acid soils.

Worked example 2: the cheapest nitrogen

Illustrative prices (not real market data): urea costs 800 Frw per kg and ammonium sulfate 500 Frw per kg. Which supplies nitrogen more cheaply? Evaluate the choice for a farmer on acid soil.

Solution. Cost per kg of nitrogen = price per kg of fertiliser ÷ mass fraction of N. $\text{urea: } \frac{800}{0.467} = 1.71 \times 10^{3}\ \text{Frw per kg N} \qquad \text{ammonium sulfate: } \frac{500}{0.212} = 2.36 \times 10^{3}\ \text{Frw per kg N}$ Urea supplies nitrogen about 28% more cheaply, although its bag price is higher. On acid soil it is also the better choice: ammonium sulfate acidifies more strongly per kilogram of nitrogen. Ammonium sulfate would be preferred only where the soil is also short of sulfur.

Warning

Common mistakes. (1) Comparing fertilisers by price per bag instead of price per kilogram of nutrient. (2) Thinking "more soluble is always better": highly soluble nitrate is easily leached. (3) Forgetting that ammonium fertilisers acidify soil through nitrification. (4) Mixing lime and ammonium fertilisers in the same hole: the base drives off ammonia gas, $\mathrm{NH_4^+ + OH^- \rightarrow NH_3 + H_2O}$ , and nitrogen is lost. Apply them separately.

Safety. Agricultural lime and quicklime (CaO) irritate the eyes and skin; quicklime reacts with water and can burn. Wear gloves and goggles, and spread with your back to the wind. Ammonium nitrate must be stored away from fuels, oils, heat and flames: stockpiles of it have exploded in fires, as at Beirut in 2020.

Tip

Think "per kilogram of nutrient", and "what will it do to the pH?" A good fertiliser is rich, soluble but not easily leached, gentle on the soil pH, free-flowing, safe, cheap per kg of nutrient and matched to what the soil lacks.

Food and Agriculture Organization of the United Nations (FAO): Use of Lime to Address Soil Acidity (4 min), a farmers' film by James Karuga. Watch for why soil pH controls nutrient availability and how lime is applied.

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Practice

List five qualities of a good fertiliser.
Explain why a fertiliser must be soluble in water, and why a very soluble one can also be a disadvantage.
Write the equation for nitrification and use it to explain why ammonium fertilisers make soil more acidic.
Write equations for the neutralisation of soil acid by (a) calcium carbonate and (b) calcium oxide.
Use Figure 15.2 to explain why adding phosphate alone did not raise bean yields much on the Burera soils.
Calculate the mass of calcium carbonate needed to neutralise the acid from nitrification of 50 kg of nitrogen applied as ammonium nitrate, assuming only the ammonium half is nitrified (N = 14.01, $\mathrm{CaCO_3}$ = 100.09).
Explain why lime and ammonium sulfate should not be mixed before they are spread.
Illustrative prices: DAP 900 Frw per kg (18% N, 46% $\mathrm{P_2O_5}$ ) and triple superphosphate 750 Frw per kg (45% $\mathrm{P_2O_5}$ ). A farmer needs phosphate only. Calculate the cost per kg of $\mathrm{P_2O_5}$ for each and evaluate which to buy.

Show answersHide answers

Any five: high nutrient content; soluble but not easily leached; suitable effect on soil pH; does not cake, easy to store and spread; safe and free of harmful impurities; cheap per kg of nutrient; supplies what the crop and soil need.
Roots absorb only dissolved ions. A very soluble ion such as nitrate can be washed below the roots by heavy rain before it is taken up.
$\mathrm{NH_4^+ + 2O_2 \rightarrow NO_3^- + 2H^+ + H_2O}$ : each ammonium ion releases two hydrogen ions into the soil.
(a) $\mathrm{CaCO_3 + 2H^+ \rightarrow Ca^{2+} + CO_2 + H_2O}$ ; (b) $\mathrm{CaO + 2H^+ \rightarrow Ca^{2+} + H_2O}$ .
In very acid soil phosphate is locked up (by $\mathrm{Al^{3+}}$ and iron) and $\mathrm{Al^{3+}}$ harms roots; lime raised the pH, so the phosphate became available and gave a large yield increase.
$\mathrm{NH_4NO_3}$ has 2 N, one in $\mathrm{NH_4^+}$ : N in ammonium = 25 kg; $n = \frac{25\,000}{14.01} = 1.78 \times 10^{3}\ \mathrm{mol}$ ; $\mathrm{H^+} = 3.57 \times 10^{3}\ \mathrm{mol}$ ; $\mathrm{CaCO_3} = 1.78 \times 10^{3}\ \mathrm{mol} = 179\ \mathrm{kg}$ .
The base in lime reacts with ammonium ions to release ammonia gas, so nitrogen is lost to the air.
DAP: $\frac{900}{0.46} = 1957$ Frw per kg $\mathrm{P_2O_5}$ ; TSP: $\frac{750}{0.45} = 1667$ Frw per kg. TSP is cheaper for phosphate alone; DAP is worth it only if the nitrogen it also supplies is needed.

Quick check

Why does repeated use of ammonium fertilisers make soil more acidic?

Key points

A good fertiliser has a high nutrient content, dissolves but is not easily leached, does not harm soil pH, stores and spreads well, is safe and pure, is cheap per kg of nutrient, and supplies what the soil lacks.
Nitrification, $\mathrm{NH_4^+ + 2O_2 \rightarrow NO_3^- + 2H^+ + H_2O}$ , makes ammonium fertilisers acidifying; lime neutralises soil acid: $\mathrm{CaCO_3 + 2H^+ \rightarrow Ca^{2+} + CO_2 + H_2O}$ .
MINAGRI reports that acidity affects nearly half of Rwanda's cropland; in its Burera trials, lime plus phosphate raised bean yields from 0.82 to 4.01 t/ha.
Compare fertilisers by cost per kilogram of nutrient, not per bag.

Sources and credits

Image: Granular Urea application - geograph.org.uk - 1219037.jpg by Michael Trolove, via Wikimedia Commons, licensed under CC BY-SA 2.0.
Rwandan data: Annual Report 2024–2025, Ministry of Agriculture and Animal Resources (MINAGRI), section 3.1.1.3 and Table 31.
Solubilities and hazards from the Wikipedia articles Urea, Ammonium nitrate, Ammonium sulfate, Diammonium phosphate and Potassium chloride, Wikipedia contributors, licensed under CC BY-SA 4.0.
Video: Use of lime to address soil acidity by James Karuga, Food and Agriculture Organization of the United Nations, embedded from YouTube.
The text, table, chart, worked examples and Practice items were written and drawn by Ganzaa. Prices in the worked examples and Practice are illustrative.
This lesson uses CC BY-SA material and is shared under CC BY-SA 4.0.

1When the soil itself is the problem

2Seven qualities, and the chemistry behind them

When the soil itself is the problem

Seven qualities, and the chemistry behind them