Fertilisers

Lessons

Senior 4 · Chemistry

Fertilisers

4 lessons

Lesson 4 of 4

Organic and Inorganic Fertilisers, and the Dangers of Substandard Fertilisers

25 min

Manure and bags, side by side

Rwandan farmers do not choose between organic and inorganic fertilisers: most use both. NISR's surveys show that in Season A of 2024, 89.1% of farmers applied organic fertiliser (manure, compost) and 64.5% applied inorganic fertiliser, and both shares had risen since 2022 (Figure 15.4). Each type has strengths the other lacks. Both can also do harm: too much fertiliser, or the wrong kind, pollutes water and soil, and a bag whose contents do not match its label cheats the farmer and can damage the crop. This lesson weighs the two types and examines what goes wrong.

Figure 15.4: Percentage of Rwandan farmers who used organic and inorganic fertilisers in Season A, 2022–2024 (NISR, Seasonal Agricultural Survey 2024, Table 0).

Definition

Manure: animal dung and urine, often mixed with bedding. Compost: plant and animal remains that microorganisms have broken down into a dark, crumbly material (humus) rich in organic matter.

Eutrophication: the over-enrichment of a lake, river or coastal water with nutrients (mainly nitrate and phosphate), which causes excessive growth of algae and loss of dissolved oxygen.

Substandard fertiliser: a fertiliser whose nutrient content or quality is below what its label states, or which contains harmful impurities. Adulteration: deliberately mixing in a cheaper material, such as sand, salt or a lower-grade fertiliser.

Organic and inorganic compared

Feature	Organic (manure, compost)	Inorganic (urea, DAP, NPK)
Nutrient content	low and variable; tonnes per hectare are needed	high and known (on the label); kilograms per hectare are enough
Speed of release	slow: nutrients are released as microbes decompose the material	fast: soluble ions are available at once
Effect on soil structure	adds organic matter: better crumb structure, holds water and nutrients, feeds soil life	none directly; long use without organic matter can harden and acidify the soil
Effect on pH	tends to buffer soil pH	ammonium fertilisers acidify
Cost and handling	made on the farm, but bulky and heavy to carry	must be bought, but easy to transport and dose
Risks	disease organisms and weed seeds if not well composted; nutrient losses if left in the rain	leaching and run-off of nitrate and phosphate; overuse; adulterated products

The best practice, often called integrated soil fertility management, combines them: organic matter to keep the soil healthy and hold water, mineral fertiliser to supply exactly the nutrients that are short, and lime where the soil is acid.

A heap of farmyard manure at the edge of a field in Derbyshire, England (2006). Manure adds organic matter as well as nutrients, but it is bulky: tonnes are needed for each hectare. Photo: Michael Patterson, CC BY-SA 2.0, via Wikimedia Commons

When fertiliser escapes: water and soil

Nitrate leaching. Nitrate is very soluble and is not held by soil particles, so rain carries surplus nitrate down to the groundwater. The World Health Organization's guideline value for drinking water is 50 mg of nitrate per litre (equivalent to 11 mg of nitrate-nitrogen per litre), set to protect bottle-fed babies from methaemoglobinaemia ("blue-baby syndrome").
Eutrophication. Nitrate and phosphate washed into lakes feed a bloom of algae. When the algae die, bacteria decompose them and use up the dissolved oxygen, so fish and other animals suffocate.
Soil acidification. Nitrification of ammonium releases $\mathrm{H^+}$ (Lesson 2); without lime, repeated use lowers soil pH.
Heavy metals. Phosphate rock contains cadmium; phosphate fertilisers can carry from about 0.1 to over 50 mg of cadmium per kilogram, and continued use of high-cadmium products builds it up in soil.
Greenhouse gas. Some fertiliser nitrogen is turned by soil bacteria into nitrous oxide, $\mathrm{N_2O}$ , a powerful greenhouse gas.

Substandard fertilisers

A fertiliser can fall short in several ways:

Low nutrient content: diluted with sand, clay, salt or a cheaper fertiliser, so the bag holds less N, P or K than the label claims.
Wrong product: a bag labelled with one grade but filled with another.
Poor physical condition: damp, caked or broken granules, from old stock or bad storage; it spreads unevenly.
Harmful impurities: biuret in badly made urea (toxic to seedlings), heavy metals such as cadmium, or excess salt.

The farmer pays for nutrients that are not there, the crop is under-fed or damaged, the soil may be harmed, and trust in fertilisers falls. The protections are to buy from registered agro-dealers, in sealed, labelled bags showing the grade, batch and supplier; to store fertiliser dry; and, for officials, to sample and analyse products.

Testing a fertiliser in the laboratory

The ions in a fertiliser can be identified with the tests you already know:

Ion	Test	Positive result
$\mathrm{NH_4^+}$	warm with NaOH(aq)	ammonia gas turns damp red litmus blue
$\mathrm{NO_3^-}$	warm with NaOH(aq) and aluminium foil (or Devarda's alloy)	ammonia gas turns damp red litmus blue
$\mathrm{SO_4^{2-}}$	acidify with dilute HCl, add $\mathrm{BaCl_2}$ (aq)	white precipitate of $\mathrm{BaSO_4}$
$\mathrm{Cl^-}$	acidify with dilute $\mathrm{HNO_3}$ , add $\mathrm{AgNO_3}$ (aq)	white precipitate of AgCl
$\mathrm{PO_4^{3-}}$	acidify with $\mathrm{HNO_3}$ , add ammonium molybdate, warm	yellow precipitate
$\mathrm{K^+}$	flame test	lilac flame

For example, pure urea gives no precipitate with acidified barium chloride; if a "urea" sample gives a heavy white precipitate, it contains sulfate, perhaps from ammonium sulfate mixed in.

Worked example 1: is the well water safe?

A laboratory reports that water from a well near heavily fertilised fields contains 13.5 mg of nitrate-nitrogen (N) per litre. Convert this to mg of nitrate ion per litre and compare it with the WHO guideline value of 50 mg per litre. (N = 14.01, O = 16.00)

Solution. Each $\mathrm{NO_3^-}$ ion ( $M = 62.01$ ) contains one N atom (14.01), so $\text{mass of nitrate} = 13.5 \times \frac{62.01}{14.01} = 13.5 \times 4.426 = 59.8\ \mathrm{mg\,L^{-1}}$ This is above the guideline of 50 mg per litre: the water should not be used to make up babies' feeds, and the fertiliser practice nearby should be examined.

Worked example 2: the cost of an adulterated bag

Illustrative case. A farmer buys a 50.0 kg bag sold as urea (labelled 46% N). Analysis shows only 35.0% N. (a) How much nitrogen is missing? (b) To supply 50 kg of N to a hectare, how much of this product is needed compared with genuine urea? (c) Evaluate the effect on the farmer.

Solution. (a) Labelled N $= 0.46 \times 50.0 = 23.0\ \mathrm{kg}$ ; actual N $= 0.350 \times 50.0 = 17.5\ \mathrm{kg}$ ; missing $= 5.5\ \mathrm{kg}$ , about 24% of what was paid for. (b) Genuine: $\frac{50}{0.46} = 109\ \mathrm{kg}$ ; adulterated: $\frac{50}{0.350} = 143\ \mathrm{kg}$ , about a third more. (c) If the farmer spreads the usual 109 kg, the crop gets only $109 \times 0.350 = 38\ \mathrm{kg}$ of N and yields fall; if the farmer buys more, the cost per kilogram of nitrogen rises by about 31%. Either way the farmer loses, and if the filler is salt or another harmful material the soil may suffer too.

Warning

Common mistakes. (1) Saying organic fertilisers cannot pollute: manure washed into a stream causes eutrophication too. (2) Saying inorganic fertilisers "poison the soil" whatever the dose: used in the right amounts, with lime and organic matter, they raise yields safely. (3) Confusing nitrate with nitrate-nitrogen: 50 mg of nitrate per litre is only 11 mg of nitrogen per litre. (4) Judging a fertiliser by its colour or smell: only analysis shows its true content.

Safety. Wear gloves when handling manure and wash your hands afterwards, because it can carry disease organisms; compost it well before use on vegetables eaten raw. In the laboratory tests, barium chloride is toxic and silver nitrate stains skin: wear goggles and gloves, and warm test tubes gently, pointing them away from people.

Tip

Organic feeds the soil; inorganic feeds the crop. Use both, lime acid soil, apply the right amount at the right time, and buy sealed, labelled bags from registered dealers.

NOAA's National Ocean Service: What Is Eutrophication? (1 min). Watch for how nutrients from land lead to algal blooms, dead zones and fish kills.

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Practice

Give two advantages of organic fertilisers over inorganic ones, and two disadvantages.
Describe, in four steps, how fertiliser run-off can kill fish in a lake.
Explain why nitrate is leached from soil much more easily than ammonium or phosphate.
Use Figure 15.4 to describe the change in fertiliser use by Rwandan farmers from 2022 to 2024 in Season A.
Give three ways in which a fertiliser can be substandard, and one consequence of each.
A water sample contains 75 mg of nitrate per litre. Calculate the concentration of nitrate-nitrogen and say whether the water meets the WHO guideline.
Describe a test that would show whether a sample sold as potassium chloride contains potassium and chloride ions.
A cooperative can buy NPK 17-17-17 from a registered dealer at 950 Frw per kg, or unbranded loose "NPK" from a roadside seller at 700 Frw per kg that analysis shows to be 10-10-10 (illustrative prices). Calculate the cost per kg of nitrogen for each and evaluate which is the better buy.

Show answersHide answers

Advantages: improve soil structure and water holding; release nutrients slowly; cheap or free on the farm; buffer pH. Disadvantages: low and variable nutrient content; bulky; slow to act; may carry pathogens or weed seeds.
Nitrate and phosphate wash into the lake; algae grow rapidly (algal bloom); the algae die and bacteria decompose them, using up dissolved oxygen; fish suffocate.
Nitrate ions are negative and very soluble, and are not held by the negatively charged clay and humus particles; ammonium (positive) is held on them, and phosphate forms insoluble compounds in soil.
Both increased: organic from 69.7% to 89.1% of farmers, inorganic from 38.6% to 64.5%; organic fertiliser was used by more farmers every year.
Low nutrient content (crop under-fed, farmer loses money); wrong product (wrong nutrients for the crop); caked or damp (uneven spreading); toxic impurities such as biuret or cadmium (crop damage, soil contamination).
$75 \times \frac{14.01}{62.01} = 17\ \mathrm{mg\,L^{-1}}$ of nitrate-N; it exceeds the guideline (50 mg of nitrate per litre).
Flame test: lilac flame shows $\mathrm{K^+}$ . Dissolve, acidify with dilute nitric acid, add silver nitrate: a white precipitate shows $\mathrm{Cl^-}$ .
Registered NPK: $\frac{950}{0.17} = 5588$ Frw per kg N; roadside: $\frac{700}{0.10} = 7000$ Frw per kg N. The cheaper bag is about 25% more expensive per kilogram of nitrogen (and of P and K), and its quality is unknown: buy from the registered dealer.

Quick check

Which is an advantage of compost over urea?

Key points

Organic fertilisers are low in nutrients and slow, but improve soil structure; inorganic fertilisers are concentrated and fast, but do nothing for soil structure and can acidify soil. Using both (with lime where needed) is best. In 2024 Season A, 89.1% of Rwandan farmers used organic and 64.5% inorganic fertiliser (NISR).
Overuse causes nitrate leaching (WHO guideline: 50 mg nitrate per litre of drinking water), eutrophication, soil acidification, cadmium build-up and nitrous oxide emissions.
Substandard fertilisers have less nutrient than labelled, the wrong composition, poor condition or harmful impurities (biuret, cadmium); they waste money and harm crops.
Fertiliser ions can be identified with the standard tests for $\mathrm{NH_4^+}$ , $\mathrm{NO_3^-}$ , $\mathrm{SO_4^{2-}}$ , $\mathrm{Cl^-}$ , $\mathrm{PO_4^{3-}}$ and $\mathrm{K^+}$ .

1Manure and bags, side by side

2Organic and inorganic compared

3When fertiliser escapes: water and soil

4Substandard fertilisers

5Testing a fertiliser in the laboratory

Manure and bags, side by side

Organic and inorganic compared

When fertiliser escapes: water and soil

Substandard fertilisers

Testing a fertiliser in the laboratory