Soil
Senior 2 · Agriculture
Soil
Lesson 4 of 5
22 min
Let's begin
Through the Girinka ("one cow per poor family") programme, many Rwandan households have received a cow. The milk is the obvious gift, but farmers often say the manure matters just as much: a zero-grazed cow in a shed produces a steady supply of dung and urine that can turn a tired plot back into a productive garden.
Organic manures are made from materials every farm already has: crop residues, weeds, kitchen waste and animal droppings. This lesson shows the main types and how to make two of them properly.
An organic manure is a material of plant or animal origin that is added to the soil to supply nutrients and improve the soil's physical and biological condition. Its nutrients are released slowly as soil organisms decompose it.
| Advantages | Limitations |
|---|---|
| Supplies many nutrients, including micronutrients | Low nutrient concentration, so large amounts are needed |
| Adds humus: better structure, water-holding and nutrient-holding | Bulky and heavy to carry up a hillside |
| Feeds earthworms and useful microbes | Releases nutrients slowly |
| Reduces erosion and makes acid soils less harmful | Poorly made manure can spread weed seeds and diseases |
| Made on the farm from waste, so it is cheap | Takes weeks or months to make |
Composting is an aerobic process: bacteria and fungi need air, water, carbon-rich "brown" materials (dry stalks, straw) and nitrogen-rich "green" materials (fresh weeds, leaves, dung). Decomposition is fastest when the carbon-to-nitrogen ratio of the mixture is roughly 25–30 to 1. As the microbes work, the heap heats up, which also kills many weed seeds and disease organisms.
Question: Gatete keeps one zero-grazed cow. Suppose he gets about 15 kg of well-rotted manure per day. He wants to apply manure at 10 tonnes per hectare to a 0.25 ha plot. (a) How much manure does he need? (b) How many days of collection is that? (c) What volume does one of his compost pits hold?
(a) 10 t/ha × 0.25 ha = 2.5 t = 2,500 kg.
(b) 2,500 kg ÷ 15 kg per day ≈ 166.7, so about 167 days, a little over five months. He should start storing well before the season.
(c) Volume = length × width × depth = 1.5 m × 1.5 m × 1 m = 2.25 m³.
Tip
Memory hook for a good compost heap: "Air, Water, Brown, Green." If any one is missing, the heap stops working.
Exam technique: when describing how to make compost, give the steps in order and give a reason for each ("coarse stalks at the bottom to let air in"). Examiners mark both the step and the reason.
Distinguish between farmyard manure and green manure.
Solution: Farmyard manure is made from animal dung and urine mixed with bedding and left to rot. Green manure is a crop, usually a legume, grown and dug into the soil while still green.
Give two reasons why a compost heap may smell bad, and how to correct it.
Solution: It is too wet, or it lacks air, so anaerobic microbes take over. Add dry (brown) material and turn the heap to let air in; cover it against rain.
Why should the farmyard manure heap be roofed?
Solution: Rain would leach out the dissolved nutrients, and strong sun would dry the heap and speed the loss of nitrogen as ammonia gas.
A farmer has three compost pits, each 1.5 m × 1.5 m × 1 m. What is the total volume?
Solution: One pit = 1.5 × 1.5 × 1 = 2.25 m³. Three pits = 3 × 2.25 = 6.75 m³.
List three signs that compost is ready to use.
Solution: Any three: dark brown colour; crumbly texture; earthy smell; cool (no longer heating); original materials can no longer be recognised.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
