Soil
Senior 1 · Agriculture
Soil
Lesson 4 of 4
24 min
Let's begin
Two farmers ask an agronomist why their beans failed. One field is grey, sticky and full of standing water after rain; the other is red, crumbly and dries out within days. The agronomist can say a lot just by looking, but to recommend the right amount of lime or fertiliser she needs a laboratory result, and a laboratory result is only as good as the sample the farmer sends.
This lesson covers the properties that make one soil behave differently from another, and how to take a soil sample that truly represents a field.
Question: Three funnels are filled with the same volume of dry sand, loam and clay. 100 cm³ of water is poured on each, and the water that drains through in 30 minutes is collected: sand 70 cm³, loam 45 cm³, clay 20 cm³ (illustrative results). (a) How much water did each soil keep? (b) Which drains best and which holds most water? (c) Suggest one reason the clay may keep less water in the field than this test suggests.
(a) Water kept = water added − water drained. Sand: 100 − 70 = 30 cm³; loam: 100 − 45 = 55 cm³; clay: 100 − 20 = 80 cm³.
(b) Sand drains best (largest pores). Clay holds most water (many tiny pores and particles with a huge total surface area).
(c) Some of the water in clay drains slowly after 30 minutes, and some is held so tightly in the tiniest pores that roots cannot take it up. So clay does not supply as much usable water as the test suggests.
Soil sampling is the collection of small amounts of soil from a field so that they can be tested in a laboratory. The test shows the pH and nutrient levels, and the laboratory recommends how much lime, manure or fertiliser to apply. Because only a few hundred grams are tested to represent many tonnes of soil, the sample must be taken carefully.
A soil auger or a clean hoe or spade, a clean plastic bucket, a strong clean bag, labels and a pen. Avoid rusty or fertiliser-stained tools and bags.
Question: Habimana's farm has a 0.3 ha terraced hillside field of beans and a 0.2 ha valley-bottom field of vegetables. He takes 18 sub-samples in each field, each weighing about 60 g. (a) How many composite samples should he send? (b) What mass of soil does he collect in each field before mixing? (c) Is that enough for a 0.5 kg composite sample?
(a) The two fields differ in slope, soil and crop, so they are separate units: 2 composite samples.
(b) 18 × 60 g = 1,080 g (about 1.1 kg) per field.
(c) Yes. He mixes the 1.08 kg thoroughly and keeps 0.5 kg of the mixture as the composite sample.
Tip
Memory hook for sampling: "Divide, Zigzag, Dig, Dodge, Mix, Dry, Label".
Exam technique: if a question asks why a sample must avoid manure heaps, ash or termite mounds, the key word is representative: these spots are richer or different from the rest of the field and would give a misleading result.
Define soil structure and name the structure best for crop growth.
Solution: Soil structure is the way soil particles are grouped into aggregates (peds). Crumb (granular) structure is best, because its rounded, porous crumbs leave spaces for air, water and roots.
What does a grey subsoil with rusty spots tell a farmer?
Solution: The soil is often waterlogged and poorly aerated. It may need drainage, or crops that tolerate wet conditions.
In a funnel test, 100 cm³ of water is poured on a soil and 35 cm³ drains through. How much water did the soil keep?
Solution: 100 − 35 = 65 cm³.
Why are sub-samples taken along a zigzag path rather than all from one spot?
Solution: Soil varies across a field. Spreading sub-samples over the whole unit makes the composite sample represent the average of the field, not one unusual spot.
Give three pieces of information that must be written on the label of a soil sample.
Solution: Any three: farmer's name; field name or number; date; sampling depth; previous crop; crop to be grown.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
