WAEC SSCE - Agricultural Science - 2002 (Practical)

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(a) Aim of the experiment

To determine the different soil particles (fractions) present in a soil sample, and hence its soil texture.

(b) Procedure

• A measured quantity of the soil is put into a measuring cylinder and sufficient water is added.
• A small quantity of hydrogen peroxide (or sodium bicarbonate) is added to disperse the particles and remove organic matter.
• The cylinder is stirred and shaken vigorously so that the soil is thoroughly mixed with the water.
• The mixture is then allowed to stand undisturbed for at least five hours to settle.

(c) Observation

• The soil particles settle in layers according to their weights and sizes.
• Gravel settles first at the bottom, followed by coarse sand, then fine sand, then silt, with the fine clay settling last on top.
• The organic matter (humus) floats on the surface of the water.

(d) How the different soil particle sizes can be estimated

• Read the total height of the sediments directly from the measuring cylinder, \(T\).
• Read the height of the sand layer, \(H_{SA}\), the silt layer, \(H_{SI}\), and the clay layer, \(H_{CL}\).
• Calculate the proportion of each fraction as a percentage of the total: \[ \%\,\text{sand} = \frac{H_{SA}}{T}\times 100, \quad \%\,\text{silt} = \frac{H_{SI}}{T}\times 100, \quad \%\,\text{clay} = \frac{H_{CL}}{T}\times 100. \]

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(a) Identity of Specimen E

Specimen E is a caterpillar (larva of a butterfly or moth, order Lepidoptera). It can be identified from the diagram by its long, soft, cylindrical, segmented body coloured green with black transverse bands and rows of yellow-orange spots, a distinct head capsule with chewing (biting) mouthparts, three pairs of true (jointed) legs on the thorax, and several pairs of fleshy prolegs on the abdomen. A common crop-attacking example is the larva of the African armyworm or the diamond-back / cabbage caterpillar.

(b) Four damages caused to crops by the specimen

1. It bites and chews the leaves, creating holes and reducing the leaf area available for photosynthesis.
2. It defoliates the crop completely in heavy infestations, so that only the stalks and midribs remain.
3. It bores into and feeds on fruits, pods, cobs and growing shoots, lowering yield and quality.
4. The wounds and faeces it leaves behind create entry points for fungal and bacterial diseases and contaminate the produce, reducing its market value.

(c) Life cycle

The specimen undergoes complete metamorphosis (holometabolous development) with four distinct stages:

\[ \text{Egg} \;\rightarrow\; \text{Larva (caterpillar)} \;\rightarrow\; \text{Pupa (chrysalis)} \;\rightarrow\; \text{Adult (butterfly/moth)} \]

The adult female lays clusters of eggs on the underside of leaves. The eggs hatch into the larva (the caterpillar shown), which feeds voraciously on plant tissue and moults several times as it grows. The mature larva stops feeding and turns into a pupa (chrysalis), often protected in soil, plant debris or a silken cocoon. Inside the pupa the tissues are reorganised, and an adult winged butterfly or moth finally emerges. The adults mate and the female lays eggs again, completing the cycle.

(d)(i) Stage most dangerous to crops

The larval (caterpillar) stage is the most dangerous to crops.

(d)(ii) Reason

It is the actively feeding stage: the caterpillar possesses strong biting and chewing mouthparts and feeds continuously and greedily on leaves, shoots and fruits, causing the direct destruction of crops. (The adult butterfly/moth feeds only on nectar and does no feeding damage.)

(e) Three control measures

1. Chemical control: spray the crop with an appropriate contact or stomach insecticide (for example a carbamate or pyrethroid) while the larvae are young.
2. Cultural control: practise crop rotation, early planting, deep ploughing to expose pupae, and destruction of crop residues and alternative weed hosts; hand-pick and destroy the caterpillars and egg masses in small plots.
3. Biological control: encourage or introduce natural enemies such as parasitic wasps, predatory birds and the bacterium Bacillus thuringiensis to attack the larvae.

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(a) Identification of specimens

• Specimen B: NPK (compound) fertilizer.
• Specimen C: Wood ash.
• Specimen D: Animal dung (organic manure).

(b) Two observable physical properties of each

(c) Two main nutrients supplied by each of specimens B and D

• Specimen B (NPK fertilizer): supplies Nitrogen (N) and Phosphorus (P), together with Potassium (K).
• Specimen D (Animal dung): supplies Nitrogen (N) and Phosphorus (P), together with Potassium (K).

(d) Two methods of application of specimen C (wood ash) on the field

1. It can be spread on the farm before ploughing, or worked into holes before planting.
2. It can be placed at the base of the plant stem, or applied in rows before ridging.

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(a) Most common source of energy in livestock rations

Specimen G (ground sorghum, guinea corn) is the most common source of energy in livestock rations. Its endosperm is rich in starch (carbohydrate), which is readily digested and supplies abundant metabolisable energy.

(b) Ingredient in specimen I with a laxative effect

Wheat offal (wheat bran). Because of its high fibre content it has a laxative effect on farm animals. (Rice bran, maize offal and brewers' dried grains act in the same way.)

(c) Three feed ingredients in specimen I essential for bone and teeth formation

• Bone meal.
• Oyster shell.
• Fish meal.

These supply calcium and phosphorus, the minerals needed for the formation of strong bones and teeth. (Dicalcium phosphate and periwinkle shell are also acceptable.)

(d) Four types of specimen I and the class of poultry to which each is fed

(e) Four farm animals that feed largely on specimen J (forage grass)

• Cattle
• Sheep
• Goat
• Rabbit

(f) Four plant protein sources used in livestock feeds

1. Groundnut cake
2. Soya bean meal
3. Cotton seed cake
4. Palm kernel cake