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Heavy Metal Contents in Soil and Plants at Dumpsites: A Case Study of  Awotan and Ajakanga Dumpsite Ibadan, Oyo State, Nigeria



1.0       Background of study

Land pollution by component of refuse such as heavy metals has been of great concern in the last decades because of their health hazards to man and other organisms when accumulated with a biological system (Adekunle et al., 2003).  Also, the use of dump sites as farm lands is a common practice in urban and sub-urban centers in Nigeria because of the fact that decayed and composted wastes enhance soil fertility (Ogunyemi et al.,2003). These wastes often contain heavy metals in various forms and at different contamination levels. Recent studies have also reviewed that waste dumpsite can transfer significant levels of these toxic and persistent metals into the soil environment. And eventually these metals are taken up by plant part and transfer same into the food chain. Consequently, higher soil heavy metals concentration can result in higher levels of uptake by plants. Although, the rate of metal uptake by crop plants could be influenced by factors such as metal species, plants species, plant age and plant part (Asuquo et al., 2004).

Food crops such as cassava, and other tuber crops grown in dumpsites or areas very close to dumpsite area take up hazardous heavy metals such as cadmium, copper, chromium, nickel from the soil.

Also, concerns about the possible increase of photo availability of bio-solids applied trace metals have been raised on the assumptions that decomposition of applied organic matter would increase photoavailabilty. It has been found that cadmium pollution affects the soil-plant pathway. Shute and Macfie’s work on cadmium and zinc accumulation in soybeans rein enforce the need to monitor the accumulation of toxic heavy metals on food crops (Ukpong et al.,  2013). Cultivation of crops for human or livestock consumption on contaminated dumpsites soil can potentially lead to the uptake and accumulation of trace metals in the edible plant parts around industrial areas with a resulting risk to human and animal health (McBride, 2007; Monika and Katarzyna, 2004). It is therefore important to express concerns and question on the state of the soil and quality of food crops, fruits and vegetables cultivated and grown in areas where refuse dumps are being found or areas close to the road side with high traffic density.

Cassava (Manihot esculenta Crantz) is a key tropical crop that is grown in the tropical and subtropical areas in the world due to its excellent climate and soil condition. It is a perennial crop of the family Euphorbiaceae, cultivated mainly for its starchy roots.Due to its tolerance to poor and harsh soil conditions and climate, it is generally cultivated by small farmers as a subsistence crop in a diverse range of agricultural and food systems. Although cassava is a perennial crop, the storage roots can be harvested six (6) to twelve (12) month after planting depending on cultivar and growing conditions (El-Sharkawy,1993). In the Scientific



1.1       Classification of Manihot esculenta

Kingdom:  Plantae(Plants)

Subkingdom:  Tracheobionta(Vascular plants)

Superdivision:  Spermatophyta(Seed plants)

Division :Magnoliophyta(Flowering plants)

Class:  Magnoliopsida (Dicotyledons)

Subclass: Rosidae

Order: Euphorbiales

Family:  Euphorbiaceae

Genus:  Manihot Mill. – cassava

Species:  Manihot esculenta Crantz – cassava

1.2 Morphologicaland Anatomical characteristics of Manihot esculenta

Cassava, which is a shrub reaching 1–4 m in height, is commonly known as tapioca, manioc, mandioca and yuca in different parts of the world. Belonging to the dicotyledon family Euphorbiaceae, the Manihot genus is reported to have about 100 species, among which the onlycommercially cultivated one is Manihot esculenta Crantz. There are two distinct plant types: erect, with or without branching at the top, orspreading types.The morphological characteristics of cassava are highly variable, which indicate ahigh degree of interspecific hybridization. Thereare many cassava cultivars in several germ plasm banks held at both international and nationalresearch institutions. The largest germ plasm bank is located at Centro Internacional de Agricultura Tropical (CIAT), Colombia, with approximately 4700 accessions.

1.2.1    Roots

Roots are the main storage organ in cassava.In plants propagated from true seeds a typical primary tap root system is developed,  dicot species. The radicle of the germinating seed grows vertically downward and develops into a taproot, from which adventitious roots originate. Later, the taproot and some adventitious roots become storage roots. In plants grown from stem cuttings the roots are adventitious and they arise from the basalcut surface of the stake and occasionally from the buds under the soil. These roots develop to make a fibrous root system. Only a few fibrous roots(between three and ten) start to bulk and become storage roots. Most of the other fibrous roots remain thin and continue to function in water and nutrient absorption. Once a fibrous root becomes a storage root, its ability to absorb water and nutrients decrease considerably. The storage roots result from secondary growth of the fibrous roots; thus the soil is penetrated by thin roots,and their enlargement begins only after that penetration has occurred.

Anatomically, the cassava root is not a tuberous root, but a true root, which cannot be used for vegetative propagation. The mature cassava storage root has three distinct tissues:bark (periderm), peel (or cortex) and parenchyma. The parenchyma, which is the edible portion of the fresh root, comprises approximately 85% of total weight, consisting of xylemvessels radially distributed in a matrix of starchcontaining cells (Wheatley and Chuzel, 1993).

The peel layer, which is comprised of sclerenchyma, cortical parenchyma and phloem,constitutes 11–20% of root weight. The periderm (3% of total weight) is a thin layer made of a few cells thick and, as growth progresses, the outermost portions usually slough off. Root size and shape depend on cultivar and environmental conditions; variability in size within a cultivar is greater than that found in other root crops (Wheatley and Chuzel, 1993).

1.2.2    Stems

The mature stem is woody, cylindrical and formed by alternating nodes and internodes. Onthe nodes of the oldest parts of the stem, there are protuberances, which are the scars left bythe plant’s first leaves. A plant grown from stem cuttings can produce as many primary stems asthere are viable buds on the cutting. In some cultivars with strong apical dominance, only one stem develops.

The cassava plant has sympodial branching. The main stem(s) divide di-, tri- or tetrachotomously, producing secondary branches that produce other successive branchings. These branchings, which are induced by flowering,have been called ‘reproductive branchings’.

Stem morphological and agronomic characteristics are very important to characterizing a cultivar. The variation of these characteristics depends on cultivar, cultural practice and climatic conditions.

1.2.3    Leaves

Cassava leaves are simple, formed by the laminaand petiole. The leaf is lobed with palmated veins. There is generally an uneven number of lobes, ranging from three to nine (occasionally). Only a few cultivars are characterized by having three-lobed mature vegetative leaves,which may represent the primitive ancestral form (Rogers and Fleming, 1973). Leaves near the inflorescence are generally reduced in size and lobe number (most frequently three-lobed),but the one closest to the base of the inflorescence is frequently simple and unlobed. The upper leaf surface is covered with a shiny, waxy epidermis. Most stomata are located on the lower (abaxial) surface of the leaves;only a few can be found along the main vein on the upper (adaxial) surface.  Of1500 cultivars studied, only 2% had stomataon the adaxial surface. The stomata on the upper surface are also functional and bigger than those on the undersurface.

1.2.4 Fruit and seeds

The fruit is a trilocular capsule, ovoid or globular, 1–1.5 cm in diameter and with six straight, prominent longitudinal ridges or aristae. Each locule contains a single carunculate seed. The fruit has a bicidal dehiscence, which is a combination of septicidal and loculicidal dehiscences, with openings along the parallel plane of the dissepiments and along the midveins of the carpels, respectively. With this combination of dehiscences, the fruits open into six valves causing an explosive dehiscence, ejecting the seeds some distance. Fruit maturation generally occurs 75–90 days after pollination. The seed is ovoid–ellipsoidal, approximately 100 mm long, 6 mmwide and 4 mm thick. The weight varies from 95 to 136 mg per seed. The smooth seed coat is dark brown, mottled withgrey. The seeds usually germinate soon after collection, taking about 16 days for germination (Allem, 1999).

1.3       Nutritional Value of Manhiot esculenta

Cassava is low in Saturated Fat, Cholesterol, and Sodium, high in Vitamin C and Manganese. Cassava tubers are rich in carbohydrates, mainly starch and are a major source of energy. With the exception of sugar cane, cassava is the highest source of carbohydrates. Cassava tubers are however deficient in protein, fats and some vitamins. Cassava leaves contain more protein than the tubers but they lack the essential amino acids, methionine (El-sharkaway, 1993).

1.4       Uses of Manihot esculenta

Cassava has been made into different derivatives including;

  • Water fufu
  • Cassava chips
  • Flour
  • Garri (white and yellow)and used for the following purposes

1.4.1 Food: Cassava-based dishes are widely consumed wherever the plant is cultivated; some have regional, national, or ethnic importance. Cassava must be cooked properly to detoxify it before it is eaten.

Cassava can be cooked in various ways. The soft-boiled root as a delicate flavor and can replace boiled potatoes in many uses such as an accompaniment for meat or fish dishes; Fufu is made from the starchy cassava-root flour.

1.4.2  Industrial Products

Cassava can be used to produce such products as High Quality Cassava Flour (H.Q.C.F), which is used in Composite Flour for Pastries and baking such as Cassava Bread. Also Cassava Starch is also produce from Cassava tubers, this is used in almost all industrial food products as stabilizers and also in paper making (Mcbride, 2007).

1.4.3 Biofuel

In many countries, significant research has begun to evaluate the use of cassava as an ethanol biofuel feedstock. In November 2008, China-based Hainan Yedao Group reportedly invested $51.5m (£31.8m) in a new biofuel facility that is expected to produce 33 million gallons a year of bio-ethanol from cassava plants.

1.4.4 Animal feed

Cassava is used worldwide for animal feed as well. Cassava hay is produced at a young growth stage at three to four months, harvested about 30–45 cm above ground, and sun-dried for one to two days until it has final dry matter of at least 85%. The cassava hay contains high protein content (20-27% crude protein) and condensed tannins (1.5-4% CP). It is used as a good roughage source for dairy or beef cattle, buffalo, goats, and sheep by either direct feeding or as a protein source in the concentrate mixtures (Momodu and Anyakora, 2010).


1.4.5    Ethnomedicine

 The bitter variety Cassava (Manihot esculenta) is a plant sometimes used as an herbal remedy. The root of the plant is also used to make tapioca, a starch found in puddings and other foods. Taking cassava in dietary supplement form is said to offer a variety of health benefits, including enhanced fertility.

The flour produced from the cassava plant, which on account of its low content of non-carbohydrate constituents might well be called a starch, is known in world trade as tapioca flour. It is used directly, made into a group of baked or gelatinized products or manufactured into glucose, dextrins and other products.

1.5 Cultivation of Manihot esculenta

1.5.1 Climate

Cassava is typically grown in tropical lowlands and requires at least 8 months of warm weather to mature. It likes full sun and requires temperatures ranging from 77 to 81 degrees Fahrenheit and at least 19.6 inches of rain annually. Cassava will not tolerate frost, so it grows best in a greenhouse or with cold frame protection in cooler areas.

1.5.2    Soil Preparation

Cassava requires the addition of a balanced 14-14-14 fertilizer to the soil before planting. This fertilizer contains equal amounts by weight of nitrogen, phosphorous and potassium. The fertilizer is covered with a thin layer of soil and irrigated. Cassava will grow in many soil types, but ideally the topsoil is at least 12 inches deep and not stony, shallow or waterlogged. Toxins in cassavas are typically higher in dry conditions and poor soils.

Cassava growers plant cuttings from the stems of recently harvested plants. They typically cut 8 inches from the bottom of the stem then slice 10-inch cuttings from the next 30 inches. Cuttings are best planted as soon as possible, although they can be stored successfully in a cool, shaded place for up to 3 months. The lower half of the cuttings are planted every 3 feet in rows that are 3 feet apart. If the soil is dry, the cuttings are planted at a 45-degree angle. If the soil is wet, they are planted vertically (Olsen, 2002).

1.5.3    Water and Fertilizer

Cassava plants like water and need regular irrigation if it is not raining. Two months after planting, urea — 46-0-0 fertilizer — is applied in a band 6 inches from the base of the plant. If the soil contains large amounts of nitrogen, continuing to add more will make the plant grow more and the edible roots grow less. In the developing world, only commercial growers typically fertilize cassava, using a balanced fertilizer. Most farmers use organic manures.

1.5.4    Harvesting

Cassava is usually not harvested until at least eight months after planting. Growers dig up a sample cassava to check its size. To harvest, the stem is cut, leaving a stub as a handle to pull the cassava roots out of the ground. Harvested roots are stored in a shaded place and deteriorate rapidly.

1.5.5    Toxins

Cassava roots, branches and leaves all contain cyanogen eticglycoside, a toxin that yields cyanide. Drought and potassium deficiency in the soil both increase the glycoside content in the plants. The toxin, which has a bitter taste, is removed by cooking or crushing the roots and soaking them in water (Alves, and Setter, 2000).

1.6       Aims of the Study

This research work is aimed at assessing the levels of some heavy metal concentration in waste dumpsites in Ekpri Nsukara, Four lanes and Use Offot area in Akwa Ibom State, Nigeria.

1.7 Scope of the Study

The scope of this study is wide, ranging from collection of samplefrom various dumpsites, sample digestion, and the use of Atomic Absorption Spectrophotometer

(AAS) to determine the heavy metals.

1.8       Limitation of the Study

The data of this project work focuses on some selected dumpsites in Uyo, Akwa Ibom State.







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