Horticultural crops are classified based on climatic requirements into temperate, tropical, and subtropical crops, and this classification is based on the temperature requirements of each crop. Some crops require low temperatures and are grown in temperate climates.
Based on climate and soil, horticultural crops are dispersed and found in different parts of the country. The article discusses the distribution of horticultural crops in Nigeria based on climate and soil.
Scope of Horticulture in Nigeria
Horticultural crops constitute a significant component of total agricultural production in any country. Environmental, soil, and biotic factors such as diseases, pests, and to a lesser extent, the demand for horticultural crops determine the level of production. In Nigeria, horticultural crops are grown all over the country.
The presence of horticultural crops in an area depends on the suitability of the climate, cultural practices, and the demand for the commodity. Fruits and nuts are scattered across the country, with a higher concentration in the southern region.
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Environmental, Soil and Biotic Factors Affecting Horticultural Crop Production
The successful production of any crop depends on the suitability of the environment. Climatic (abiotic), edaphic (soil), and biotic factors determine the distribution of horticultural crops in Nigeria and worldwide.
Climatic Factors Influencing Horticultural Crop Distribution
1. Rainfall: Rainfall is a natural source of water for plants. Water is vital as it constitutes 70–80% of the fresh weight of herbaceous plants and about 50% in woody plants.
It serves as a medium for nutrient uptake and transportation of substances within the plant. It is a raw material in photosynthesis and maintains turgor pressure in stomata, which enables gas exchange. Plants require water to replace what is lost through transpiration.
Plants lose about 98% of absorbed water through transpiration. Water needs vary with species and age, increasing as plants grow. Moisture deficiency lowers air humidity, increasing its drying power and affecting transpiration, diffusion, and evaporation rates.
Excess moisture encourages diseases and causes lodging. Indoor horticultural plants are sometimes misted to raise humidity, especially during winter when heaters are in use.
2. Temperature: Plant response to temperature varies by species. Each crop has an optimal temperature range for growth and reproduction. Most horticultural crops grow between 15°C and 30°C.
3. Hardy Plants: These withstand low temperatures but cannot tolerate high heat. Examples include cabbage, cauliflower, and apple.
4. Tender or Tropical Crops: These tolerate high temperatures but are sensitive to cold. Examples are banana, mango, and okra.
The stage of growth influences temperature sensitivity. For instance, seedlings are more vulnerable to heat. Different plant parts also respond differently; flowers are more heat-sensitive than vegetative parts, and roots are more sensitive to cold than aerial parts.
High night temperatures raise respiration and reduce food reserves. Tuber initiation in potatoes and root development in carrots are hindered by heat.
Additionally, high temperatures increase pest and disease activity and reduce flowering and fruiting, as seen in tomatoes, which require a night temperature of 21°C for optimal fruit set. Low temperatures slow germination, growth, and cause fruit damage.
5. Light: Light primarily comes from the sun and is vital for photosynthesis and other functions like seed germination. Its role in horticulture depends on quality, quantity, and duration.
6. Quantity: Refers to light wavelength, influenced by cloud cover. Low light quality results in poor fruit colour. Visible light ranges from 390–735 nanometers.
7. Intensity: Refers to light strength. Sun-loving plants like maize and tomato thrive in full sunlight, while others like asparagus prefer shade. Cocoa seedlings require partial shade. At midday, sunlight intensity can reach 10,000 foot-candles, but most plants utilize only about 50%.
8. Duration: Refers to daily light hours. Based on this, plants are:
9. Long-Day Plants: Flower under more than 14 hours of light, e.g. onion, peas, lettuce.
10. Short-Day Plants: Flower under less than 10 hours of light, e.g. citrus.
11. Day-Neutral Plants: Flower at 12 hours of light, e.g. tomato, maize.
12. Humidity: This is the water content in the air, measured as Relative Humidity (RH) using a psychrometer. It depends on water vapour pressure and temperature. RH decreases as temperature rises if water vapour is unchanged.
Crop water needs are closely linked to humidity. High humidity favours growth in crops like banana. Some crops like mango need high humidity for growth and low humidity for flowering. Low humidity helps in drying crops like maize. High humidity also promotes pests and diseases.
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Soil Conditions Influencing Horticultural Crop Production
Soil is the main medium for crop growth. Climate significantly influences soil type through weathering, a process that breaks down rocks into particles. The soil type affects the vegetation it supports, which in turn contributes to soil formation by adding organic matter and nutrients.
Soil provides physical support and supplies nutrients and moisture. Soils are categorized as fertile, marginal, or infertile. Nutrient levels decline over time and need replenishment. Even if soil lacks nutrients, it must retain water and nutrients temporarily.
Otherwise, growers must apply supplements to avoid deficiencies. Good soil must support root development and provide adequate anchorage and nutrition for plants.
Biotic Factors That Affect Horticultural Crop Development
Biotic factors are living organisms that can harm or benefit crops. Climate influences pest and disease occurrence. The presence of a susceptible host, a pathogen, and a favourable environment is necessary for disease development this combination is known as the disease triangle.
Local weather can encourage disease outbreaks by weakening plant resistance. Insects with short life cycles are influenced by seasonal changes. Some pests increase in number during specific seasons and decline in others.
Soil contains microscopic pathogens, while larger organisms may help or harm crops. Birds assist in seed dispersal, and bees and butterflies pollinate flowers. Conversely, rodents damage crops by consuming seeds, fruits, and foliage. Herbivores like deer, rabbits, and monkeys can also cause serious damage in the field.
Distribution of Vegetables and Fruits Grown in Nigeria According to Climate and Soil
Rainfall in Nigeria is highest in the southern region, averaging about 3,000 mm annually. Rainfall and its duration decrease further inland. The north receives less than 400 mm of rainfall annually. Temperature variation also increases inland.
In the south, the average maximum annual temperature is around 30°C, and the minimum is 22°C. In the north, the maximum temperature averages 19°C. Generally, southern Nigeria experiences higher temperatures year-round than the north.
The cool dry season in the north, from November to February, is the best period for vegetable production. Southern Nigeria has high humidity year-round, while northern areas may have relative humidity levels below 10% during the dry season.
The country is thus divided into two zones:
1. Forest (Southern) Zone: Rainfall ranges from 1500–3000 mm, and temperatures are high. This zone favours tropical fruits like banana and pineapple, and root vegetables such as cassava and yams. However, excessive rainfall, high humidity, and temperature can promote diseases. Limited sunlight due to cloud cover can reduce the growth of sun-loving vegetables.
2. Savanna (Northern) Zone: Suitable for horticultural crops like citrus and many tropical plants. However, temperatures may not support temperate crops like carrot, lettuce, and cabbage, except during the cool harmattan season. The main challenge is water scarcity during the dry season. Irrigation is required to fully utilize favourable conditions during this period.
Horticultural crops are cultivated across Nigeria. However, the success of any crop depends on climatic, soil, and biotic factors. Growers must consider these before choosing crops that are best suited to their local environment.
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