Plant Growth

Germination

Germination is the change that takes place in the seed when it first begins to grow. Seeds are dormant before planting. Seeds are alive but show no signs of life and are hard and dry. The word dormant means asleep. When seeds come in contact with water, water is absorbed, and the seeds swell up. This normally happens when seeds are in damp soil but will begin to germinate even if not in soil. During germination, the embryo inside the seed (the plumule and the radical) begins to grow and push through the seed coat or testa). Once this happens, germination is complete.

 
Germinating seed.

 

Conditions are Needed for a Seed to Germinate

Seeds require certain conditions to germinate and start growing. As young plants get older and bigger and need suitable conditions to continue growing. The requirements for plant growth are similar to those for germinating seeds, but plants also need sunlight and nutrients from the soil.

 

A. Water

Plants cannot grow without water. Almost 90% of the plant’s mass consists of water.

Plants normally contain many substances, but the majority is water. All chemical reactions that take place inside the plant cells happen in a solution (in water), controlled by organic catalysis molecules, called enzymes. No reactions can happen without water. Seeds are dormant. Unless a dry seed receives water, it cannot start to grow. When a seed absorbs water, its dormancy is broken, and the normal life cycle of a plant can start again. The amount of water in the soil affects germination. Too much water prevents air from reaching the seeds. Too little water and the seedlings may die. Farmers irrigate to ensure that seedlings have just the right amount of water.

• Water provides nutrients: A plant needs water to absorb food. Mineral salts in the soil dissolve in the water and enter the roots in the solution.
• Water moves through plants: Food substances are carried in solutions to all parts of the plant through tiny tubes called xylem vessels. Water enters plants through the roots, passes up through the stem’s xylem vessels to the leaves, and then is lost to the air by a process called transpiration. It is a continuous movement of water through the plant it is called the transpiration stream.
• Water gives plants shape: A plant can keep its normal shape when the cells of cells are full of water. When it is short of water, the plant wilts and the leaves droop down.
• Water is necessary for photosynthesis: Plants make sugar or starches from a mixture of water and carbon dioxide when they are exposed to sunlight. Oxygen is released, through the leaves as part of this reaction and sugar or starch acts as a nutrient and allows the plant to grow and stores extra starches in its roots and fruits. Photosynthesis is an important process, which cannot happen without water. Both plants and animals, which eat them, depend on photosynthesis to survive.
• Sugar moves in the water: Plant sugar is made in the leaves. It is then moved in solution to other parts of the plant to be stored. It moves through other tiny tubes that are called phloem vessels. This process is called the translocation of food materials.
• Water helps to cool the leaves: Water evaporates from leaves during transpiration and acts as a cooling mechanism for plants. Heat is lost when water evaporates, and this cools the plant when the weather is hot.

How plants adapt to shortages of water:

Water is lost from the soil through evaporation and when it drains away. Water is also lost from plants themselves through transpiration. Sometimes this water loss combined with no irrigation or rain may lead to a shortage of plants. Plants have ways of guarding against water shortage.

• Deep roots: Some plants like sorghum grow very deep roots to reach water lying deep below the soil surface.
• Storing water: Some plants have storage organs in their roots or stems. These are big and swollen because they hold a lot of water. The spineless cactus is an example of a plant with large water storage organs in its stems.
• Reduced transpiration: Plants have small holes or pores on the underside of their leaves for transpiration. These pores, called stomata, can be opened or closed. When there is a shortage of water, the plant closes its stomata and reduces water loss by transpiration.
• Wilting: Leaves droop down when they are not full of water or turgid. They then show a smaller surface area to the sun, which helps to reduce evaporation. Wilting leaves on soft plants are warning signs that they need water.
• Curling of leaves: Many types of grass curl up their leaves during drought. This means that less of the leaf surface is exposed and this reduces the loss of water by evaporation.
• Waxy cuticles: Some plants have leaves with shiny surfaces. The shine is caused by a covering made of wax that helps to reduce transpiration. The leaves of mango and citrus trees are examples of this.

 

B. Air

Germinating seeds need oxygen to respire. As they respire, they release the energy required for growth. Respiration uses up food stores. Seeds in waterlogged soil cannot respire because they are short of oxygen and will die if the excess water does not drain away.

Air is a mixture of gases consisting of:

• 21% oxygen.
• 78% nitrogen.
• 1% carbon dioxide and other rare gases.

Oxygen and carbon dioxide are the most important gases for plants and are used in respiration and photosynthesis. Cultivation of the surface soil while the crop is growing breaks up the hard cap and allows air to enter the soil. This helps roots and soil organisms respire.

• Oxygen: Oxygen is essential for respiration. This process takes place in all living cells, including those of plants. Sugar is broken down and energy is released. Sugar contains a lot of energy. In plants, respiration sugar combines with oxygen and a reaction takes place in the solution in water, under the control of enzymes. In this way, plants slowly and steadily release the energy from sugar to help them build up their tissues and grow bigger.
• Carbon dioxide: Carbon dioxide is a by-product produced by respiration in plants and animals. This gas is used in photosynthesis to make glucose and other sugars. The sugars are then built up by plants into carbohydrates such as starch and cellulose. Starch is used for food storage in roots stems and seeds. Cellulose builds up the cell walls of plants.

 

C. Suitable Soil Temperature

Chemical reactions are faster when it is warm and slower when it is cold. This is also true of the chemical reaction that takes place inside a germinating seed. As previously mentioned, enzymes control all chemical reactions that happen inside plants. The rate of enzyme-controlled reactions is determined by the temperature. Seeds will thus germinate quickly when it is warm and slowly when it is cold.

Plants cannot control body temperature as some animals can. As mentioned previously, the rate of growth is determined by enzyme-controlled reactions in their cells. This rate of growth depends on the temperature at which the plant grows.

Controlling soil temperature:

In African countries, the soil temperature often gets too hot. Farmers can do the following things to keep the soil cool.

• Mulching: Germinating seeds are protected from the hot sun by putting mulch on the soil surface. This is a covering of dry grass, straw, or compost. When the seedlings emerge, the mulch is moved to let the seedlings have light. Once the seedlings have germinated, the soil temperature can be controlled by adding some mulch around the plants. Sheets of black plastic can also be used as mulch to keep the soil cool and reduce evaporation. However, organic mulches are better because they mix with the soil to improve the quality of the soil.
• Shading: Seedbeds are protected from the sun with a frame made of bush poles covered with grass. The grass is removed, a little at a time, as the plant grows stronger. Transplanted seedlings are shaded by pushing leafy twigs into the soil beside them. The twigs are taken away after a few days.
• Watering: The soil is cooled by rain or irrigation water. It takes a lot of heat to evaporate this water, so the wet soil stays cool.

 

D. Plants Need Sunlight for Photosynthesis

Photosynthesis is the process by which green plants use sunlight to make sugar. Sugar provides energy for growth. Plants also build up other food materials such as carbohydrates, proteins, and fats.

• Conditions necessary for photosynthesis:

Photosynthesis happens when carbon dioxide gas is absorbed by the leaves through the leaf pores (stomata) and water is taken in through the roots and passed up to the leaves. This water and carbon dioxide combine to make sugar. This can happen only in the presence of chlorophyll, the green-coloured substance in plant leaves. Sunlight, shining on the leaves, provides energy for photosynthesis and is released into the air to be used for respiration by animals, plants and people.

• Making chlorophyll:

Plants need sunlight to produce chlorophyll and to photosynthesise so seedlings should never be over-shaded by taller plants or trees. The seedlings need sunlight to grow properly. If they do not get ample sunlight, they will become too tall, and pale green.

• Length and direction of growth:

Plants need light to control the length of their growing season. In most of the world, the length of days and nights changes during the year. This affects the time of year when plants produce their flowers and seeds and ensures that the plant produces its seeds at the best time of the year. Plants also need light to control the direction of growth. Plant shoots always grow towards the light.

• Plant nutrients:

Besides air, water, sunlight and warmth, the plant also needs nutrient elements from the soil. The three main elements (macro-nutrients) – Nitrogen, Potassium and Phosphorus – and how each affects plant growth. Plants need trace elements (micro-nutrients) from the soil in very small amounts. If any of these nutrient elements are in short supply, a plant cannot grow properly. Deficiencies in soil nutrients can be corrected by adding manure, compost, or fertilisers.

• Photosynthesis and nutrition:

Nutrient elements alone cannot cause plants to grow but also need photosynthesis. Photosynthesis is also a form of nutrition for plants because water and carbon dioxide are plant nutrients. Most of the increase in weight shown by plants as they grow comes from these two nutrients. They are the raw materials for photosynthesis.

 

Seedling Emergence

Emergence means “coming out”. This happens straight after germination when a small root comes out of the seed and grows down into the soil. After this, a shoot appears and grows upwards to emerge above the soil surface. At this stage seedling emergence is complete.

 

A. Two Types of Germination

There are two types of germination:

1. Hypogeal germination:

The word hypogeal means underground. In this type of germination, the whole seed remains underground while the embryo develops inside it, and then it grows out. First, the radical grows down from the root then the plumule grows up to form the shoot. Once the shoot shows above the ground hypogeal germination is complete. Maize is an example of a plant that germinates in this way. All the cereal grains and the seeds of the grass family germinate like this.

2. Epigeal germination:

The word epigeal means above the ground. In this type of germination, the radicals first grow down into the soil to form the root. Then the plumule grows up and lifts the two cotyledons of the seed completely out of the ground. The cotyledons then open out and turn green to become the first seed leaves for photosynthesis and make food (sugars and starch) for the plant. After this, a bud develops between the cotyledons and grows to form the first pair of true leaves.

 

B. Germination in the Field

1. Seedbed preparation:

Farmers can provide all three conditions necessary for germination by preparing a seedbed into the topsoil to make a fine tilth. This forms a good seedbed that provides seeds with:

• Water or humidity.
• Air (oxygen).
• Suitable temperatures.

When moist soil particles are in close contact with the seeds and thus have all around them, it is absorbed.

Air for seedling respiration can easily penetrate the open crumb structure of a seedbed.

2. Planting depth:

The depth of planting is important to achieve the best germination. Seeds should be planted at a depth five times their diameter. If it is planted too deep, the young shoots may never reach the surface. If it is not planted deep enough, the seedlings may die due to a lack of moisture. Whatever method of planting is used, the aim should be to ensure the even distribution of seeds and planting to the correct depth. Random spreading of the seed is not a very successful method. The seeds are not evenly spread or planted at the right depth. Mechanical planters must be carefully adjusted to place the seed at the right depth.

 

Systems of Crop Production

Crops can be grown in many ways. Some farmers grow just one type, others grow a different crop in each of their fields. Others may grow two different types of crops at the same time in the same field. Some farmers grow crops one after the other in a definite order from year to year. These different ways of growing crops are known as systems of cropping.

The main systems of cropping are:

• Monoculture.
• Monocropping
• Mixed cropping.
• Crop rotation.
• Continuous cropping.
• Inter-cropping.

A. Monoculture

Monoculture is the practice of producing or growing crops singly over an area of land. Under this system, the same crop is grown all over the farm every year. This is how some crops are grown on big estate farms for processing at a factory or the export market. Crops like sugarcane, pineapples and citrus fruits are grown in this way. The monoculture system has advantages and disadvantages.

Advantages and disadvantages of monoculture.

Advantages

Disadvantages

It is simpler if there is just one crop to think about. Farmers become skilled in managing their one crop. Mechanisation is possible. This means that machines like tractors can be used for soil preparation, fertilising, weeding and harvesting. They can do the work very quickly. Labour costs per hectare are low when machines do most of the work. Irrigation is possible on a big scale. The crop then does not depend only on rainfall. Overseas markets can take large quantities of produce. This increases the wealth of the country because the money comes in from abroad.

Mechanisation is expensive. Tractors and other farm machines are expensive. Irrigation is expensive when carried out on a big scale. Monoculture is not usually worthwhile on a small scale unless the farm is near the processing factory. Pest and diseases that attack the crop can build up where just one copy is grown. An intensive crop spraying programme is therefore needed every year to control these pests and diseases. Risks cannot be spread when there is just one crop, i.e., all the eggs are in one basket. If the demand from overseas buyers for the single crop drops, it can be a big problem for the farmer.

 

B. Monocropping

Monocropping is growing the same crop year after year without crop rotation. Some commercial farmers do that because they know they can sell that crop in that area. They may keep growing the same crop year after year, or they may change it if they think that another crop would be better for them.

Advantages and disadvantages of monocropping.

Advantages

Disadvantages

Farmers become skilled at looking after their one crop. Machines can be used for tasks such as planting, fertilising, weeding and harvesting. Mechanisation makes the work go quicker and more easily.

Pests and diseases can increase rapidly where just one crop is grown. Total crop failure can result if the crop is attacked by pests and diseases or by unusual conditions (hail, frost or drought). More fertilisers may be needed as the crop uses the same nutrients from the same level in the soil each year. Labour is not well spread out over the growing season. A single crop needs a lot of attention at certain times and temporary labour is needed at those times.

 

C. Mixed Cropping

Subsistence farmers grow a mixture of different crops to provide food for the family. They have more than one way of doing this. They may spread the seeds of two types of crops together so that the two crops grow in the same field at the same time. This is a common method. Beans and sorghum may be planted together. Plant alternate rows, first of one type of seed and then the other and grow mixed in that way. This is called intercropping.

 

Advantages and disadvantages of mixed cropping.

Advantages

Disadvantages

Total crop failure can be avoided. When different crops are grown together, one of them may fail but others may succeed. Pests and diseases are specific to one crop and not the other. Again, one of the crops may survive better than the others. Conditions of temperature and rainfall in the growing season could favour one crop more than the others. The workload is spread throughout the year because the different crops need attention and are harvested at different times. A balanced diet is provided for the family. Cowpeas (a high protein crop) may be planted with maize (a high carbohydrate crop). This provides body-building food and energy-producing food.

Mechanization is not possible when plants of different types are grown in the same field. This is because machines are designed to work with rows of plants of the same type. The two crops may compete with each other for water and nutrients. If there is any shortage of water or nutrients neither of them will do well. Farmers must be skilled in looking after several different crops.

 

D. Crop Rotation

Rotation means going around. Crop rotation means that crops are moved each year from one part of the farm to another.

 
Crop rotation.

 

The diagram shows that arable land is divided into four almost equal parts. In this rotation, a different crop is grown on each of the three parts of the farm. The fourth part is left fallow – without a crop. This is a rotation method followed on many commercial farms. After four years, the rotation comes back to where it started. In the fallow year, the land has a chance to rest. Then micro-organisms help to decompose organic matter and help the soil to recover some fertility. The crops shown in the diagram are not the only ones that can be grown in rotation. Sunflowers (an oil-producing crop), could, for instance, be replaced by potatoes (a root crop). The cowpeas could be replaced by another legume such as beans. The main thing is to keep moving the crops to different areas of the farm. Where the soil is fertile, a fourth crop could be included instead of leaving the land fallow. It is also possible to rotate groups of crops.

Four suitable crops are:

• Leafy crops: Cabbage, spinach, or Swiss chard.
• Root crops or an oil crop: Potatoes or sweet potatoes.
• Cereal crops: Maize or sorghum.
• Legume crops: Cowpeas, beans, peas, or groundnuts.

The best rotation to use will depend on where the farm is and what crops grow best in the area. Local farmers or the local Extension Officer will know what rotation is most suitable. Grass can also be grown as part of a rotation. On some farms, grass seeds are planted just like any other crop seed to grow a good pasture. The grass can then be used for grazing or for making hay or silage. A grass break in a rotation may stay in place for three or four years. Then it can be ploughed up again and the land returned to cropping. This is a way of controlling troublesome crop pests and diseases and providing food for livestock at the same time. You will see why a crop rotation of some kind is a good idea when you look at the advantages of the rotation described.

Advantages and disadvantages of crop rotation.

Advantages

Disadvantages

Total crop failure can be avoided. If one crop fails for any reason, the others may succeed. Pests and diseases can be controlled. Remember that the enemies of the crop are specific. They usually attack just one type of crop and no other. When moving crops year by year, pests and diseases are stopped and cannot build up. Weeds can be controlled. Some weeds grow very well among certain crops but not among others. Moving crops will control weeds much better. Nitrogen, a plant nutrient, is increased in the soil when legumes are grown. Some of this nitrogen is left in the soil when the legume crop is harvested. It then can be used by the crops that follow in the rotation. Nutrients are used from different levels in the soil because some crops have deeper roots than others do.

Following can be wasteful, although it is sometimes practised. It would be better to grow a fourth crop than to leave the land fallow. The profit of rotated crops may not be as great as from monoculture cash crops.

 

E. Continuous Cropping

In this system, the land is never left without a crop. As soon as one crop is harvested, another is planted. This system can work only where there is good rainfall throughout the year or with a very good irrigation system.

Advantages and disadvantages of continuous cropping.

Advantages	Disadvantages
The land produces crops all the time. There are no fallow periods. More money is made from sales of cash crops. Erosion is controlled when the land is under crop cover. The crop protects it from erosion by rain and wind.	None, but the system will not work without good rainfall or irrigation. The land must also be kept fertilised and supplied with plenty of organic matter.

 

F. Intercropping

In this system, two different crops are grown at the same time on the same piece of land, usually alongside each other in alternate rows. This is another form of mixed cropping. Examples of inter-cropping would be growing beans or groundnuts between rows of sugarcane or growing cowpeas between rows of maize or sorghum. This can work well as the smaller plants mature earlier than the cereals and can be harvested early in the season.

 

Commercial Soil Preparation and Planting

The factors to consider when looking for good land to plant crops on are listed below. Not many sites have all the features described here, but the more they have the better the sites will be.

• A fertile, medium loam with deep topsoil: This is the best soil for growing crops. The best soils have a good texture and structure, are well drained and aerated, and have a good humus content. Clay soils or sandy soils are not so good but can be improved.
• Flat or gently sloping lad: This helps to prevent soil erosion and makes irrigation easier. Water running over the ground after heavy rain can do tremendous damage to cultivated land. If a sloping site must be used, it should be terraced to hold back the water. Terracing enables you to have level stretches of land across a sloping hillside. The terraces hold back rainwater until it has had time to infiltrate (drain down into) the soil.
• Water supply: If rainfall is not enough, then irrigation is needed. This means having a reliable water supply such as a spring, a stream that does not dry up, or a borehole.
• No big rocks or stones: These make it hard to dig or plough.
• No big trees: Large trees could shade the crops too much and compete with them for water and nutrients. An open unshaded site is best.
• A strong fence: A good, strong fence is needed to keep animals out that can damage the crops.
• Shelter from strong winds: Strong winds might damage the plants. A hedge planted beside a garden makes a good shelter belt. A mat of thatching grass or maize stalks attached to the fence will also provide shelter. Shelter belts of trees can be planted around larger fields.
• Nearness to the market: If crops are being grown for sale, you have to consider how far you will have to transport them to the market and how much this will cost.
• Access to inputs: For proper crop growing, you need access to tools, implements, seeds, fertilisers, and chemical sprays. It is important to have a source of supply nearby.
• Unusual climatic factors: Knowledge about locality is imperative. Some places may be subject to flash floods after heavy rain or even frost damage. Factors like these will affect both the choice of a site for planting and the choice of crops to grow.