Seeds Sowing and Planting Practices
Seeds and other planting materials are the basic foundation upon which successful crop production is laid. Failure in the choice of correct seeds, seed storage, time of planting, depth of planting, rate of seeding and spacing could all lead to failure in the crop production enterprise.
In this article, all aspects of seeds and planting materials as they relate to quality, germinations, storage of seeds, seed treatments, time of planting, seed rate, depth of planting, seed placement, seed emergence and seedling vigour are treated.
The establishment of field crops could either be from seeds or vegetative parts. The terms sowing refers to seeds while planting is used when vegetative parts are involved.
The following are examples of crops established by sowing seeds: maize, groundnuts, sorghum, millet, wheat, cowpeas and melon. Crops such as yams, sugarcane, ginger, sisal, cocoyam, sweet potatoes and cassava are normally established by planting vegetative parts.
A seed consist of a miniature plant (the embryo) enclosed in a wrapper (the testa). Sometimes a quantity of nutritive material (the endosperm) may also be enclosed in the wrapper.
The embryo represents the new generation of the plant and it is the most essential part of the seed. It consists of a shoot axis (the plumule), a root axis (the radicle) and one or more seed leaves (the cotyledons).
In seeds where the endosperm is absent, much of the food materials are stored in the cotyledons. The testa is the outer covering of the seed. It usually encloses the seed entirely except for a minute pore.
Germination of Seeds
The process of germination involves the growth of the miniature plant (embryo) contained within the seed into a larger plant. The main function of the endosperm, where present, is to supply food materials to the growing embryo.
The first event that occurs after a seed is sown in a moist soil is that the seed imbibes water. This process causes the seed to swell and also results in the activation of various enzymes systems in the seed.
Following the intake of water, the food material present in the seed is hydrolyzed. The products of hydrolysis are then translocated to the growing points, where they are used as building blocks for new cells and as substrates for respiration to supply energy for various processes.
Conditions Necessary for Germination
For seeds to germinate certain environmental conditions must be fulfilled:
Water must be available so that when it is imbibed by the seed, metabolic processes within the seeds are enhanced
Oxygen should be present for aerobic respiration to occur so as to supply energy for germination process
Thirdly, there should be an appropriate temperature.
Types of Seed Germination
The morphological changes occurring during germination vary greatly from seed to seed. In some, the cotyledons are carried above ground level due to elongation of the region just below the cotyledons (hypocotyl).
Seedlings whose germination occurs in this way are classified as epigeal germination. The seedlings of groundnuts, melon, cowpea, onion, okro, and castor are all epigeal.
In contrast to epigeal seedlings are those in which the cotyledons remain at the level where the seed was planted and are not carried above ground.
Such seedlings exhibit hypogeal germination. Rice, maize, guinea corn, rubber and broad bean seedlings are all examples of hypogeal germination.
Seed Dormancy
When a living seed fails to germinate even when provided with the normal conditions necessary for germination, such a seed is said to be dormant.
Causes of Seed Dormancy
Presence of an impermeable testa that may prevent intake of water and probably oxygen.
The presence of growth inhibitors in the seed.
Alternatively, it may be caused by the need for cold treatment or for exposure to certain photoperiods before the seed can germinate.
The embryo is still immature and has not yet reached its full development at the time of harvest.
Very high temperatures during seed maturity may induce dormancy in some species.
Measures of Overcoming Seed Dormancy
1. After Ripening Treatment
One type of primary dormancy is characterized by immature embryos. Although the seeds are shed by the plant, the embryo must continue to develop before germination will occur.
Problems of immature embryos will be overcome if the seeds receive appropriate after ripening treatment. Often high temperatures are required for after-ripening of certain palm seed require 38°C to 40°C for three months.
2. Stratification
Seeds of many plants require moist chilling conditions for a period of time to render them capable of germination. This process is called stratification.
Chilling is usually 0°C- 10°C for 7 to 180 days. For example, apple seeds require up to 60 days in moist medium at 3°C to 5°C to overcome dormancy.
3. Scarification
Seeds of some plant species have a very hard covering that may prevent them from germination unless treated.
Hard seed covering can prevent absorption of water and gaseous exchange or may physically prevent the embryo from growing and emerging through the seed coat.
It is therefore necessary to make these covering weaker or pervious to water and gases through the process of scarification.
4. Light
Is essential for seed germination of certain species of lettuce (positively photoblastic), if light is required seeds should be sown shallowly.
Some species such as tomato and some lilies are negatively photoblastic (their germination is inhibited by light) and should be sown more deeply for good germination.
5. Vernalisation of seeds
Temperatures affect the time of flowering in many plants. Winter varieties of cereals crops head normally from spring sowing and behave like spring varieties when the seeds are germinated at temperatures slightly above freezing before they are sown. This process is called vernalisation.
Methods of Seeds Scarifications
Seeds can be scarified by soaking them in concentrated sulpuric acid for a period ranging from few minutes to an hour or more.
A hot water soak is another method of scarifying seeds, first heating water to boiling point add seeds and remove them when the water is cool 12-24 hours later.
Dry heat can be used to rupture seed coats of some species.
Mechanical scarifier can be used to scratch the seed coats. When scarified seed deteriorate rapidly, they should be planted immediately.
Aging brings about slow natural deterioration of the seed coat in dry storage. In an experiment with alfalfa, one half of the impermeable seeds germinated after one and half years while all germinated after eleven years in storage.
Alternate freezing and thawing sometimes stimulates germination of hard seeds of alfalfa and sweet clover.
The germination of hard seeds of alfalfa or clover can also be achieved by exposure for 1 to 1.5 seconds or less to infrared rays of 1180 millimicrons wave length or by exposure to a few seconds to high frequency electric energy.
6. Seed Quality
The factors affecting seed quality; the factors affecting seed quality includes the following: maturity, wholesomeness, diseases and pests and foreign matter.
6a. Maturity
Immature seeds tend to store poorly, and in many instances may fail to germinate. Both the stage of maturity when harvested and the conditions prevailing during maturity are important factors in seed quality.
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6b. Wholesomeness
Injury, cracking or breakage of the seed would result in reduced germination. The extent to which a seed’s ability to germinate is impaired depends on the part of the seed that is injured.
Relatively large amounts of injury occurring in the endosperm or at the edges of the cotyledons may impair germination only slightly, while minute injury to the plumule-radicle axis may cause failure of germination.
Mechanical injury to seeds may also make them susceptible to diseases and pest and therefore reduces their storability. The market value of broken but otherwise good seed is lower than that of whole seeds. Thus, wholesomeness of seeds is important, whether or not they are to be sown.
6c. Diseases and pest
The presence of diseases and pests reduce the quality of seeds both for consumption and for sowing. Diseases impart an unpleasant odour and taste to the seed, while pest may consume the seed and degrade it with excrement.
Diseased seeds germinate poorly and pests such as cowpea weevil may damage the seeds and also cause poor germination. Seeds that are to be stored are commonly protected from diseases and pests by treating them with appropriate pesticides. This procedure is referred to as seed dressing.
6d. Foreign matter
The presence of foreign matter in the seed lot reduces its quality. Inert foreign matter such as stones and dried plant material are objectionable, particularly in seeds destined for consumption.
Even more objectionable are weed seeds which may pose grave problems when the seeds are sown. Moreover, weed seeds may promote spoilage of the seed lot during storage.
7. Seed Certification
In each country, there is an agency responsible for overseeing and supervising the quality of seeds.
Mostly, the activities of the agency include the monitoring of the locations where seeds is produced, grading the seeds, carrying out viability and germination tests, certifying the seeds, and in some cases distributing certified seeds.
8. Classes of seeds
8a. Breeder seeds
This provides the source for the initial and recurring increase of foundation seed. They are directly controlled by the originator or in some cases by the sponsoring plant breeder or institution.
8b. Foundation seeds
Foundation seed is the source of all other certified seed classes, either directly or through registered seed. They are handled so as to maintain specific genetic identity and purity as prescribed by the agricultural experimental station.
8c. Registered seeds
These are the progeny of foundation or registered seed. It is carefully handled so as to maintain satisfactory genetic identity and purity that has been approved and certified by the certifying agency. This class of seed should be of quality suitable for production of certified seed.
8d. Certified seed
This shall be the progeny of foundation, registered or certified seed that is so handled as to maintain satisfactory genetic identity and purity. Such seeds have been approved and certified by the certifying agency.
9. Seed Storage
The optimum conditions for storing seeds that would endure long term storage are:
Drying the seed to 5 -7 per cent moisture content;
Sealed storage in the absence of oxygen;
A storage temperature of 7.5° to 15°C.
Most seeds stores well in cool dry condition. However, some seed may lose viability rapidly when dry. Within limits, for most species, for each 10 per cent decrease in seed moisture the life of the seed is doubled.
Likewise the life of the seeds doubles for each 10°C drop in temperatures. Most seeds can be stored at – 18°C for considerable length of time.
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Seeds of some plants including citrus and chestnut should be stored moist under refrigerated condition. Moist chilling condition is necessary to stratify seeds of many woody temperate species which help meet their dormancy requirement.
If stored in relatively small quantities, most seeds should be kept dry in a tightly sealed container.
10. Seed Treatments
Both insects and diseases may be seed borne. Disease and pest control by seed treatment is sometimes necessary.
Methods of Seed Treatment
1. Surface treatment
The organisms causing some seed-borne diseases are on the surface of the seed and may be controlled by chemical treatment e.g. damping off of seedlings.
Seed treatment may also protect young seedlings from soil-borne diseases at the time of germination, when they are especially vulnerable. Copper sulphate, formaldehyde and oxide of copper are among the chemicals commonly used.
2. Hot water treatment
When the disease causing organism is found beneath the seed coat, surface application of chemicals may not be effective. Some of these seeds may be treated with hot water for a period of time sufficient to kill the disease without destroying the viability of the seeds.
3. Fumigation
Weevils in beans may be controlled by fumigation with formaldehyde. When seed is not treated, the weevils may develop within the seeds and render them incapable of germination.
4. Pelleted seeds
Even distribution of very small and irregular seeds is often a problem. A pelleting procedure has been developed whereby individual seeds are coated with a material which may contain a fungicide and chemicals to stimulate growth and enclose the seed in spheres of uniform size. Pelleted seeds are much more easily sown at regular intervals and may produce better result.
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