The following points highlight the two types of forest nursery. The types are:- 1. Bareroot Nursery 2. Container Nursery.
Type # 1. Bareroot Nursery:
i. Direct Sowing in Beds in the Nursery:
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Growing bare-rooted seedlings is the simplest form of tree propagation in a nursery. This method is used if the species is easy to plant out, which means seedlings are not prone to heavy losses after field planting. Ample quantities of (cheap) seed and high germination percentage are also favourable for direct sowing. Seeds are sown in a well-prepared bed and the seedlings are grown till they are ready for field planting. Tree seedlings have to stay in the nursery bed for two months to one year.
ii. Sowing In Seedbeds and Further Growth in Plantbeds:
The above method is not used much for tree nursery work. It is only suited to seedlings which will be ready within a few months and will survive easily after field planting. A more common method is to sow the seed closely in seedbeds and the young seedlings are pricked out into plant-beds at the right spacing for their further growth in the nursery. Pricking out is the term used for transplanting young delicate seedlings by lifting them gently.
iii. Air-Pruning of Roots in Seed Trays:
A final improvement in the germination stage is the use of seed trays with an open-mesh bottom. Wooden trays with a wire mesh bottom (like a sieve) can be made or heavy netted plastic sheets can be cut and folded for using as windscreens. Cheap plastic trays with mesh sides and bottom are available in the market.
Sand would fall through, so trays have to be filled with friable soil or – better still – with a good rooting medium such as shredded coconut fibre. The trays need to be placed on a bench with an open top, e.g. made of wire netting or slats, so that the underside of the trays is exposed to the air. The seeds are sown in the seed trays and often covered with loose material, such as coarse sand or very fine gravel, which are easier for the seedlings to break through.
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iv. Preparation of Seedbeds and Sowing in Nursery:
The soil in seedbeds must be compacted to ensure good contact between the top layer in which the seeds are sown and the deeper layers. In loose soil, the topsoil may dry out quickly because moisture cannot move upwards between the soil particles. The soil is firmed, for instance using a board over which one can walk.
The soil is sufficiently compacted in seedbed if only a slight imprint is left when pressed it with the knuckles of a fist. Light watering after sowing also helps to settle the soil around the seed. The soil should not be compacted in a bed when it is too wet as this can ruin the soil structure.
Broadcasting is scattering the seed by hand and it requires practice to achieve a fairly even distribution. If experience is lacking, it is better to sow thinly. The seedlings have to be thinned if there is overcrowding, which is a waste of planting material.
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Broadcasting is limited to small seeds, because large seeds cannot be placed at the right depth by broadcasting. Broadcast seed is either raked in or covered immediately with loose material, such as coarse sand or very fine gravel. Light watering of the covered seeds helps to settle the soil, ensuring good contact between seeds and soil.
Row-seeding makes it easier to ensure an even distribution, although dibbling also requires practice and some thinning out may be needed. Sowing in rows also facilitates weeding and pricking out. The rows can be made by pressing a slat across the bed to make small V-shaped furrows in which the seed is sown.
If seeds are sown directly in their final position in the nursery (no pricking out) they are sown in pits at the proper spacing in a regular pattern. The spacing depends on the tree species and the required seedling size for planting out. A drilling board, fitted with pegs at the desired spacing is a very useful tool to make holes the correct distance apart and in the correct pattern. Both large and small seeds can be sown in this way.
Depending on the germination percentage, a single seed or several seeds (usually 3 to 6) are sown in each position. The spacing ranges from 7 x 7 cm (ending up with 200 seedlings per m2) for slender conifers to 14 x 14 or 20 x 20 cm for species with large leaves such as teak (resulting in 50 or 25 seedlings per m2). After emergence of the seedlings, they are thinned leaving only a single seedling in each grid position. If thinning is inadequate, the seedlings are crowded which grow spindly and may not survive after planting out.
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Sowing depth is generally twice the diameter of the seed. For example, seed with a diameter of 3 cm is sown at a depth of 6 cm. If seed is sown too deep, the food reserves in the seed may be depleted before the seedling has formed green leaves for its nourishment. Seeds are covered immediately after sowing.
If germination takes several weeks, it is worthwhile to mulch seedbeds. The mulch softens the impact of watering and keeps the topsoil moist. However, mulch should be removed as soon as the first signs of germination are observed; otherwise sprouts growing through the mulch will be blanched, spindly and prone to disease.
v. Pricking Out:
Seedlings in seedbeds or seed trays have to be transferred to the plant-beds. Pricking out is usually done when the seedlings have one or two normal leaves. If seeds are sown at the right spacing, the seedlings fill the bed or tray by the time they have reached the stage for pricking out. For most species this is within a few weeks from emergence of the sprout. Simultaneous germination makes it possible to prick out all the seedlings at the same time.
But even with proper pre-germination treatments, it is often necessary to prick out repeatedly at an interval of several days. In that case, the spacing of the seeds in the tray or bed needs to be wide enough to prick out seedlings without damaging the others. It should be noted that the seedlings are held by a leaf, not by the stem. This is to avoid damaging the tender stem or transmitting damping-off.
In most species, seedlings are large enough for pricking out about 10-20 days after germination, but conifers are often pricked out sooner, even 2-4 days after emergence. If the seedlings are too young they will be very delicate and likely to be damaged. However, waiting too long will result the seedlings become overcrowded and separating the roots becomes difficult. Only few seedlings should be pricked out at a time just to avoid drying out.
It should be made sure that the seedlings will have enough room when pricked out to attain the size desired for planting out in the plant-bed. The planting distances may be the same for direct seeding, ranging from 7 x 7 cm to 20 x 20 cm. But to facilitate weeding and root pruning, rows are often spaced about 20 to 25 cm apart, and the seedlings are set closer together in the rows, for instance 5-10 cm apart.
If wildings have to be nursed before field planting, they are set in plant-beds at the proper spacing. They can be graded according to size, paying particular attention to the amount of roots, to improve uniformity of the planting material.
Type # 2. Container Nursery:
i. Polybags (Polypots) and Polysleeves:
Sowing or planting in container is an alternative to sowing or planting in beds. This requires extra work in the nursery and greatly increases the cost of transport if the seedlings are to be planted in a remote site. Making container from local materials is time-consuming and earthenware pots are expensive, but with the introduction of cheap plastic containers (polybag, poly sleeves, root trainers etc.) potting up has become a lot more popular.
Polybags are supplied flat; filling with soil gives them their pot shape. Polybag and polysleeve are made of black or clear polyethylene and available in various volume sizes (polybag 0.3 to 45 L volume and polysleeve 0.3 to 1.5 L volume). Polybags have several drainage holes at bottom and the bottom of poly sleeves is open.
A common problem with polybags is that plant roots tend to grow in spirals and this will lead to plants with restricted growth, poor resistance to stress and wind-throw. Seedlings growing in polybag for longer period may penetrate the bottom and roots go deep into the earth. So during shifting, the roots are damaged and result in low survival per cent in field planting.
Poly sleeves allow for air pruning when placed on raised propagation beds. But they are less resilient in transport, because the potting substrate can easily fall out and damage the roots. Polybags and polysleeves are not very durable and can break prematurely. For plants with a long nursery cycle, this can be a problem. They are normally used only once. The discarded polybags and polysleeves are a problem for nursery waste management, as they do not decay and are often burned, producing serious air pollution.
ii. Root Trainers:
Root trainers are usually rigid containers with internal vertical ribs, which direct roots straight down to prevent spiral growth. The containers are set on frames or beds above the ground to allow air-pruning of roots as they emerge from the containers. The latest developments also encourage lateral air root pruning through vertical slits.
Seedlings grown in root trainers have more vigorous and rapid root growth than seedlings grown in polybags. Out planting survival and long-term survival are much better. Plants grown in root trainer systems are often ready for planting out when they are substantially smaller than those from conventional polybags. This helps to reduce space requirements in the nursery and transport costs to the field.
iii. Filling Polybags:
Soil for filling the polybags should be light and loose-textured for good aeration and drainage, so that root development is not impeded. But a ball of moist potting soil should not fall apart when picked up. The soil is sieved with wire mesh (with 5-10 mm square holes) to get rid off lumps and stones. Polybags should be well filled with sieved soil. The bag should be tapped on the ground to settle the soil, leaving no air pockets.
Funnels should be used to fill narrow bags and this works well if the soil is sufficiently loose and dry to flow down the funnel. Filled bags should be watered and left to stand for a few days till the soil has settled.
It is difficult to give a standard mixture for potting soil, because what is used depends on what is available locally. As a rule of thumb, when soil is to be part of the growing medium, the following mixtures are used (topsoil: fine gravel/sand: well decomposed organic matter such as manure or compost).
a. Commonly used mixture – 3:1:1
b. For heavy (clayey soils) – 1:2:2
c. For medium (loamy) soils – 1:1:1
d. For light (sandy) soils – 1:0:1
iv. Beds for Polybags:
Polybags are placed close together in blocks of similar length and width as the plant beds in the nursery. It is very important that the ground has been properly leveled and firmed, so that the bags do not fall over and water does not erode the ground. A frame of wood, bricks or earth can be set up along the sides of the bed to keep the bags upright and to prevent overheating of the outer row of pots (black bags can get very hot in the sun).
On a slope, the bags can be placed in trenches – again with carefully levelled bottom – to be inundated periodically by surface irrigation. The water cascades from the upper trench to the lower trenches. If water is scarce, the trenches can be lined with polythene.
The bags are placed together in straight lines of equal length and this makes counting and transplanting easier. The bags are always placed upright even though it is often easier to lean them against each other. If the bags lean to one side, even only slightly, water sprinkled over the bags tends to run off the lower edge instead of penetrating into the bags. It also results in a one-sided root system.
v. Sowing or Planting in Polybags:
Polybags can be seeded directly, or they can be planted with seedlings pricked out from seedbeds or seed trays. Wildings or cuttings can also be planted in bags. Following direct seeding, it is not possible to stimulate branching of the roots by (air) pruning. So this method is only suitable for species that naturally form a root system branching sufficiently to hold the potting soil together during field planting.
Larger size bags must be chosen to enable the seedling to grow to the desired size. Also, the germination percentage of the seed lot should be known, because this determines how much seed can be sown in each bag. If nearly every seed produces a good seedling, as is the case for Acacia species, one seed per pot is enough and there will be no need to thin out.
For very small seeds, sand-seed mixture (2:1) should be used. Small (paint) brush is dipped in water, then in the sand-seed mixture and the mixture is spread carefully over the bags that are filled with soil. For Eucalyptus seeds, this will produce a maximum of 4 or 5 seedlings per bag. In case several seeds are sown per bag, only the strongest one is left after emergence.
Thinned seedlings can be pricked out into bags in which none of the seeds germinated. For most tree species, pricking out seedlings into bags is much preferred to direct seeding in the bags for two reasons. Pricking out stimulates branching of the root system, so that the bags are better filled with roots to hold the soil during field planting. Because the pricked out seedlings are uniform, each batch of bag plants is uniform and problems with delayed germination are avoided.
vi. Growing Medium or Substrates:
Good plant development depends to a large part on the growing medium used. Most nurseries use mixtures of topsoil with organic and inorganic additions. As an alternative to native soil, organic and inorganic soil-less substrate have been receiving increased attention in tropical countries. The soils-less material have optimal physical and chemical properties and do not have weed seeds, fungal spores or insects or they can be heat sterilized without losing their properties.
The two major groups and components of soil-less media are:
i. Inorganic Substrates – gravel, sand, vermiculite, perlite, tuff and pumice, polystyrene
ii. Organic Substrates – peat, charcoal, softwood and hardwood barks, rice hulls, sawdust and other organic waste products
vi. Fertilizers:
Fertilizers provide plants with nutrients necessary for healthy growth. Apart from macronutrients N, P, K, Ca, Mg, and S, there is a known suite of micronutrients (Fe, Mn, B, Cu, CI, Zn and Mo) that play important roles in the plant’s metabolism. When soil or compost-based media is used, the substrate might contain enough nutrients for good plant growth. However, it is advisable to analyze the substrate for available plant nutrients regularly.
The different types of fertilizers include:
i. Organic Fertilizers:
FYM, compost, vermin-compost, animal waste (poultry litter, bone meal, fish meal, hoof and horn meal)
ii. Inorganic Fertilizers:
Ammonium Sulphate, Urea, Ammonium Chloride, Ammonium Nitrate, Ammonium Sulphate Nitrate, Calcium Ammonium Nitrate (CAN), Sodium Nitrate, Calcium Nitrate, Potassium Nitrate, Rock Phosphate, Single Superphosphate, Double Superphosphate, Triple Superphosphate, Basic Slage, Dicalcium Phosphate, Muriate of Potash, Potassium sulphate, Ammonium Phosphate, Urea Ammonium Phosphate, NPK fertilizers
iii. Biofertilizers:
Rhizobium (RHZ), Azotobacter (AZT), Azospirillum (AZS), Blue Green Algae (BGA), Azolla, Phosphate solubilising biofertilizer (PSB), Mycorrhiza such as VAM