Nursery Techniques - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 34

source. If clay is very heavy, or very light, its proportion in potting mix can be
adjusted by decreasing or increasing it suitably.
Sand is abundantly available in most places in arid and semiarid areas. Clean sand
may be rarer to find, but it should not be aimed at. Sand with loam will serve the
purpose as well. If the available loam is heavy, its proportion in potting mix can be
corrected accordingly. Sand should be free from salinity, weed seeds, and colonies
of termites. If the soil within or near the nursery site is free from salinity and other
deleterious factors, it can be used and cost of transportation can be saved.

2.6 Procurement and Storage of Polypots

The ubiquitous polyethylene tube (polypot) has become the standard container for
raising plants in nurseries in many tropical countries. Polypots are made of different
grades of materials and different gauges of thickness. Low-density polyethylene
(LDPE) is the commonly used material. Costs may be saved by opting for recycled
material or high-density polyethylene (HDPE), but this involves compromises on
strength, durability, and other desirable properties of the polypots. Alternatives like
bricks, earthen pots, and metal containers can be used if considered suitable. Polypots
are, however, compact and easy to store and handle.
An alternative to polypots are rigid containers called root trainers which consist
of tubular cells in groups of 20s or 30s formed into a frame of moulded plastic sheet.
The cells have vertical ribs on the inner surface designed to direct growth of roots
vertically down so that root coiling is minimised. The containers are suspended on a
wire frame at a height of 15-20 cm above ground. As roots emerge out of the container
through the open bottom, they get dried up (air-pruned). Smaller volume of these
containers—usually in the range of 150-300 ml—leads to savings on procurement
of potting mix and on operations such as irrigation, pot turning, and transportation.
On the other hand, capital cost incurred on procurement of root trainers can be about
10 times as high as that of polypots. In terms of cost-effectiveness, therefore, a root
trainer must be used 20 times over—in those many seasons—before it can compete
with the polypot. Although a superior method on technical grounds, adoption of root
trainers should be accompanied by training of workers in use of these containers in
nursery and in field at the time of outplanting.
Quantity of polypots in terms of kilograms or tonnes can be determined on the
basis of their size. Common sizes and their unit weights can be found in Table 2.6 .
Larger sizes are used if taller plants are required. For most of the xerophytic plants
to be used in arid and semiarid areas, the size 10 25 cm should be considered,

whereas for timber, fruit, and other slow growing tree species requiring more than a
year's growth in nursery, the size 20

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40 cm should be preferred. Larger polypots
mean higher costs in terms of the pot itself, the volume of potting mix, and the labour
required in handling the pots. On the other hand, larger pots provide better nutrient
availability and more room for the roots to grow and therefore taller plants.

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