The selection of the area for a nursery is critical and it should be sited as centrally as possible to the field(s) to be planted. In addition, the following should also be considered:
The selected area should be flat to gently undulating with slopes between 0 and 30 and preferably, with a reliable/permanent source of water supply for irrigation purpose.
Water requirements (quality and quantity) must be determined prior to starting site preparation. The easiest source of water is where there is a large natural pond or lake whereby all that is required is to place an intake pipe to connect with a pump unit. A back-up system should be considered, particularly in isolated areas or areas of lower or unreliable rainfall.
The site chosen should not be prone to flooding, which will damage seedlings and buildings (stores).
To achieve good growing conditions with minimal risk of etiolation, a main nursery planting density of 13,800 polybags per ha with 0.91m(3ft) x 0.91m(3ft) D planting is recommended excluding allowance for accessibility. The spacing should be increased by another 0.15m (0.5ft) if the seedlings are anticipated to be kept in the nursery for longer than 12 months.
Accessibility and Nursery Roads
Roads within the nursery and their alignments will need to be carefully planned and laid out depending on the placement distance of the polybags and the type of irrigation to be utilised. Access roads to the nursery should be sufficiently wide to allow vehicles to manoeuvre during peak planting periods to facilitate supervision and movement of materials.
The preparation of the area for a nursery is important to allow optimum seedling growth, maintenance of nursery site, unimpeded access and to provide hygienic conditions for plant growth. Four main activities are involved in preparing a site for nursery, namely nursery design, clearing, fencing and lining.
A well-designed nursery allows for access of many vehicles during evacuation of seedlings for field planting especially for large-scale plantings. This objective can be achieved through the drawing up of a plan to show all paths, roads and irrigation points. An example is shown in Figure 1.
With the boundaries determined, felling and clearing should be carried out at least 2 months before the arrival of the seeds. Once clearing is completed, proceed to fence the area, fill the polybags and install the irrigation system.
The major types of fences utilised for nurseries are the conventional barbed wire fence and the electric fence.
The Conventional Fence
The specifications for the conventional fence depend on the species of animals that it is required to keep out. For example, a four-strand barbed wire fence, with wires spaced at 0.3, 0.6, 0.9 and 1.2 m from ground should be adequate to control cattle and goats.
Where there is threat of wild mammalian pests, an electrical fence is possibly the best form of defence. Wires of 250-300 kg breaking strain have been found to be the most suitable. The heights at which the electrified wires are placed above ground level are critical, being dependent on the mammalian pests to be kept out, e.g. 10 cm to repel porcupines.
Lining is carried out to space the polybags evenly in the nursery, so that seedlings have good and uniform access to sunlight and to achieve the most cost effective irrigation system .
Polybags are lined at 0.91m (3 ft) x 0.91m (3 ft) triangular spacing to give each seedling the optimum growth space. All seedling rows must be straight along the axis at 60o to each other and parallel to irrigation lines.
In the Sumisansui irrigation system, MK II tubes are laid down between the polybags at every 4 rows, a 1 m wide path is provided at every 8 rows for easy access. When the seedlings are about 7 – 8 months old, additional MK II tubes are added between the existing MK II tubes to give a final layout of 1 tube for every 2-seedling row. This is illustrated in Figure 2.