Design of a fully-automated Nursery Table

This article describes a simple non-mechanical growing system to sow, germinate, raise and strike cuttings of fruit trees without the need for hand watering or reliance on electrically operated irrigation. The two main physical principles exploited in the system are pressure and capillary action. Items needed for assembly are easy to source and relatively cheap.

Assembly of the system:

The area where seeds are raised / young trees are grown is a common plastic outdoor table. To protect from extreme weather conditions and pests, a lid assembled with aluminium tubing and 50% shade cloth sits on top of the table. It is secured with two or three door hinges at the long side of the table to open and close whilst the opposite side has a string with hook to hold the opened lid in place when working on the table.

Watering is achieved by connection to a rain water tank and drip feeding from one end of the table, using a 4mm drip irrigation poly pipe regulated by a 4mm in-line tap and an adjustable dripper at the end, as the water outlet.

As the water moves from one to the other end of the table, it is important to adjust the table legs in such a way that a slight downward slope is created. The lowest point should be only on one distant corner of the table, so run-off can be collected in a 60 litre bin for recycling. Drip frequency is adjusted by the in-line tap, whilst the adjustable dripper at the end needs to be almost fully open to prevent algal growth blocking the dripper.

A 5000 litre rain water tank provides enough water pressure to feed the dripper, however smaller volumes might be sufficient as the key variable to drive flow is the vertical hydrostatic pressure. As the water tank level falls, it is important to top up the water storage if it approaches the same height as the table.

Fertilising is continuous by adding a complete water-soluble fertiliser (containing all essential macro-, meso- and micronutrients) in the rainwater tank.

Before placing pots and cell trays on the table, a thin layer of water-absorbing material is scattered across the table to allow water drops to spread equally over the whole table as a film surrounding this material. Examples of suitable materials are vermiculite, coco husk products and sand. Cell trays and plant pots with their base drainage holes placed on the table will then take up the fertiliser solution by capillary action.

The seed raising medium or potting mix in which the plants are growing should be equally well absorbent to ensure the capillary action supplies the plants with enough moisture throughout the entire volume of the medium. Root growth will be enhanced by high oxygen levels, so the texture can be improved by adding coarse, yet also absorbing material, such as perlite. When striking cuttings it may be an advantage to further increase the coarse texture.

Recycling of the run-off fertiliser solution can be used to water and fertilise fruit trees in the garden during watering days. The water-absorbing layer on the table should be replaced approximately once a month to avoid potential build-up of pests small enough to pass through the shade cloth lid such as fungal gnats, mites and aphids. However, used vermiculite and coco coir can be recycled by spreading around fruit trees in the garden which will act as a soil improver. Ideally two tables can be set up side-by-side, so one table can be sterilised (and during hot weather left for solarisation for a few days) whilst plants are shifted to the clean table.

Key features of the Nursery Table:

• Reliable, automated watering and fertilising system with no moving parts or electricity
• Low cost and maintenance
• Multipurpose: Sowing and germination, striking cuttings, and establishing fruit tree and vegetable seedlings
• Protection from pests that are larger than the shade cloth mesh size (eg. snails, slugs, moths and their caterpillars)

Michael Crone