The nutrition of these plants is based on the use of slow release fertilisers. Liquid fertilisers are generally applied only occasionally but under conditions where leaching may occur they may need to be applied more frequently.

TESTING SOIL-LESS MEDIA
It is important that the nutrient levels in the growing media are monitored closely to prevent the development of nutrient disorders. Desirable nutrient levels for soilless media are given in Table 1.

For bark based compost, half the P value given in Table 1 may be adequate. For seedlings, the desirable range is half the values for all elements. A pH of 5.3-6.5 measured in the unfiltered extract is considered desirable for most crops. It may be necessary to reduce N to very low levels in autumn for outdoor nursery stock, including container grown kiwifruit, so as to harden them against winter frosts.

Bark used in potting mixes should be well stabilised (composted), as indicated by a Nitrogen Drawdown Index (NDI) of at least 0.2.

FERTILISER REQUIREMENTS
The quantity of base fertiliser for bedding plants, nursery stock and pot plants grown in bark and peat media is given in Table 2.

Plants growing in bark mixes generally require more Fe than those grown in peat. If an Fe deficiency is suspected, chelated iron (50-75 g/100 l) can be applied at a rate of 20-30 ml/pot.

The quantity of Controlled Release Fertiliser (CRF); eg, Osmocote®, to be applied depends on the type of plant. For bedding plants and seedlings growing in a mix of four parts by volume of bark or peat to one volume of medium grade pumice, a CRF with a release period of 3-4 months and an N:P:K formulation of 14:6:12 or similar should be applied at a rate of 2-3 kg/m³ of growing media. This rate can also be used for sensitive pot plants such as African violets or bougainvillea. A rate of 5 kg/m³ of growing media is satisfactory for other pot plants. For container grown nursery stock, a CRF with a release period of 8-9 months and a formulation of 18:5:8 or similar can be used. A rate of 3-5 kg/m³ is recommended in this instance.

For acid requiring plants such as azaleas, rhododendron, and ericas the amount of dolomite should be reduced to 1 kg/m³ of potting mix. Other plants such as macadamia, kauri and silver beech (Nothofagus menziesii) also need a low input of lime, as well as the lower rate of CRF (2-3 kg/m³).

For Proteas (Leucandendron solignum 'Red Bird' and L. 'Safari Sunset'), a potting mix of 9 volumes of bark or peat and one volume of sterilised soil has been found to be successful with the base fertiliser input given in Table 3.