Fertilizing Outdoor Plants Growing In Containers – Which Type Of Fertilizer To Use

In principle, the best way to provide nutrients to established garden plants growing in the soil, is to add on a consistent basis, organic matter in the form of compost. For as well being a source of the mineral nutriment essential to plant growth, organic matter improvements and develops the health of the habitat in which the plants grow, previously the soil. There are circumstances though when the use of chemical fertilizer is preferred to compost. One of these relates to plants grown in pots. As there are a number of methods by which chemical fertilizer can be applied, the question arises as to which is the most appropriate.

Plants in pots or containers must always be grown in some form of artificial potting medium, in order to ensure the correct air / moisture balance in the root zone. One such medium, highly regarded by landscape professionals today, is Perlite, which among its various properties, excels both in its capacity to retain moisture on the one hand, and sufficient oxygen on the other. Chemically, it is almost entirely inert, which is an advantage in one sense, as the problem of salt build-up is avoided. Yet the other side of the coin is that mineral nutrient is liable to be absolutely lacking for the plants, unless supplied on a constant basis. This is why regular applications of chemical fertilizers are necessary. How should it be applied?

Clearly, the old method of manually spooning read soluble fertilizer is impractical at least in the case of Perlite, as the work would have to be carried out every few days or so. An elegant way round this is by installing an automatic fertilizer pump, where a specially balanced blend of liquid fertilizer is injected through the drip irrigation lines. Today, the pumps are calibrated by the manufacturer so that low and safe concentrations are supplied to the plant's roots, during every watering. Watering a bit to excess on each occasion ensures that the salinity of the medium is kept in check.

Despite its many benefits, there are a number of drawbacks to this method, not least of which, is the legal obligation required by most countries to install an instrument preventing the backflow of water. However in Mediterranean climates, typified by hot dry summers, and mild cool and wet winters, there is another disadvantage with fertilizer pumps, which should be considered.

It is often forgotten that the garden plants require some access to nutrients during the mild Mediterranean winter, albeit at greatly reduced levels. For plants growing in the ground, the compost added in the autumn should suffice. When it comes to containers however, especially if the potting medium used is a chemically inert product like Perlite, it is necessary to ensure that a supply of fertilizer is available to the plants. As fertilizer pumps supply the nutrients through the irrigation system, one is forced to open the taps, even if the plants do not need watering. Baring in mind that Perlite can hold sufficient moisture during the winter for a good 3-4 weeks, it follows that this great water conservation benefit is lost.

It is for this reason amongst others, that I advocate the use of slow release fertilizer as the principle means of feeding pot plants that are grown outdoors. (The hydroponic method applicable to indoor plants requires a different type of fertilizing) There are a number of products available, which release the nutrients over periods of time ranging from 3 to 12 months, thereby reducing labor to a couple of times a year or less . Many brands also contain trace elements, which are generally not missing in garden soils, but may well be so in the artificial potting media.