Whenever he moved home, an uncle of mine who worked as a soil scientist always kept a spade in the boot of his car. Once a cursory tour of any potential house was complete, he would take the spade and explore the garden to gauge the property’s true value. For him, the choice of house was always ultimately dictated by the garden’s potential and this, in turn, was determined by the quality of the soil. When his spade struck poor soil that was likely to be too difficult or expensive to improve he would immediately disqualify properties that had otherwise seemed perfect to his (often exasperated) family.
As well as the obvious difficulties posed by extreme heat, wind-exposure and humidity, gardeners in the UAE also face a very particular set of challenges in the form of naturally occurring local soils. Apart from the fact these are often rubble-strewn, they are also low in nutrients and laden with salts that are eventually lethal to many of the plants most popular with gardeners and municipalities alike. Add to this the fact that groundwater tables are invariably high and almost always brackish and UAE gardeners have a formidable number of obstacles to consider and overcome. Even container-gardeners like me are forced to consider soil-related issues carefully because we need to strike the right balance in our potting mixes between drainage, aeration and water-retention while also maintaining levels of vital nutrients that are quickly leached away by repeated irrigation.
As with all things, there are several ways to tackle these problems which vary greatly in terms of both labour intensity and cost. On most of the large-scale projects I work on as a landscape architect, issues of salinity, low water holding capacity and high groundwater levels are dealt with by completely excavating the existing ground, replacing this with ‘sweet soil’, adding organic material to improve soil fertility and even by installing sub-surface geo-textile membranes to combat rising groundwater. While this may sound like an expensive, Sisyphean task for the domestic gardener, each stage can be scaled back and recreated at home if you want to be able to garden flexibly with the widest, most exotic palette of plants possible.
To my embarrassment, ‘sweet soil’ came as something of a surprise to me when I first arrived in Abu Dhabi. My discomfort was further compounded by the fact that the UAE version was not soil at all as I understood it but was, largely, a form of plant-friendly dune sand. Deposited on the tops of dunes, this wind-blown sand has a better structure and is lower in salts than its earthbound counterpart and, when mixed with organic material like rotted manure or tree bark and a slow-release fertiliser, it makes an excellent planting medium.
The depth of sweet soil required by different types of plant is determined by their eventual size and also by the structure of their root systems but a general rule of thumb is that large trees should be planted in a pit of sweet soil with a depth of 1.5m, shrubs 60cm, groundcovers 45cm and turf laid over a layer 30cm deep. To this layer, water-retentive additives can be added that will slow the rate at which irrigation water infiltrates. One of the most widely used in the UAE is a product called Zeoplant, a locally-produced, mineral-based amendment that has proved popular with private gardeners and landscape professionals alike. Surface mulches can also be added that help keep the soil cool while also reducing the rate at which water is lost through evaporation. Chipped bark is an inexpensive mulch that will be familiar to many gardeners from overseas however, in the strong sunshine and heat of the UAE, it deteriorates quickly and will need frequent replacement. A longer-term and more attractive solution is to apply a 5cm-thick layer of stone chippings or gravel as a surface dressing to planting beds but this requires a more substantial investment at the outset.
Although it’s impossible to overestimate the role of well-dug, well-fed, well-structured soil in the success of any garden, it’s also possible to approach the subject from the perspective of plant selection and still achieve a garden that is verdant and highly attractive. Many of the plants used in local hotels and resorts are chosen for their ability to thrive in highly-saline, maritime conditions and gardeners can learn a lot from a weekend visit to the coast or beach club. Some of my favourites include the black-fruited, fan-flowered Inkberry (Scaevola plumieri), dwarf Japanese mock orange (Pittosporum tobira), paper-flowered Cottonwood (Hibiscus tiliaceus) and Sea grape (Coccoloba uvifera), which can be grown very attractively as a tree but is more commonly planted as a hedge or shrub.