This is one of the most important posts here, as many of the plant problems that are experienced in the region are a result of pH imbalances.
pH is the measure of free Hydrogen atoms in a substance; it is measured on a negative logarithmic scale from 1 to 14. The amount of free Hydrogen atoms in a substance determines its acidity or alkalinity. A low reading such as 1 indicates that a substance is highly acidic. A pH of 14 is extremely alkaline and 7 is neutral.
As the pH scale is logarithmic a pH of 8 indicates that it is 10 times more alkaline than a reading of 7 and 100 more alkaline than a reading of 6.
The norm for good, healthy plant growth is in the region of 6.5 to 7.5, again due to the adaptations by the plants themselves to cope with the variations of pH. In this range, about 80% of plant species will be able to survive.
Once the pH of the soil moves out of the accepted "bracket" for plant growth (either higher or lower) odd things begin to happen. The availability or scarceness of Hydrogen in the soil causes normal chemical reactions to halt. Once available for plant growth, nutrients become chemically bound to form compounds that the plant cannot either absorb or assimilate - this is sometimes called "nutrient locking" in a horticultural and landscaping context. This causes plants to show signs of nutrient deficiencies such as chlorosis (yellowing), stunting, premature flower and fruit drop, disfigurements of young foliage, or death.
Let's look at a practical example
Referring to the chart above, let's look at a common local problem, Iron deficiency; if you have plants such as Ixora you will be only too aware of this problem, yellowing of the interveinal areas of the leaves, and stunting of the plant.
You will note that Iron is freely available within the pH range 4 to 6, by the time the pH has reached 7 the availability in the soil is half of what it was, and by 7.5 it is about a quarter of the original. By the time the pH is in the range of 8.5, the amount of available iron for plant growth is virtually non-existent!
It shouldn't then come as a surprise to learn that the local, native soils are in the region of 8.5 to 9; let's compound the problem a little more as well, what do you think the pH of the local municipal potable water is? It's in the range of 8.5 to 9! This means every time you irrigate your garden you are simply compounding the problem.
And this is only Iron, the same applies to the other trace elements: Manganese, Boron, Copper, and Zinc.
Even the macro-elements Phosphorus and Potassium become deficient at higher pH values; affecting root formation, flowering, fruiting, and drought tolerance of plants.
If we look at our Ixora example a little more closely we can add a few more pieces to the puzzle, Ixora originates in tropical/ jungle regions; due to the large volumes of leaf litter and decomposing organic matter that you would find naturally in these areas, these soils are naturally acidic as they have a surplus of Hydrogen atoms, so when the Ixora plant is grown in an alkaline soil the pH is between 100 and 1000 times what would be considered the "norm" for its native habitat.
This holds for all plants originating in tropical or jungle areas, they are adapted to low-pH soils - to get them to thrive you need to improve the natural soils and maintain this over time to prevent the soil pH from becoming alkaline again.
Symptoms of plants grown in soil with an unbalanced pH
The following symptoms can be expected to be found in plants growing in soils where the pH falls outside of the "optimal range" - either too high or too low, although in the GCC the expectation is that this will be too high pH (alkaline).
Yellowing and other symptoms of nutrient deficiencies (or in some cases excess)
Stunting
Wilting (not associated with under or over-watering)
These will be chronic symptoms, meaning that you will be able to treat the symptoms short-term by applying dry fertilizer applications or even trace-element spray applications (e.g. Iron Chelate), however, as soon as the effects of the fertilizer have worn off, the symptoms will return unless more fertilizer is applied.
Depending on the type of plant it may be subject to one or even all of these factors. It must also be noted that this will apply more to plants planted directly into the soil as containerized plants are normally grown in a more organic-rich medium which naturally has a more neutral pH - unless, of course, you are using generous amounts of native soil in your containers.
So what is the long-term solution?
The only natural, effective way to reduce the pH of the soil is to add organic material in the form of compost. Practically this is not always easy if you have an existing garden, as you don't necessarily want to remove all your existing plants or disturb the roots of healthy plants simply to add compost to the area. The easiest way to accomplish this is to add compost as a mulch to the top of the soil and keep it damp, over time the compost decomposes and the small particles will migrate deeper into the soil profile. If you are lucky you may be able to encourage earthworms under the right conditions, these will speed up the movement of organic matter into the lower levels of the soil (as well as increase aeration in the ground).
This is a slow process, as you simply can't add 20cm of compost to the top of your flower beds and expect to be absorbed overnight into the ground.
Start by applying 3 to 4cm of compost in the open areas of the planting beds, do not apply it up against the stems of the shrubs as this can result in stem rot. Over a few months, this will break down and the level will drop, then add another 3 or 4 cm and repeat the process - Ideally you want to try and get 20-30cm of compost into the soil over time. Make sure that you keep this compost damp when you are watering the garden as if it dries out it will simply blow away, and you need water/ damp soil conditions for decomposition to work.
New gardens are easier, add as much compost to the planting areas (and work it into the top 20cm of the soil) as you can afford - the more the better. Ensure all trees are planted with a generous helping of compost as well, and don't skimp on the size of the planting hole!
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