How to Fix Problem Soils in the Tropics

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Healthy soil is a prerequisite to a lush tropical garden.

You can’t grow a good garden unless you understand your soil. In the tropics, you don’t necessarily have to have good soil, but you must know what is wrong with it and how best to handle it if it is to produce luxuriant garden plants.

It is certainly easier to grow plants if you have good deep soils that are friable and soft and easy to dig, but with a bit of know-how almost any soil can be made to produce a beautiful garden.

It has been a popular misconception that only the topsoil is important. This is the upper 10-20 cm of slightly darker, more organic soil that people dig in, apply water and fertiliser and expect plants to grow. This has led to the destruction of large areas of bushland and seasonal lakebeds, and quite often, severe disappointment to the gardener who has paid dearly for this often unsuitable silty, often salty material.

The most important part of the soil in tropics is the subsoil because of its ability (or otherwise) to store moisture for the plants. Shallow soils cannot store much moisture, and in dry periods they may need daily watering. Very gravelly, stony, or coarse sandy soils also have a low ability to store moisture and also need frequent watering.

Under natural conditions shallow soils support only shallow rooted trees and if you want to grow a tree that will have a tap root you will need to dig a substantial hole. Even a 10-20 cm wide hole, a metre or so deep, allows the taproot to extend into moisture bearing layers of the soil and improves its chances of survival. A good deep taproot will also allow the tree to anchor itself against strong winds.

African Mahogany (Khaya senegalensis and allied species) is a popular garden shade tree in many tropical countries because of its fast growth characteristics. However, it became a much-maligned tree in many regions when it was discovered how easily it would blow over in cyclones or typhoons. That’s because it is a naturally shallow rooted tree species with a distinctive plate root system and a huge, dense canopy, and when planted on shallow soils, it had no resistance to cyclonic winds, particularly after soaking rains.

However, there is a solution to making the species cyclone-proof.  A large deep hole (about metre deep and the same square) is dug and a seedling mahogany is planted at the bottom of the hole. Over the next few years, the hole is gradually filled in with soil and compost. As each layer is consolidated, the tree will grow secondary plate root systems from adventitious roots produced from the trunk. 

Depending on the time taken to fill in the planting hole, 3 or 4 layers of roots can be developed, effectively giving the tree a considerable anchor in the soil. As another way of cyclone proofing this magnificent shade tree, heavy pruning or even pollarding of the branches should be practiced every few years to reduce the canopy load in strong winds.

Mulching conserves moisture

Soil moisture retention can be improved tremendously by mulching the topsoil with organic material. This reduces evaporation of moisture at the surface and hence slows down the rate of moisture loss from the subsoil.

ln the tropics, old leaves, cuttings from shrubs, lawn clippings, and so on. should never be thrown away or burnt. These are essential inputs to all our soils. They can be laid on the soil around shrubs, under banana plants, between rows of vegetables, or in a compost heap. lf possible, animal manures should be added as well. All these materials rot into the soil and help to conserve moisture, improve soil structure, and recycle plant foods at the same time. Well-rotted organic materials are the best soil conditioners available to home gardeners.

Organic materials are important as fertilisers also. They rot away slowly and release small quantities of plant nutrients and thus provide a continuous supply that keeps pace with uptake and use by plants and loss by leaching. Blood and bone and fish-based plant foods have a similar, but more concentrated effect.

The chemical fertility of the soil is also important. Whether organic or inorganic fertilisers are used it is a good policy to add a dolomite lime. Dolomite is preferable to straight garden lime or gypsum as it contains equal amounts of both calcium and magnesium. These are essential requirements in the tropical garden soil. Tropical soils are usually acid and chemical fertilisers can accentuate this effect, making the sweetening aspect of the dolomite even more essential.

Dolomite or lime will bring the soil acidity, or pH closer to neutral, and will also supply calcium and magnesium for the plants.

The term pH refers to a scale that is used by chemists to measure acidity and alkalinity. The scale runs from 0 (extremely acid) to 14 (extremely alkaline) with a pH reading of 7 representing the neutral level.

Correct soil pH is important

Some plants like an acid soil, others like an alkaline soil, whilst the majority grow best in soil that is close to a neutral value. Correct soil pH can make all the difference between thriving, healthy plants and stunted, disease prone plants that struggle to stay alive.

Many tropical plants do best in neutral to slightly acid conditions, say 6.5 to 7.5 on the pH scale, but many tropical soils are much more acid than that. Therefore, it will be necessary to add dolomite to these soils, even for plants that require slightly acid conditions.

The magnesium in dolomite also serves a second, vital function in the tropics. Nitrogen can be considered to be a prime mover of all other essential minerals used by the plant. However, when the temperature reaches the low 30s °C, nitrogen slows down or becomes inactive, meaning that those minerals needed for continued active growth are not delivered on time or not at all.

Fortunately, magnesium takes over the role as prime mover as nitrogen fails in this role and it can remain active till the temperature reaches the low 40s °C. This means that magnesium is a critical component in a healthy tropical garden and luckily it can be easily supplied by using dolomite in the compost and as a top dressing twice a year.

Soil pH can be measured with a pH meter or a soil test kit or you can do a very rough test with neutral litmus paper, or test strips, available from pharmacies.

Lawns often lose their vigour because the soil has become too acid. Under very acid conditions the grass may start dying back. Liming or top dressing with dolomite once a year at the start of the Wet season, is usually sufficient to correct any excess acidity.

Dolomite lime is the best form of lime to use as it contains more magnesium than ordinary agricultural lime. Never use slaked lime or quick lime as they are so strongly alkaline that they may injure your plants.

Heavy clay soils will also require applications of gypsum, a mineral soil conditioner that increases soil permeability. Dug into the soil at a rate of 1-2 kg per square metre, it will help to improve aeration and drainage of the soil and help to break up clay crystals that can lock up much of the applied plant food.

Gypsum also supplies calcium and sulphur for the plants but does not alter the pH of the soil so there is no danger of over application.

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