All erica family plants have very fine, hair-like roots that form a densely packed root-ball that is seldom more that 24 in (60 cm) deep, but which is at least as wide as the plant.
Many rhododendrons are adapted to growing under deciduous trees where, with an annual dressing of fallen leaves, the soil, over time, becomes composed almost entirely of leaf mold. This type of soil may not be high in nutrients but it is extremely humus-rich and is moisture-retentive yet well-drained. It is not a deep soil (even if it were, the tree roots would remove many of the nutrients from the lower levels), but it is open and easily penetrated by fine roots, water and air. This sort of compost-based soil is usually acidic, and this is why rhododendrons have the preference they do for acidic soils.
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Under these conditions rhododendrons have developed highly specialized roots. Most plants have roots with very fine hairs at their tips, which greatly expand the surface area of the root tip and absorb the essential minerals and moisture from the soil. Rhododendron roots do not have root hairs. Instead, the very fine roots perform the functions of root hairs so that the whole surface of the root ball is composed of feeding roots, thus enabling rhododendron roots to make the best use of the small volume of soil they occupy.
There are disadvantages, though. Delicate hair-like roots are the first to suffer in periods of drought or flood, so steady moisture and good drainage are essential. Fine roots suffocate in compacted soils, cannot penetrate heavy soils and cannot move obstacles, such as large stones that may obstruct their progress. All this means that a loose, well-aerated, easily penetrated soil is essential. Avoiding problems comes down to one thing: humus. For rhododendrons, it is virtually impossible to work too much compost or other humus-containing materials into the soil.
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Alpine rhododendrons are more adapted to growing in mineral-based soils than in humus-rich leaf mold. However, they tend to occur naturally only in areas with fairly high rainfall and are rarely moisture stressed. The fine roots are an advantage in this environment too, because they enable the plants to make the best use of soils that are frequently leached of nutrients by rain and melting snow.
Many vireyas are epiphytes (they grow on other plants as opposed to in soil). Although they generally only grow in rainforests or in areas that are frequently enveloped in cloud, they are more subject to moisture stress than other rhododendrons and do have somewhat heavier roots as a result. Likewise, deciduous azaleas, which can be found growing in quite dry environments, often have a few woody roots.
Grown under good conditions, rhododendrons are remarkably trouble-free. Apart from direct damage from wind, insects or sunburn, nearly all rhododendron disorders can be traced back to problems with the roots and ultimately the soil, so take the time to get it right.
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Nothing improves the soil more than natural compost. It is both a fertilizer and a soil conditioner and adds the all-important humus that opens up and aerates the soil while retaining moisture. You really can't overdo the compost -add as much as you can and dig it in as deeply as you can. Leaf mold, rotted conifer needles, rotted straw and garden compost are the best materials because they are full of vegetable humus and also provide good levels of nutrients. Stable manure and other animal manures are also good, but they should be mixed with straw and well rotted. Peat, bark chips and rotted sawdust are acceptable but they tend to break down quickly, have little humus and are low in nutrients. Avoid mushroom compost as it is usually quite alkaline.
Making your own compost is the best, and ultimately the cheapest, way of building up your soil's humus content and thereby improving its nutrient levels, aeration, structure, moisture retention and drainage. No garden should be without a compost pile and no serious gardener should be without compost.
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As moisture is filtered through the soil, the presence of minerals and vegetable matter in the soil will cause variations in its acidity or alkalinity, which are measured on the 14 point pH (potential of Hydrogen) scale.
Rhododendrons prefer an acidic soil with a pH in the range of 4.5-5.5, but most will grow perfectly satisfactorily with a pH as high as 6.5. Rhododendrons in the wild sometimes occur on limestone, which is alkaline. However, lime is very soluble and usually travels downward through the soil, so if a humus layer builds up on top of the limestone, the soil may not necessarily be alkaline.
If your soil is alkaline, it may be possible to imitate this type of situation by constructing raised beds for your plants. The lime is unlikely to migrate upward into raised beds, whereas making acidic pockets in limestone soils is only a temporary solution as the lime eventually seeps back.
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Rhododendrons have very efficient root systems. Good soil with plenty of compost, combined with regular surface mulching, will usually keep the plants growing well. Establishing the proper nutrient balance not only ensures that your plants will thrive, it also helps them resist climatic extremes, pests and diseases. Regular applications of mild liquid fertilizer and slow-release fertilizer pellets mixed into the mulch will overcome any minor deficiencies.
Plant growth requires that certain elements (nutrients) be present in the soil. Nitrogen (N), phosphorus (P) and potassium (K) are regarded as the "big three". You may have seen the letters NPK on fertilizer packets. They refer to the percentage of these elements in the fertilizer blend. A fertilizer with an NPK of 20-10-15 has 20% nitrogen, 10% phosphorus and 15% potassium by volume.
The pH is also important in determining how efficiently soil nutrients will be used. In general, trace element deficiencies are more apparent in acidic soils but very few soils are so acidic that the effect is greatly noticeable unless the soil is regularly cropped. On the other hand, plants will have difficulty taking up iron and magnesium if the soil becomes too alkaline.
There are two main groups of plant fertilizers, organic and chemical, or to put it another way, natural and artificial. Both forms are available in solid and liquid forms. Solid fertilizers are almost always worked into the soil or used as a soil dressing. Liquid fertilizers are often applied to the soil too, but many are intended to be applied to the foliage. These are known as foliar fertilizers or foliar feeds.
Naturally occurring organic fertilizers, such as animal manures, tend to be relatively mild unless they are very fresh or applied in large quantities. Many add humus as well as nutrients, but some offer only a limited range of nutrients and with repeated use deficiencies may occur. You may need to add chemical fertilizers to ensure a good supply of all the essential nutrients.
Although organic products are better at adding humus, chemical fertilizers are more suited to providing a balanced supply of nutrients and for correcting specific deficiencies.
Chemical fertilizers come as general balanced nutrient blends or as nutrient-specific fertilizers that supply one element or a selected group of elements. All-purpose fertilizers are ideal as a dressing before planting and as a booster in general cultivation, while nutrient-specific fertilizers are primarily intended to correct deficiencies. An example would be the use of iron sulfate or chelated iron to correct iron chlorosis.
The most common nutrient deficiencies that occur with rhododendrons are a lack of nitrogen or iron and magnesium chlorosis.
Lack of nitrogen leads to slow growth and an overall yellowing of the foliage that affects the old growth first. Nitrogen is most effectively added by using urea. However, urea is strong; use it at no more than 1 oz per 1 1/2 gallons (25 g per 5 liters) of water or severe burning may result. Milder sources of nitrogen include ammonium sulfate and ammonium nitrate. Make sure these fertilizers are thoroughly watered in.
Chlorotic leaves are usually yellow with distinctly green veins, indicating a lack of iron and/or magnesium. This may be due to deficiencies in these elements (more likely in container-grown plants) or to excess alkalinity preventing the plant from making the best use of the available nutrients.
Because it is often difficult to tell the cause of chlorosis, aim to cover all the possible causes. Mix about 1 oz (20 g) each of iron sulfate and magnesium sulfate (Epsom salts) in 1 1/2 gallons (5 liters) of water. Thoroughly soak the soil around the plants with this mixture. This should correct any deficiencies and the sulfate base of these fertilizers helps to neutralize alkalinity.