The not so Mellow Yellow story of the simple leaf

Many healthy plants can suddenly worry us as their healthy green leaves turn to a sickly looking yellow. The leaves of many plants can suddenly turn yellow and this may happen for a number of reasons. The first thing to consider is the weather. If it has turned particularly cold or very dry, then the yellowing of the leaves could be a natural progression of the plant life cycle, it is preparing to drop its leaves for the colder season ahead. If this is the case there is nothing to worry about. The leaves will fall and new ones will grow the following year. If there is a drought, then water the plant and hopefully all will be well, with new buds forming soon, or perhaps in the following season, depending a little on how severe the drought and the stage the plant is at in its season.

If cold and lack of water can be ruled out and there is no chance of herbicide damage, then the yellowing of a plant’s leaves are usually caused by a nutrient deficiency. There are nine essential nutrients that plants require for growth and when the plant is lacking even one of these vital elements, it will try and tell you about it, in the only way a plant can. By either turning its leaves yellow, or partly yellow, or defoliating. At this stage, identifying the element missing and replacing it, or increasing the plants access to it in the soil, will be necessary to resolve the yellowing.

The not so Mellow Yellow story of the simple leaf

Iron (Fe) is required by all plants and without it plants cannot function as they should. Iron plays a part in many of the vital functions of the plant. Iron deficiency, known as Iron chlorosis, can affect many plants, and yellowing of the leaves, whilst the veins remain green, is a good indicator of iron deficiency. Typically yellowing starts at the tips of new growth and works its way to older leaves as the deficiency gets worse.

Iron chlorosis in plants is typically caused by one, or more, environmental factors. A soil with a pH that is too high, making it more alkaline.

Some plants prefer a neutral to slightly acidic soil and so a slightly alkaline soil restricts the ability of these plants to absorb the levels of iron they need. Rarely is it the soil itself that is lacking in iron, just the plant’s ability to absorb it. PH levels of the soil can easily be checked at home with a simple soil pH testing kit, available from many garden centres and online sellers.

Soils with low levels of organic matter also inhibit the plant’s ability to absorb iron from the soil. Organic matter is full of trace nutrients that the plant uses to take iron into its roots. So mulching with an organic soil improver should increase the levels of trace nutrients available in the soil and increase the plants ability to take iron in through its roots. This can be especially important in clay soils, soils that are particularly poor because of other plants nearby taking water and nutrients, and soils in newly built houses that may have high levels of building waste or subsoil near to the surface.

Over watering, or poor drainage, can also manifest itself through limp, yellowing leaves. Checking the soil around yellowing plants will soon show if this is the case. If it is, improve the drainage if possible or reduce watering. A heavy, wet soil can mean that certain plant roots do not have enough air to take up iron for the plant’s needs. If drainage cannot easily be improved an application of sulphate of iron may prove helpful.

Magnesium (Mg) is another nutrient deficiency that can cause yellowing of leaves. Plants lacking magnesium will usually turn a pale green or yellow colour, again between the leaf veins and often with red, purple or brown hues and early leaf fall. First signs of a magnesium deficiency are the yellowing of older leaves near the bottom of the plant. Common in apples, cherries, camellias and roses, amongst others.

Similar to iron deficiency there is not usually a shortage of magnesium in the soil, unless you have a light sandy soil, but over application of potassium rich fertilisers can cause a problem as the plant will take up the potassium in preference to the magnesium. Reduce application of potassium fertilisers and apply a foliar feed during the summer, of Epsom salts. Or for a longer term solution apply to the soil around the roots.

The not so Mellow Yellow story of the simple leaf

Manganese (Mn) is a nutrient that is only used in very small amounts by plants, but is one of the  most important for healthy plant growth. Symptoms of deficiency are similar to iron deficiency, leaves will turn yellow and there will be inter veinal chlorosis, but where as iron deficiency progresses to a paling of leaves, between the veins anyway, prolonged manganese deficiency results in tan areas developing between veins. Manganese is also less mobile in a plant than magnesium and deficiency appears first on younger leaves. A useful identifier as it’s the opposite of magnesium deficiency. Manganese deficiency commonly occurs when the pH of the soil is in excess of 6.5, as the nutrient is tied up and unavailable for uptake by the plant. Deficiency may also occur from under fertilising or use of general purpose fertilisers, which generally have low levels of micro-nutrients. As with magnesium deficiency a foliar feed, with manganese sulphate, can help clear up a problem in the short term and can be applied to the soil around the plant.

As a starting point, ensuring that soils are not starved is a good preventative measure. Ensuring annual mulching with well composted manure, home compost or other organic material will solve many issues before they arise. The correct rate of water is a second important factor that will assist in preventing many deficiencies. Understanding the pH of your soil is also important, and knowing how it changes around your garden. PH testers are readily available and easy to use and will tell you a lot about the soil in your garden, which could be very different to the soil in next door’s garden.

It is also important to note that over-application of any nutrient can be just as detrimental to under application. Although the information here will hopefully give you a guide to identifying your plants needs, a test of plant material and soil may be necessary to ensure that you are doing the best thing for your plant.

The not so Mellow Yellow story of the simple leaf