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Most species of palms prefer acid soils well enriched with humus; especially palms that occur in rainforest areas. Palms such as Brahea species, Phoenix dactylifera, P. theophrasti and Chamaerops humilis will tolerate a wider range of soils, sometimes even alkaline, that is soils containing lime. Soil acidity or lack of it controls the solubility of nutrients and their availability to the plant may be reduced if the acidity is not correct. Palms can tolerate a reasonable range of soil acidity levels and palms growing in the ground generally do not develop mineral deficiencies.
Lack of a particular mineral or nutrient is sometimes visually obvious. It can appear as a change in colour of the leaves, or leaves that are reduced in size, sometimes deformed or with necrotic leaf margins.
A deficiency in nitrogen is evident when a general yellowing of the leaves occurs. Palms lacking nitrogen produce slower growth and make fewer and often smaller leaves. First the oldest leaves begin to turn yellow and this eventually spreads through all of the foliage. In extreme cases, the foliage can be almost white. Nitrogen can become unavailable to the plant if the soil is either very acidic or is alkaline. With palms it is most likely that lime has been applied to the soil reducing the availability of nitrogen. Alternatively lack of nitrogen can occur because there is insufficient nitrogen for the palm in the soil and this can be corrected with a supplementary feed of urea.
A deficiency of phosphorus will also cause growth to be slow and in some species of palms the foliage changes to a light olive-green or even a blueish colour, sometimes even yellow, and the palm may lack lustre and general vigour. Lack of phosphorus is relatively uncommon and is most likely to occur if a palm is planted in poor soils where crops or other plants have removed that mineral from the soil. It can be corrected by the application of superphosphate: However it is unlikely to occur if a fertilizer containing the three elements nitrogen, phosphorus and potassium is used.
Potassium deficiencies commonly cause the lower leaves of some palms to become necrotic at the leaf margins. In others this may develop as spots, blotches or streaks. The necrotic or dying areas may often be surrounded by pale zones. A potassium deficiency can occur in wet sandy soils; potassium, or potash as it is commonly called is very soluble and is readily leached from the soil. In climates with wet winters, potash can be leached from the soil and if feeding does not occur in spring or summer, this deficiency may become apparent,
A lack of magnesium is evident with signs of interveinal chlorosis in the oldest leaves. That is the areas of the leaflets between the veins tend to become yellow. This is often progressive from the edges of the leaflets inwards. The rachis and veins of the leaflet may remain green except in severe cases where the whole leaf may turn yellow.
In some Phoenix species, particularly P. canariensis and P. reclinata, the lower leaves sometimes turn brown and prematurely die off. This has been found to be due to a deficiency of magnesium and can be corrected by the application of magnesium sulphate (Epsom salts).
Some palms lose magnesium from their foliage during winter especially during cold wet periods and the foliage becomes yellow in colour. A supplementary feed or foliage spray with magnesium sulphate (Epsom salts) will benefit the plant particularly in early spring.
Many palms have a high requirement for iron and a lack of this element will also appear as interveinal chlorosis or an overall yellowing where it is apparent in the newest leaves first. When iron is unavailable, the leaves yellow because they cannot make chlorophyll the essential material needed by all green plants. Iron becomes less available if the soil is alkaline, that is when lime has been applied and for palms this can be corrected by the application of iron chelates or with sulphate of iron.
An iron deficiency can sometimes occur with a potassium deficiency as both of these nutrients can become unavailable if the soil is too alkaline. Coconut Palms growing on sandy beaches at the edge of coralreefs can often suffer from a lack of iron or iron and potassium together. In some Pacific islands, a common remedy is to drive iron nails into the trunks of Coconut palms so that the palm can absorb some of the iron as the nails rust. This method works to some extent but: would be better corrected by the application of the nutrient to the roots.
Frizzled emerging leaves in Syagrus romanzoffiana (Arecastrum romanzoffianum) have been found to be due to a lack of manganese and can be corrected by the addition of manganese sulphate. This problem can occur if the palm is grown in alkaline soil. It has been found that a deficiency of manganese in Phoenix canariensis, Livistona chinensis and Caryota species, appears with the new leaves emerging pale green in colour and failing to open properly. In cases where this problem has been occurring for some time, the older leaves usually also have a yellowish discoloration and the palm tends to become water spotted in wet weather.
A zinc deficiency in many palms, particularly Dypsis lutescens, has been reported to be apparent with yellowish undersized leaves that have short pinnae which are closely spaced and the plants assume a stunted appearance.
A calcium deficiency in Howea and Chamaedorea species has been reported to also produce stunted new leaves that are deformed in appearance.
Palms grown in pots and containers are more prone to feeding deficiencies than palms grown in the open grouncL In either case, it is easier to prevent the problems from occurring than to try to correct them later. Regular feeding with a balanced N.P.K. fertilizer containing magnesium and the other trace minerals will keep a palm healthy and free from these problems. When deficiencies appear in palms growing in the ground, particularly large specimens, the most difficult task is to identify the problem. The application of trace minerals such as iron, manganese and zinc to the roots of a large palm may take months to reach the leaves and correct the problem. Weak solutions of the trace mineral particularly in the form known as a chelate can be applied by spray to the foliage during cloudy weather and as they are absorbed, the palm responds to the corrective measure more rapidly. When trace minerals are applied, they should be used at the rate recommended by the manufacturer. Toxicities can be caused by applying too much of a trace element.
Palms & Cycads Beyond the Tropics, Keith Boyer.