Fertilizer Management: Toxicity

Aluminum toxicity is widespread in the humid tropics where soils are acid and highly weathered. Low pH results in the accumulation of soluble Al in the soil that reduces root growth and impairs root function. Aluminum toxicity is thus manifested in the appearance of Mg and K deficiency, particularly in young palms but can be corrected by applying rock phosphate or dolomite at planting to ameliorate soil pH. Magnesium fertilizers have also been shown to reduce the effect of Al toxicity on plant growth for a variety of crops (Grimme and Härdter, 1991).

Nickel toxicity can be a difficult problem in soils derived from ultrabasic or ‘serpentine’ rocks. Palms affected by Ni toxicity exhibit narrow, fish-net like chlorotic patterns on younger leaves, and growth can be severely retarded. Nickel toxicity is difficult to correct and, whilst mulching with EFB (where available), liming, and additional K fertilizer may help to alleviate the symptoms, it is not recommended to plant oil palm on ultrabasic soils. Some sources of poor quality rock phosphate are also known to contain a large concentration of Ni, and these fertilizer materials should not be used.

Micronutrient toxicities are rare under humid tropical conditions and are usually only found after excessive application of micronutrient fertilizers. Great care must be taken when choosing the application rate for micronutrient fertilizers since the difference between deficiency and toxicity is often quite small. Copper toxicities may develop as a result of excessive and careless fertilizer application, the latter especially from excessive use of Cu-containing fungicides.

Copper toxicity symptoms first appear as small, oval or round, light brown spots on the leaf surface. The spots have depressed centers and may coalesce into extensive necrotic areas with yellow margins on the leaf surface.

Boron toxicity symptoms start on the younger leaves where interveinal chlorotic streaks appear on leaf tips. Chlorosis is rapidly followed by necrosis, developing from the distal to proximal end of leaves. Boron toxicity may be corrected by the application of N fertilizers, which precipitate B and improve palm growth by diluting the concentration of B in palm tissue.

The application of molybdenum to palms as foliar spray resulted in a decrease in yield in young palms on an acid sand soil in Nigeria (Ataga et al ., 1982). Bunch yield was also reduced when palms were treated with a foliar spray of manganese. In both cases, however, no leaf symptoms were reported.

The hyperacidity disorder can be found on palms planted on excessively drained acid sulfate and peat soils. Typical symptoms include the gradual necrosis and desiccation of leaflets on lower fronds, but the spear and young leaves are not affected. Liming with dolomite or BA is not very effective and hyperacidity can be better prevented by maintaining the water-table just above the jarosite layer or 30 cm from the surface, whichever is lower.