(Indicative Annual Yield = 6-8 t ha-1) (see also Chapter 7) Visible symptoms of zinc deficiency in rice include:
wilting due to loss of turgidity in the leaves, basal chlorosis of the leaves, delayed development of the plants,
"bronzing" of the leaves and, in some ases, death of the rice seedlings (5). Neue et al. (6) stated that the common symptoms of zinc deficiency in rice are: the midrib at the base of the youngest leaf of zinc deficient rice becomes
chlorotic 2-4 weeks after sowing or transplanting. Then brown spots appear on older leaves. The spots enlarge, coalesce and give the leaves a brown colour. Zinc-deficient plants show stunted growth and reduced tillering. If the deficiency is not too severe the plants may recover after 4-6 weeks but maturity is delayed and yields of
susceptible cultivars are reduced (6).
In California, it is reported that symptoms of zinc deficiency in rice are usually observed after a flood is established. The most noticeable symptom is the plant’s loss of turgidity, where the plant falls over and floats on the surface of the water. The basal leaves become pale green and after 3-7 days the leaves become chlorotic.
However, although the symptoms of zinc deficiency in rice are most noticeable after flooding, more subtle symptoms can be observed before flooding. The symptoms do not normally appear in seedling rice until around 10 days after emergence but may possibly take several weeks to appear. These include: basal leaf chlorosis (beginning in the youngest leaf), loss of turgidity in the leaves and floating on the surface, bronzing (red-brown splotches on the surfaces of the oldest leaves, bronzed leaf tissue may eventually turn brown) and stacking of leaf sheaths or joints (leaf collars do not open out) (7).
It is important to note that visual symptoms of zinc deficiency in rice vary, to a certain extent, with: soil type, cultivar and growth stage. Symptoms can be mistaken for those of deficiencies of nitrogen, magnesium, manganese or iron, or a virus disease called ‘tungro’. Sulphur deficiency is often combined with zinc deficiency and it is difficult to distinguish between the symptoms of the two and so plant analysis is required for confirmation.
Treatment: Several alternatives are currently practiced: In the Philippines, it is the normal practice to dip seedlings in 2% zinc oxide solution (2 g ZnO/100 ml water) before transplanting, or broadcast 10-20 kg ZnSO4 ha-1 into the flooded field after the first irrigation (8).
In Arkansas the usual treatment is 11.2 kg Zn ha-1 as zinc sulphate or 1.12 kg Zn ha-1 as a Zn-chelate.
In California, farmers are advised to make pre-flood applications of 0.9-7.2 kg Zn ha-1 (2-16 lbs Zn) from whatever the source (usually ZnSO4.7H2O). For maximum Fig. 3.1
Typical symptoms of interveinal chlorosis in zinc deficient rice plants
chelate is broadcast or sprayed on the soil surface after the last seedbed tillage. For rice, the zinc treatment should never be incorporated into the soil (9).
Mikkelson and Brandon (10) recommended broadcasting 9 kg Zn ha-1 as either zinc sulphate or oxide before flooding. In Missouri, when rice growing soils contain less than 1 mg kg-1 DTPA-extractable Zn, 5.6 kg Zn ha-1 is applied. However, experiments have shown that soaking rice in a proprietary zinc compound could be used for water seeded rice production (5). In India, 25 kg ZnSO4 ha-1 is generally applied at puddling (where the previous crop was shown to exhibit symptoms of deficiency) followed by the same amount again to the current crop if symptoms of zinc deficiency appear again. If the symptoms of zinc deficiency appear even after the application of the recommended dose of zinc sulphate, as in the case of sodic and flood plain soils, 25 kg ZnSO4 should be applied mixed with an equal amount of dry soil (as a diluent) to the affected area.
3.5.1.2 Wheat (Triticum aestivum L - bread wheat, Triticum durum Desf - durum wheat)
(Indicative Annual Yield in tropics = 4-6 t ha-1) (see also Chapter 7)
Durum wheats tend to be more sensitive to zinc deficiency than bread wheats but varieties of both types differ
considerably in their ability to tolerate low levels of available zinc (zinc efficiency).
Zinc deficiency in wheat reduces grain yield and nutritional quality. Symptoms appear first on young leaves as zinc is relatively immobile under conditions of deficiency. Light green to white chlorotic and necrotic streaks developed on either side of the leaf mid-rib are characteristic of mild deficiency in wheat. Where the deficiency is more severe, the lower leaves tend to be totally chlorotic and short, but of normal width. Sometimes they have an oil soaked appearance. As the necrosis proceeds the leaves often collapse in the middle (4). Leaf mid-ribs and margins tend to remain green but, in some cases, leaf edges appear to be tinted red or brown.
The leaves remain small, cup upward and develop interveinal chlorosis. On the upper leaf surface necrotic spots appear which later join each other to form brown necrotic and brittle patches. The necrosis is often more noticeable on middle-aged leaves which eventually wilt, bend and collapse. Zinc deficiency in fields is typically patchy and the symptoms can develop rapidly but depend on the degree of stress.
Treatment of deficiencies with zinc fertilisers or foliar sprays can increase yields and also improve the plant’s resistance to ‘foot rot’ fungus (F. graminarum) (12). Treatment: Application rates of zinc fertilisers vary in different parts of the world. In Australia, wheat and other cereals are normally treated with 0.6-2.4 kg Zn ha-1 on light soils and 1.8-3.9 kg Zn ha-1on heavier soils (13). Fig. 3.2
Yara-Phosyn Chlorosis and necrotic spots on the leaves of zinc deficient wheat.
Fig. 3.3
Interveinal chlorosis and necrotic patches on leaves of wheat.
Various authors recommend applications of zinc sulphate in the range 2.2-11.2 kg Zn ha-1 depending on the soil conditions, crop variety and local agronomic factors.
Recent recommendations for the treatment of zinc deficiency in wheat in India are 50 kg ha-1 zinc sulphate for acute cases and 25 kg ha-1 for moderate
deficiencies (11).
3.5.1.3 Maize (Zea mays L) (also called ‘Corn’) (Indicative Annual Yield = 6-9 t ha-1)
(see also Chapter 7)
Maize can be grown on a wide variety of soils over a pH range of 5.0-8.0 but pH 6-7 is optimal. Maize has medium salinity tolerance(3).
Symptoms of zinc deficiency in maize appear as a yellow striping of the leaves.
Maize is highly susceptible to zinc deficiency. Areas of leaf near the stalk may develop a general white to yellow chlorosis (‘whitebud’). In cases of severe deficiency, the plants are stunted due to shortened internodes and the lower leaves show a reddish or yellowish streak about one third of the way from the leaf margin. Plants growing in dark sandy or organic soils usually show brown or purple nodal tissues when the stalk is split. This is particularly noticeable in the lower nodes (14).
Treatment:
In India, if symptoms of zinc deficiency were noticed in the preceding maize crop, or where a soil test indicates zinc deficiency, 25 kg ZnSO4 ha-1 is usually broadcast at sowing. As with rice, if ZnSO4 is to be applied to a current crop after the appearance of deficiency symptoms, 25 kg ha-1 of zinc sulphate should be applied, mixed with an equal quantity of dry soil as a diluent, along the rows and hoed in before irrigating the field. When the symptoms are observed late in the growing season and inter-row cultivation is not possible, a spray application of neutralized 0.5% ZnSO4 solution is recommended (11). In the USA and other areas with more intensive agriculture, zinc treatments for maize range from 2.2 to 34 kg Zn ha-1 broadcast (as zinc sulphate) and 1.1-4.5 kg Zn ha-1 banded (as zinc sulphate) or 0.6-3.3 kg Zn ha-1 banded (as ZnEDTA)(15).
3.5.1.4 Barley (Hordeum vulgaris L.)
With zinc deficiency, the leaves show uniform chlorosis, dry-up and tip growth decreases (4).
Treatment: The same as for wheat crops.
3.5.1.5 Sorghum (Soghum bicolor L.) (Indicative Annual Yield = 3-5 t grain ha-1) Sorghum, like maize, is highly susceptible to zinc
deficiency and the symptoms in grain sorghum are similar to those in maize, but less pronounced. Although not quite so susceptible as maize, zinc deficiency retards development and maturation of the seed heads in sorghum (4).
Treatment: Widespread deficiencies of zinc in sorghum growing on cut soils (topsoil removed) in Kansas (USA) have been treated with zinc sulphate at the rate of 11.2-16.8 kg Zn ha-1(10-15 lb Zn /acre) broadcast over the field. On calcareous soils in Texas and Western Australia, the deficiency in sorghum was treated with 0.7-2.24 kg
Fig. 3.4
International Plant Nutrition Institute Maize plants showing interveinal chlorosis and
“white bud” symptoms.
Zn ha-1 (0.6-2 lb Zn /acre) as either zinc sulphate or zinc oxide. In Colorado and Nebraska soil application of zinc sulphate at 5.6-22.4 kg Zn ha-1 (5-20 lb Zn/acre) or using a zinc chelate at 0.56-1.12 kg Zn ha-1 (0.5 lb Zn/acre) is used. In Texas, 11.2-33.6 kg ZnSO4 ha-1 or 2.24-6.7 kg ha-
1 (2-6 lb/acre) of zinc chelate is recommended(4). 3.5.1.6 Oats (Avena sativa L.)
The leaves become pale green; older leaves show collapsed areas at the margins and tips are greyish in colour. Necrosis extends down the leaf and the remainder of the leaf is grey to bronze-green (4).
Treatment: Deficiencies in oats have been widespread in Western Australia and were treated with zinc oxide applications to the soil: 0.7-2.7 kg ha-1 (0.6-2.4 lb/acre) zinc oxide on lateritic sandy soils, 1.34 kg ha-1 zinc oxide (1.2 lb/acre) on non-calcareous sandy loams, and 3.36 kg ha-1 zinc oxide (3 lb/acre) on calcareous soils (4).