Nitrogen deficiency

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Characteristics and occurrence

Nitrogen (N) is one of the most abundant elements in plants and animals, as it is a major component of proteins. The amount of nitrogen required by a crop is large compared with the natural nitrogen reserves in most soils, and so most crops respond positively to additional nitrogen, whether from animal manures or inorganic fertilisers such as urea. However, this pattern does not always hold for sweetpotato. In some studies, nitrogen application has been reported to reduce sweetpotato yields. More commonly, the pattern is for low rates of nitrogen to increase yield to some extent, but higher rates to cause a yield decline.

The reason for this confusing response is that nitrogen supply has a strong influence on the distribution of dry matter within the plant, particularly affecting root growth relative to top growth. When nitrogen supply is high, plants tend to grow more tops relative to roots. In the case of sweetpotato, high nitrogen may cause luxuriant growth of the vines at the expense of storage root yield.

Cultivars vary greatly in the level of nitrogen required to maximise yield, and in their tendency to reduce yield at higher levels of nitrogen. In particular, negative responses to nitrogen are more common in cultivars developed in low-fertility areas where soil amendments are not traditionally used. One study reported that an application of 60 kg nitrogen/ha increased yields of three USA cultivars but decreased the yields of three African cultivars.

There is a common belief in developing countries that nitrogen fertilisers are bad for sweetpotato. This is unfortunate because nitrogen deficiency is very common. Large responses to nitrogen are often obtained on soils which have been heavily cropped in the past, or those subject to heavy leaching. The response to nitrogen may be poor, however, if deficiencies of other nutrients such as potassium are overlooked and left untreated. Sweetpotato tends to respond better to composts of plant materials which contain high potassium relative to nitrogen, than to animal manures, which are lower in potassium. However, this depends on the balance of nutrients present in the soil.

Improved nitrogen nutrition of the crop also leads to higher protein concentrations in the storage roots, and this may be of considerable significance in communities which obtain much of their protein from sweetpotato. In the Kaintiba District of Papua New Guinea, for instance, the mean protein concentration in storage roots sampled, at 0.62%, was less than half the average for the South Pacific region, suggesting that these crops were nitrogen deficient. The protein intake of these Papua New Guinea Highlanders is typically suboptimal unless supplemented by imported foodstuffs.

Sweetpotato has been shown to form root associations with the nitrogen fixing soil bacterium Azospirillum brasilense. Inoculation of crops with the bacterium has been shown to increase the root yield and the nitrogen concentration in leaf tissue when no nitrogen fertiliser was applied. The prevalence of such symbiotic associations in the various regions and environments in which sweetpotato is grown, and its significance for yield of subsistence crops, have not yet been investigated.


Deficiency of nitrogen causes dramatic reductions in growth of sweetpotato plants, and yet it is not easily recognised in the field, unless there is a well-fertilized crop for comparison. General symptoms are a uniform light green chlorosis of the leaves, and slow growth resulting in a delayed or sparse ground cover.

The development of nitrogen deficiency symptoms vary according to conditions experienced by the crop. When nitrogen is initially adequate during the establishment phase but becomes depleted during crop growth, plants may appear normal or near normal in colour and habit, except for yellowing and premature shedding of older leaves due to remobilisation of nitrogen from these tissues. In this case, the oldest leaves become uniformly yellow and slightly wilted. A light brown necrosis may spread from the tip or margins, but often the leaf is shed before it develops extensive necrosis. Necrotic tissue is supple rather than brittle.

Alternatively, if nitrogen supply is low throughout the growth of the crop, no senescence of older leaves may be evident. Symptoms of chronic nitrogen deficiency include uniformly pale colour, reduced leaf size, loss of the normal sheen resulting in a dull appearance of the leaves, thin spindly vines and reduced activity of axillary buds leading to less branching. In severe cases, small purple-pigmented flecks or ringspots have been observed on the surface of older leaves of some cultivars.

Increased anthocyanin pigmentation of the young leaves and especially the leaf veins is a noticeable symptom of nitrogen deficiency, which has been observed on all cultivars studied. However, it is not unique to nitrogen deficiency, since phosphorus- or sulfur-deficient plants may show a similar symptom. This symptom is observed on plants suffering both types of nitrogen deficiency described above. In cultivars in which young leaves are normally pigmented, the purple colour is deepened and is retained for longer in the veins, whereas the leaves of healthy plants change colour uniformly from purple to green. In cultivars which normally display little or no anthocyanin pigment, veins of the young leaves become red or purple. In some cultivars, the pigment may be most obvious on the upper surface of the leaves; in others, it may be almost absent from the upper surface but distinct on the lower leaf surface. The red pigmentation usually also extends to the petiole and stem.

Possible confusion with other symptoms

Red veins on young leaves and yellowing of older leaves may also indicate phosphorus deficiency. However, most leaves on phosphorus-deficient plants remain dark green. Purple pigmentation on the older leaves before senescence is seen in some cultivars suffering phosphorus deficiency but not nitrogen deficiency.

Sulfur deficiency also induces a general chlorosis of the plant, and may be difficult to distinguish from nitrogen deficiency. Sulfur deficiency is indicated if the chlorosis is greater on the young leaves than on the old, if leaf veins are paler than the interveinal tissue, or if red-purple pigmentation is at least as great on the oldest leaves as on the youngest.

Diagnostic soil and plant tissue tests

A critical concentration of 4.2% N has been determined for the 7th to 9th youngest leaves of sweetpotato, on the basis of vine growth in solution culture. This agrees well with data from several field studies, when leaves were sampled early in the season (8-10 weeks). However, at least for some cultivars, the nitrogen concentration in leaves tends to decline with plant age, and it is likely that the critical concentration varies with the age of the crop. A critical concentration in the range 3.0-3.8% N may apply to a more mature crop.

The concentration of nitrate-N in the petiole is often used to assess the nitrogen status of crops. One study quoted nitrate-N concentrations in the petiole of the sixth leaf of sweetpotato in mid season, as 1500, 2500 and 3500 mg/kg for deficient, intermediate and sufficient nitrogen status, respectively. However, another found that, while petiole nitrate-N concentration was sensitive to nitrogen supply, it was highly variable among cultivars and with age of sampling, and was a poor predictor of root yield.

In the field, fertiliser test strips may be used to confirm a deficiency of nitrogen, and to distinguish nitrogen from sulfur deficiency. For example, urea (containing only nitrogen) and sulfate of ammonia (containing both nitrogen and sulfur) can be applied to separate strips of one or two ridges, at a rate of about 50 kg N/ha (eg. 1 kg N for a 2 m x 10 m strip). If plants green up equally with either sulfate of ammonia or urea, it is likely that they were suffering from nitrogen deficiency; if only the sulfate of ammonia produces a response, the problem is most likely sulfur deficiency.

Soil nitrogen measurements are difficult to interpret, as they donít distinguish between sources of nitrogen with differing availability to plants. As a rough indication, concentrations below 0.1% N (Kjeldahl method) are regarded as very low, while concentrations of 0.5-1.0% N may be adequate for maximum crop growth. As an alternative to total soil nitrogen, soil nitrate-N can be measured. On a sandy-loam with moderate organic matter content, a critical soil nitrate-N concentration for sweetpotato was estimated to be 37 mg N/kg soil, sampled 29 days after transplanting.


Cultural control

A sweetpotato crop of 20 t/ha removes approximately 87 kg N/ha, if roots and vines are harvested. The optimum rate of fertilisation will depend on the amount of plant-available nitrogen in the soil, and on yield potential, which may be dictated by the available soil water and rainfall. Reported recommendations for application of nitrogen fertilisers to sweetpotato generally lie between 30 and 90 kg N/ha (De Geus, 1967). Overfertilisation with nitrogen may lead to reduced yields, as it may encourage excessive vine growth at the expense of the storage roots.

Organic mulches of various materials including foliage from leguminous trees and animal manure may be used to provide nitrogen to the crop. Nitrogen also accumulates in organic material during a fallow period, particularly if the fallow is dominated by leguminous species. This becomes available to the subsequent crop gradually as the organic material decomposes. Alternatively, burning the fallow residue rapidly releases the nitrogen from the organic matter, but much of this is lost to the atmosphere as nitrogen oxides (NO2, N2O) during the burn. The remaining nitrate is readily lost through leaching and denitrification, and the long-term effect of burning is to reduce the supply of nitrogen and other nutrients, as well as degrading soil physical properties. Nitrogen supply can be increased by growing a leguminous crop, such as peanut in rotation with sweetpotato, but only if the crop residues remain in the field.

Nitrogen deficiency is sometimes associated with waterlogging of the soil. Under anaerobic conditions, soil bacteria quickly convert soil nitrate into nitrogen gas (N2), which is lost to the atmosphere. Improved drainage can be achieved by increasing the height of ridges or mounds, keeping the troughs between them clear of weeds, and providing adequate channels for excess water to leave the field. Sweetpotato is sensitive to waterlogging, and crops tend to yield poorly if waterlogging occurs for even a short period, particularly in the early stages of crop growth. Therefore, good drainage is necessary regardless of nitrogen supply.


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Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



Small, pale, dull nitrogen-deficient leaf (left) compared with a healthy leaf (J. O'Sullivan).


General light green colour and sparse canopy, compared with N-fertilized crop behind.


Yellowing, wilting and drying up of oldest leaves, and purple veins on young leaves are common symptoms (J. Low).


Typically oldest leaves turn completely yellow before necrosis spreads.  Numerous yellow leaves on a young crop are indicative of N-deficiency (J. O'Sullivan).


A crop with chronic N deficiency, showing small, dull leaves and sparse ground cover, but few yellow leaves (J. O'Sullivan).


Small purple ringspots on mature leaves is occasionally seen (J. O'Sullivan)


Red or purple pigmentation of young leaves, especially veins and petioles, is a common response to N deficiency, and may be apparent on upper or lower surface (J. O'Sullivan).


Small light green leaves and red veins and petioles on a young N-deficient plant (J. O'Sullivan).