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.

Why Nitrogen is it important?

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.

Symptoms

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.

How to manage

• Apply N fertilizer efficiently.
• Do not apply large amounts of N to less responsive varieties.
  Hybrid rice absorbs mineral N more efficiently than inbred rice varieties.
• Choose a suitable plant spacing for each cultivar. Crops with suboptimal plant densities do not use fertilizer N efficiently.
• Adjust the number of splits and timing of N applications according to the crop establishment method. Transplanted and direct-seeded rice require different N management strategies.
• Maintain proper water control, i.e., keep the field flooded to prevent denitrification but avoid N losses from water runoff over bunds immediately following fertilizer application.
• Establish a dense, healthy rice crop by using high-quality seed of a high-yielding variety with multiple pest resistance and a suitable plant density.
• Control weeds that compete with rice for N.

Where possible,

• Correct deficiencies of other nutrients (P, Potassium, and Zinc) and solve other soil problems (shallow rooting depth, toxicities).
• Over the long term, maintain or increase the supply of N from indigenous sources through proper organic matter management:
  • Apply available organic materials (farmyard manure, crop residues, compost) on soils containing a small amount of organic matter, particularly in rainfed lowland rice and intensive irrigated rice systems where rice is rotated with other upland crops such as wheat or maize.
  • In irrigated rice-rice systems, carry out dry, shallow tillage (5−10 cm) within two weeks of harvest. Early tillage enhances soil oxidation and crop residue decomposition during fallow and increases N availability up to the vegetative growth phase of the succeeding rice crop.
  • Increase the indigenous N-supplying power of permanently submerged soils by periodic drainage and drying.