Phosphorous deficiency

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Characteristics and occurrence

The phosphorus (P) availability to plants may be limited by its low abundance in the soil, but also, and very commonly, by its adsorption onto various soil minerals. In acidic soils, phosphorus may be adsorbed by iron or aluminium oxides, and various clay minerals. Many of the most fertile and productive soils in tropical zones are derived from volcanic material containing allophane minerals, which have a large phosphorus fixing capacity. Phosphorus deficiency is often the major limitation to crop growth on these soils, particularly where previous cropping has caused a depletion of soil organic matter and increased acidification. Phosphorus deficiency is also common on highly weathered tropical soils and siliceous sands; in fact, few soils are naturally well endowed with this nutrient.

In calcareous soils, phosphorus may be adsorbed by calcium carbonate, or precipitated as calcium phosphate. The ability of sweetpotato to take up phosphorus may also be reduced by high pH.

Sweetpotato is reputed to be relatively tolerant of low soil phosphorus status. A number of fertiliser trials in the USA have shown little or no yield responses of sweetpotato to the application of phosphorus fertilisers. In these trials, it appears that residual phosphorus from previous crops was sufficient to supply the needs of the sweetpotato crop.

Sweetpotato’s efficiency in obtaining soil phosphorus is due in part to its association with vesicular-arbuscular mycorrhizae (VAM). These ubiquitous soil fungi invade the plant roots and feed on its sugars, and in return they assist with the capture of phosphorus from soils with low phosphorus availability. Mycorrhizal infection has been shown to increase sweetpotato growth and yield in a number of studies. The extent of mycorrhizal infection has been positively correlated with yield, and negatively correlated with the crop response to phosphorus fertiliser, over a range of soils in the Highlands of Papua New Guinea. The effect is greatest under low phosphorus fertility, and V

AM may have no benefit or even a negative effect on crops which are well supplied with phosphorus.

Although sweetpotato may yield relatively well under low phosphorus conditions, phosphorus deficiency is still a very common cause of reduced yields. Many cases have been reported where phosphorus fertilisers have dramatically increased yield. It is clear from these examples that wider use of phosphorus fertilisers will play an important role in improving sweetpotato production, as it has for other crops. However, it is also evident that rates of phosphorus fertilisers recommended for other crops may be excessive and wasteful when used on sweetpotato.


Mild to moderate phosphorus deficiency may be difficult to recognise in the field. Growth may be reduced to less than one half that of well nourished plants, without the appearance of any identifiable symptoms of phosphorus deficiency. Mild phosphorus deficiency is often associated with a darker than normal, bluish green colour of the foliage. Unlike nitrogen deficiency, young to mature leaves remain dark green at all levels of severity. In a young crop, only severely stuntedplants will show obvious symptoms on the older leaves. However, on less severely deficient crops symptoms may develop as the crop matures.

The first sign of phosphorus deficiency is usually the premature senescence of older leaves. In most (but not all) cultivars, yellowing is preceded by the appearance of purple anthocyanin pigments, producing a range of autumnal colours in the senescing leaves. Yellowing may spread from discrete interveinal patches, which typically become cleared of anthocyanin pigment, or may be more general, but often affecting one half of the blade more than the other. In this case, the chlorotic areas may appear orange or red due to the overlying anthocyanin pigments. Necrotic lesions may develop in the chlorotic zones, and the necroses spread as irregular patches until the leaf blade is entirely brown and dry. In some cultivars (eg. Markham) no yellow or purple phase precedes the necrotic lesions, which appear on green tissue. However, as in other cultivars, those parts of the leaf blade which are not yet necrotic turn yellow in the final stages of senescence.

Some cultivars may develop purple pigmentation on the upper surface of the youngest leaves, particularly on the veins. This may resemble nitrogen deficiency, although in phosphorus deficiency it is less common among cultivars, and is less strongly veinal.

Possible confusion with other symptoms

Potassium and magnesium deficiencies also cause chlorosis on older leaves, but in phosphorus deficiency chlorosis usually does not retain a distinct interveinal pattern, as is typical of potassium or magnesium deficiencies. The appearance of red pigmentation on the veins of young leaves may resemble nitrogen deficiency. However, in the case of phosphorus deficiency, there is no general chlorosis of the plant.

Sweetpotato feathery mottle virus (SPFMV) may also induce chlorotic spots surrounded by purple tissue on older leaves. Lesions caused by the virus are randomly scattered over the leaf blade, and are not restricted to the oldest leaves on the vine. The symptoms do not progress to cause necrotic lesions or orange and red colours on the senescing leaves, as seen in P deficiency.

Diagnostic soil and plant tissue tests

In solution culture experiments, a critical concentration of 0.22% P has been estimated in the blades of the 7th to 9th youngest leaves. Concentrations between 0.26 and 0.45% P were associated with maximal growth.

Little work has yet been done to calibrate soil tests for predicting yield response of sweetpotato to applied phosphorus. Available phosphorus levels in the range 5 - 7 mg/kg (Olsen’s method) have been suggested as the threshold for deficiency in crops with a low phosphorus requirement, such as sweetpotato. This is supported by a report of positive regressions between available phosphorus and sweetpotato yield, on soils ranging from 0.6 to 5 mg/kg available phosphorus (Olsen’s method), indicating that phosphorus is yield-limiting within this range. Soil solution phosphorus concentrations may also be measured. It has been suggested that 0.01 mg/kg is the minimum soil solution concentration of phosphorus needed to maximize yield of sweetpotato, and a soil solution concentration of 0.003 mg/kg corresponded to a yield 70% of the optimum. It should be noted that these measurements include only part of the phosphorus in soil organic matter, which may represent a considerable proportion of the phosphorus available to the crop in some soils.

Measurements of phosphate binding capacity, in addition to plant-available phosphorus, have been used to estimate the quantity of phosphorus fertiliser required on phosphate-fixing soils. Phosphate binding can be estimated using phosphate sorption isotherms (Fox and Kamprath, 1970), or phosphate retention (Saunders, 1974). These methods have been described by Rayment and Higginson (1992).


Cultural control

Phosphorus deficiency can be corrected by broadcast, band or spot application of soluble phosphorus sources, such as single or triple superphosphate, ammonium phosphate, or mixed fertilisers (containing N, P and K, with or without other nutrients). Band or spot application of phosphorus fertilisers is recommended on strongly P fixing soils.

Rock phosphate is a relatively cheap alternative to more soluble fertilisers. Rock phosphate should be well incorporated into the soil, and is usually only effective on acidic soils, due to its very low solubility at neutral to high pH. Being slow to dissolve, rock phosphate may have a greater residual effect on subsequent crops.

Single superphosphate (10% P) also contains sulfur and calcium, and is recommended in soils where these nutrients are also low. Triple superphosphate (TSP, 24% P) also contains calcium, but not sulfur.

A sweetpotato crop will remove about 8-40 kg P/ha from the soil depending on yield, but on phosphorus-fixing soils, much higher rates of application (>100 kg P/ha) may be needed in the first year of application. In subsequent years, lower rates may suffice to maintain an adequate supply of phosphorus to the crop.

Traditional subsistence methods of cultivating sweetpotato, including the incorporation of substantial amounts of organic matter and using large, healthy runners for planting, add significant amounts of phosphorus, as well as other nutrients. Decomposition of organic matter provides a steady supply of plant-available phosphorus even on highly P-fixing soils. An important part of the response of sweetpotato crops to organic matter additions is due to improved phosphorus nutrition.


Bingham, F.T. 1962. Chemical tests for available phosphorus. Soil Science 94, 87-95.

Bouwkamp, J.C. 1985. Production requirements. In: Bouwkamp, J.C. (ed), Sweet Potato Products: A Natural Resource for the Tropics. CRC Press, Boca Raton, Florida. pp 9-33.

de de Geus, J.G. 1967. Fertilizer Guide for Tropical and Subtropical Farming. Centre d'Etude de l'Azote, Zurich.

Dowling, A.J., Konabe, B. and Tigat, R. 1994. Nutritional assessment of steeply sloping soils from Aiyura in the Eastern Highlands of Papua New Guinea. Papua New Guinea Journal of Agriculture, Forestry and Fisheries, 37(2), 23-29.

Floyd, C.N., Lefroy, R.D.B. and D’Souza, E.J. 1988. Soil fertility and sweet potato production on volcanic ash soils in the highlands of Papua New Guinea. Field Crops Research 19, 1-25.

Fox, R.L. and Kamprath, E.J. 1970. Phosphate sorption isotherms in evaluating the phosphate requirements of soil. Soil Science Society of America Proceedings 34, 902-907.

Fox, R.L., Hashimoto, R.K., Thompson, J.R. and de la Peña, R.S. 1974. Comparative external phosphorus requirements of plants growing in the tropics. Tenth International Congress on Soil Science (Moscow) 4, 232-239.

Goodbody, S. and Humphreys, G.S. 1986. Soil chemical status and the prediction of sweet potato yields. Tropical Agriculture (Trinidad) 63, 209-211.

Halavatau, S., Asher, C.J. and Bell, L.C. 1996. Soil fertility and sweet potato research in Tonga - Nitrogen and Phosphorus. In: Craswell, E.T. Asher, C.J. and O’Sullivan, J.N. (eds.) ACIAR Proceedings No.65: Mineral nutrient disorders of root crops in the Pacific. pp 58-64.

Ila’ava, V.P. 1997. Effects of soil acidity factors on the growth of sweet potato cultivars. Ph.D. thesis, The University of Queensland, Australia.


Jones, A. and Bouwkamp, J.C. 1992. Fifty years of cooperative sweetpotato research 1939-1989. Southern Cooperative Series, Bulletin No. 369, Louisiana Agricultural Experiment Station, Baton Rouge, USA.

Kabeerathumma, S., Mohankumar, B. and Potty, V.P. 1986. Efficacy of rock phosphate as a source of phosphorus to sweet potato. Journal of the Indian Society for Soil Science 34, 806-809.

Kandasamy, D., Palanisamy, D. and Oblisami, G. 1988. Screening of germplasm of sweet potato for VA-mycorrhizal fungal occurrence and response of the crop to the inoculation of VAM fungi and Azospirillum. Journal of Root Crops 14, 37-42.

Khasa, P., Furlan, V. and Fortin, J.A. 1992. Response of some tropical plant species to endomycorrhizal fungi under field conditions. Tropical Agriculture 69, 279-283.

Leonard, O.A., Anderson, W.S. and Gieger, M. 1949. Field studies on the mineral nutrition of the sweetpotato. Proceedings of the American Society for Horticultural Science 53, 387-392.

Mulongoy, K., Callens, A. and Okogun, J.A. 1988. Differences in mycorrhizal infection and P uptake of sweet potato cultivars (Ipomoea batatas L.) during their early growth in three soils. Biology and Fertility of Soils 7, 7-10.

Negeve, J.M. and Roncadori, R.W. 1985. The interaction of vesicular-arbuscular mycorrhizae and soil phosphorus fertility on growth of sweet potato (Ipomoea batatas). Field Crops Research 12, 181-185.

Nishimoto, R.K., Fox, R.L. and Parvin, P.E. 1977. Response of vegetable crops to phosphorus concentration in soil solution. Journal of the American Society for Horticultural Science 102, 705.

O’Sullivan, J.N., Asher, C.J. and Blamey, F.P.C. 1997. Nutrient Disorders of Sweet Potato. ACIAR Monograph No. 48, Australian Centre for International Agricultural Research, Canberra, 136 p.

Paterson, D.R., Taber, RA., Earhard, D.R. and Cushman, K.E. 1987. Response of "Jewel", "Topaz", and "Cordner" Ipomoea batatas cultivars to endomycorrhizal fungi in greenhouse and field experiments. HortScience 22, 1107.

Paula, M.A., Urquiaga, S., Siqueira, J.O. and Dobereiner, J. 1992. Synergistic effects of vesicular-arbuscular mycorrhizal fungi and diazotrophic bacteria on nutrition and growth of sweet potato (Ipomoea batatas). Biology and Fertility of Soils 14, 61-66.

Rayment, G.E. and Higginson, F.R. 1992. Australian laboratory handbook of soil and water chemical methods. Inkata Press, Australia.

Saunders, W.M.H. 1974. Effect of superphosphate topdressing on phosphate and sulphate retention. New Zealand Soil News 22, 15-22.

Tisdale, S.L., Nelson, W.L. and Beaton, J.D. 1993. Soil fertility and fertilizers, 5th Edition. Macmillan, New York. p 189.

Weir, RG. and Cresswell, G.C. 1993. Plant Nutrient Disorders 3. Vegetable Crops. NSW Agriculture; Inkata Press, Australia.


Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



Reduced growth in P-deficient plant (right) without obvious symptoms of disorder.


Purpling and yellowing  of older leaves in a young, stunted plant, but new leaves are dark green (J. O'Sullivan).

Autumn leaf colours and increased flowering are typical signs of P deficiency (C. Asher).

Purpling on older leaves, in this case widely distributed on interveinal tissue (J. O'Sullivan).

Purpling is often concentrated on veins.  Yellowing and necrosis is often asymmetrical (J. O'Sullivan).

In some varieties, P deficiency induces purpling of tips which are normally green (J. O'Sullivan).