Phosphate and potatoes

New Zealand's commercial potato growers face a wide variety of challenges in their day-to-day business. Whether growing for the export or domestic market, processed or fresh, growers need to meet stringent guidelines in order to get maximum value for their crop.

According to VegFed, the retail and export value of the New Zealand potato industry to year-end on 30 June 2004 was $422 million. Of this, nearly half was for the local processing industries and just under 20 percent for export. The export market has grown dramatically over the past ten years — in 1994 export earnings were $10.6 million; in 2004 they were nearly $80 million. Yet the total land area used for potato production has not increased by anywhere near the same amount — going from 9,405 ha in 1994 to 11,717 ha in 2004.

Part of the increase in export earnings can be accounted for by improved prices for these products, but there must also have been heightened productivity by growers. This will have been fuelled by increasingly scientific crop management practices, including plant nutrition.

While potato variety, soil type, soil pH, water and canopy management and crop protection all play a part in determining the yield and quality characteristics of a potato crop, there is no doubt that plant nutrition is also essential. The provision of adequate levels of all nutrients — at the time they are required and in a form that is available for plant uptake — influences tuber size, number and quality. Increasing these three characteristics is one of the keys to improving production without expanding the area under cultivation.

 Nutrient Main functions in potato crops
 Nitrogen Fuels growth and high yields
 Phosphorus Promotes early shoot and root development; needed for optimum tuber initiation and set
 Potassium Controls plant water flow; allows plants to tolerate frost stress
 Calcium Builds strong cell walls resistant to disease
 Magnesium Essential for photosynthesis
 Sulphur Helps reduce levels of common and powdery scab

Table 1: Key functions of the macronutrients in potatoes

The benefits of phosphorus

When looking at nutrient uptake and removal by potatoes, it is clear that potassium is the most abundant nutrient involved in potato growth. Data from Holland showed that in excess of 6 kg of potassium is removed with every tonne of potato tubers harvested. The second most prominent nutrient is nitrogen (4 kg removed in every tonne of tubers), followed by magnesium. By contrast, only 0.5 kg phosphate is typically removed in each tonne of tubers harvested.

This relatively low level of nutrient removal belies the importance of phosphate to the yield potential of potato crops. An adequate supply of phosphate is essential for early root and shoot development. The more vigour displayed by a plant during this early growth stage, the more yield potential increases, unless environmental conditions conspire against it.

Phosphate is also important at the initiation of tuber formation. The reasons for this are not entirely clear, but are likely related to the increased enzyme activity that occurs in the apical region of the stolon at this time.

The importance of potatoes to the worldwide human diet means that considerable funds have been put towards the study of many aspects of its growth, including tuber formation. Such studies frequently involve analysis on a molecular level, examining the roles of various enzymes and plant hormones and the factors that influence their activity.

What factors influence tuber initiation?

Some of the factors that drive tuber initiation have long been elucidated, but others remain uncertain. However, there is definitely a combination of exogenous and endogenous factors involved. Long nights, cool temperatures and low levels of nitrogen in the soil all encourage the development of tubers. Inside the plant, falling levels of gibberellic acid mark the onset of tuber initiation. High levels of cytokinins have also been reported to increase the formation of tubers, though the effect is not as clear as that of gibberellic acid. Two other phytohormones — tuberonic acid and jasmonic acid — have also been investigated recently. Both are thought to play a role in tuber initiation and early development, high levels stimulating tuber set, though their exact mode of action is not quite clear yet. On the other hand, high levels of phytochromes inhibit tuber formation.

Energy the key

Running all this enzyme action and activating growth systems requires energy. In plants, as in animals, the compound that is responsible for delivering energy at a molecular level is called adenosine triphosphate, or ATP for short. ATP contains three phosphate molecules, and the release of each of these generates a certain amount of energy that is used to drive cellular processes. Removing one phosphate molecule generates energy plus ADP (adenosine diphosphate); removing a second phosphate molecule generates energy plus AMP (adenosine monophosphate).

When the phosphate molecules are removed from ATP (or ADP) they are usually transferred to another compound — an enzyme or a sugar, for instance. The cell then has to remake the ATP, by attaching more phosphate to it.

It's clear, then, that in order for cell growth and differentiation to occur, the plant must have an adequate supply of phosphate. It's not just required for ATP, either — phosphate has many different uses in cells. If insufficient phosphate is taken up by the plant, then growth in general will be hindered.

P is for potatoes

The question facing growers is, how much phosphate is sufficient? Will residual phosphate in the soil meet early growth requirements? Is it worth building up the soil Olsen P so that crops have a large phosphate reserve to draw on? Do foliar phosphate applications offer any benefit, or is it sufficient to use solid fertiliser only?

Not surprisingly, over the years there has been a substantial amount of research into the effects of various plant nutrients on potato growth and yield. However, some of the results obtained may not be relevant today, as changes in horticultural practices mean that parameters used are no longer valid. For instance, in the UK, a lot of early research was done on clay soils, but today commercial potato crops are grown on lighter soils with a lower clay content. The fate of phosphate in these soils is different, so results obtained in one system cannot necessarily be extrapolated to other systems. Likewise, irrigation is common today; it tends to increase availability of soil phosphate to plants by desorbing it from soil surfaces.

Nonetheless, recent research reports note that there is a positive correlation between the amount of applied phosphate and tuber yield. US researchers Rosen and McNearney conducted a two-year field study examining the impact of fertiliser phosphate on potato crops grown in a loamy sand.¹ They found no difference between phosphate source (having examined only mono-ammonium phosphate and di-ammonium phosphate) but they did show that adding up to 70 kg P/ha tended to increase the total tuber yield of Russet Burbanks, and that it also increased tuber number per plant.

Similarly, Maier et al. found that phosphate application resulted in significantly greater tuber yields at 16 out of the 33 sites tested in the main potato growing areas of South Australia.² Where loamy sand and clay loam soils were phosphate deficient, Maier's team showed that to achieve 95 percent of the maximum yield, it was necessary to apply 48-73 kg P/ha, banded at planting. For coarse-grain sand soils, applying 27-59 kg P/ha banded at planting was necessary if 95 percent of maximum yield was to be attained.

Allison et al. recently addressed the issue of phosphate levels as applied to the UK potato industry.³ An earlier review had suggested that in the UK, recommended application rates were high and Allison queried whether these high rates were actually producing production gains.

In a series of experiments, the team showed that it was possible to get a yield increase by adding fertiliser phosphate only if the Olsen P was less than 26. Similarly, the number of tubers on a plant increased with fertiliser phosphate only if the Olsen P was less than 16. However, Allison's experiments contained one potential flaw, in that the fertiliser was broadcast, rather than banded. Banding — with a spacing of 10 cm to the side and 10 cm down from the seed on low CEC (cation exchange capacity) soils, and 7 cm out and 7 cm down on high CEC soils — helps ensure that phosphate is within easy reach of plant roots, facilitating uptake by the plant. By broadcasting phosphate fertiliser, Allison et al. reduced the actual effect of the applied nutrient on potato plants, so the potential for a strong response to eventuate was also lowered. In addition, the yields achieved in Allison's experiments were lower than those of many commercial growers. In the six sites that showed a significant yield response, applying 90 kg P/ha resulted in yields of 44.0 t/ha (Record), 21.2 (Pentland Dell), 42.3, 68.1, 36.5 and 35.2 t/ha (all Estima). In New Zealand, maincrop potato yields are more typically between 60 and 80 t/ha.

Regardless of the method of application, Allison's improvements in yield and tuber number appeared to be related to an increase in ground cover. Given that tuber initiation occurs two to three weeks after 50% emergence and lasts for two to seven days, it is easy to see that for phosphate fertiliser to have an effect it must be present relatively early the plant's lifecycle.

For this reason, Allison proposed that foliar applications of phosphate are likely to be of little help enhancing tuber initiation, since at the crucial time the canopy is small and of variable area - meaning that much of the applied foliar phosphate is likely to end up on the soil.

Humic acid

In Idaho, much of the potato-growing industry uses soils that are both relatively alkaline (pH 8.0 to 8.2) and low in organic matter (1.1 to 1.3%). Organic matter plays many important roles in soil-plant interactions, including raising the water- and nutrient-holding capacity of the soil. Soils low in organic matter, therefore, are likely to contain low reserves of phosphorus. Bryan Hopkins and Jeff Stark conducted an interesting experiment to determine whether this could be remedied by the application of one particular fraction of organic matter — humic acid.4 They discovered that banding humic acid along with phosphate fertiliser increased total yield by 18 cwt/acre (330 kg/ha) and yields of US No 1 tubers by 22 cwt/acre (404 kg/ha), compared to not applying the humic acid. The mode of action of humic acid is likely to be multifaceted, but increased phosphate retention in the root zone is undoubtedly a key factor in these results.

This work — and the work of Allison et al. —underlies the importance of ensuring that applied phosphate is actually available to plants. How much phosphate fertiliser needs to be applied to produce the optimal result from a potato crop depends in part on how much residual phosphate is already in the soil. Timing of application is also clearly important, as sufficient phosphate needs to be available to drive optimum tuber initiation and canopy development.

Soil type, cultivar, end use of the crop, and a host of cultivation practices all interact to determine the optimum phosphate levels for a potato crop - every grower needs to find the ideal condition for their crop, balancing maximising yield and economic benefit with minimising environmental impact.

References

1. Rosen, C. and McNearney, M. (2003), Potato yield and tuber set as affected by phosphorus fertilization,
2. Maier, N.A., Potocky-Pacay, K.A., Jacka, J.M. and Williams, C.M.J. (2001), Effect of phosphorus fertiliser on the yield of potato tubers (Solanum tuberosum L.) and the prediction of tuber yield response by soil analysis,
3. Allison, M.F., Fowler, J.H. and Allen, E.J. (2001), Effects of soil- and foliar-applied phosphorus fertilizers on the potato (Solanum tuberosum) crop, Journal of Agricultural Science, 137: 379-395.
4. Hopkins, B. and Stark, J. (2003), Humic acid effects on potato response to phosphorus, Idaho Potato Conference, January 22-23.

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