Fertilization management
The search of an optimal productivity and quality of patatoes involves, among other things, a good nutritional management of plants.
Deficiencies in macronutrients, which are required in large quantities or in micronutrients can reduce the growth of the vegetative system (haulm and underground organs) and the length of the growing cycle, and hence can reduce tuber growth and crop productivity. Some deficiencies (e.g. potassium) increase tuber susceptibility to damage such as internal blackspot.
Conversely, excessive application of fertilisers can also upset the nutritional balance and reduce tuber productivity.
Good nutritional management is required for the quality and the productivity of the potato crop and it must be adapted to the crop’s characteristics. These include the soil type, the type of production (seed, table or processed potatoes) and the expected yield.
The quality of fertilizer distribution in addition of the soil preparation and water management are important for the nutrient availability to the plants. This is particularly true for nutrients which are not very mobile in the soil. These factors are often predominant as severe direct nutrient deficiencies are quite rare in potato production.
Nutritional management of potato crops includes an adequate supply of fertilizers with macro and micronutrients, according to the nutrients availability in the soil, the soil type and the crop requirement. It is also related to a good soil structure, favourable to an efficient root system, to absorbable forms for optimal nutrient uptake and to chemical balance between the different nutrients. Acid or basic soils are more susceptible to nutrient deficiences.
Macronutrient
The availability of mineral elements may cause symptoms on potato plants that are associated with either a deficiency or an excess (phytotoxicity) of certain nutrients.
Symptoms of deficiency are observed chiefly in filtering or sandy soils, which are prone to severe leaching, or in certain countries where crops are grown in pure sand and are fertigated.
The deficiency may be due to an insufficient supply of certain nutrients or because their availability is reduced by another factor (pH, chemical balance between elements, etc.).
The symptoms described in this chapter are given as a guideline and the confirmation of a deficiency or of an excess of nutrients usually requires further plant and/or soil analysis in a laboratory.
Nitrogen deficiency or toxicity
- Role in the plant : nitrogen (N) is involved in the production of proteins, nucleic acids and in the photosynthesis. A sufficient supply of nitrogen is therefore important during the early stage of leaf formation and haulm growth but also later during tuber bulking. Conversely, an excessive application of nitrogen can inhibit tuber initiation, reduce the yield and limit the dry matter content. At the end of the vegetative period, an over-supply of nitrogen can delay the maturation of the tubers and favours skin peeling, which in turn can impair the quality of the tubers and their storage ability.
- Symptoms of deficiency: in the case of nitrogen deficiency, the colour of the foliage is lighter. In severe shortage, leaves are pale yellow or chlorotic (photos 1 to 3) and the leaf mass is reduced (photo 2). As nitrogen is a mobile nutrient, symptoms of deficiency usually concern the whole plant.
- Excess nitrogen delays maturity and favours the development of the canopy. A high haulm volume increases the humidity around the basal parts of the plants and may enhance the plant susceptibility to certain diseases such as late blight, blackleg or white mould.
Phosphorus deficiency
- Role in the plant: phosphorus (P) is a structural element (proteins, nucleic acids…) and provides energy for plant processes such as ion uptake and transport. An adequate supply of phosphorus is needed for good plant growth and tuber formation and a lack of phosphorus can reduce the number of tubers. This nutrient is not very mobile in the soil and deficiencies may occur even when soils are rich in phosphorus in the case of poor root systems or reduced nutrient uptake due to various factors such as bad soil structure, drought, low temperatures, or when diseases damage the root development. In soils with a high pH level (above 7,5-8), e.g. in some volcanic soils, the availability of phosphorus can be highly reduced, even when high amounts of fertilisers are present in the soil.
Good phosphate nutrition is beneficial for the formation of thick and resistant skin on tubers as well as for starch quality (viscosity).
- Symptoms of deficiency: the deficiency is initially manifested by an elongation of the leaflets, sometimes accompanied by slight deformation of the leaf margins, followed by brownish spots and a dull, dark green appearance of the leaf blade (photos 4 and 5). Severe deficiencies can lead to stunted plants with thin stems and smaller upwards curling leaves.
Potassium deficiency
- Role in the plant: the potato needs a large amount of potassium (K), as this element is essential for metabolic functions such as the transfer of leaf sugars to the tubers and the conversion of the sugars into starch. It is also important for the osmotic potential of cells and related water balance. Potassium deficiencies adversely affect the yield, the size and the quality of the tubers. A lack of potassium also reduces the dry matter content of the tubers. Potassium deficiencies also lessen the plant’s resistance to biological stresses (diseases or pests) and to climatic conditions such as drought and cold.
- Symptoms of deficiency: this deficiency is manifested firstly on the older leaves because potassium is highly mobile inside the plants. Symptoms begin with discoloured then necrotic brown spots firstly at the leaf margins, then spreading across the leaf blade within the veins. Leaves become deep veined, deformed and with a dark-green colour and shiny and metallic aspect (“bronzing”) (photos 6 to 8).
The availability of potassium is essential to the quality of the tubers, particularly regarding their resistance to mechanical damage like internal black spot bruising.
Eléments secondaires et microéléments (micronutriments)
The main secondary elements or micronutrients include magnesium, calcium, iron, manganese, boron and sulphur. Although they are needed by the plants in much lower amounts than macronutrients, adequate supplies of these nutrients to the plants are required for quality crop production.
The availability of these nutrients in the soil depends on the pedoclimatic environment. For example, zinc is a not very mobile element and is concentrated in the soil’s organic matter close to the surface of the soil. Therefore, cool and humid weather reduces the availability of zinc and may cause deficiency.
The availability of micronutrients usually diminishes with a soil pH of over 7. Consequently, soils with a high pH level are more likely to suffer from a deficiency. Similarly, sandy soils are more likely to suffer from a deficiency than clay soils.
The main symptoms of deficiency in micro elements nutrients are described on the page opposite.
The use of micronutritional elements must be justified (by soil and plant tissue analysis or from meticulous visual examination of the plants) in order to avoid any extra or unnecessary cost, as well as limiting the possible toxic effects to the environment and adverse negative interactions with other elements. The choice of an effective method of application depends on the deficient micronutrient, local soil conditions and the phenological stage at which the deficiency is detected.
Boron deficiency
- Role in the plant: boron regulates the transport of sugars throughout the membranes and also plays a key role in cell wall formation and cell division and for the metabolism of auxins.
- Symptoms of deficiency: a typical symptom consists of the death or delayed development of the growing buds on foliage or tubers. The terminal bunches of the plants turn yellow and necrotise (photo 9). The plants have a bushy appearance and the top leaflets are thickened and may curl upwards (photos 10 and 11). The plants may have a slow emergence and foliar development or may even be stunted, because of the shortening of the internodes. In severe cases, yield reduction may occur.
Toxicity: strong leaf deformation, similar to that caused by hormones, may be induced by severe boron toxicity.
Calcium deficiency
- Role in the plant: calcium is an essential constituent of the cell walls contributing to their structural rigidity.
- Symptoms of deficiency: the young leaves and the leaf tips become chlorotic, smaller and slightly curled (photo 12). In the case of severe calcium deficiency, plants may be stunted. As calcium is not very mobile in the plant, symptoms are typically located on the apical parts of the leaves, stems and sprouts. Necrotic and black tips on the sprouts (photo 13) as well as tuber internal rust spot are sometimes related to a lack of calcium availability.
Iron deficiency
- Role in the plant: iron is essential to plant respiration, electron transfer and the photosynthesis reaction.
- Symptoms of deficiency: young shoots and leaves are pale yellow or even whitish because the chlorophyll formation is impaired, however the veins remain green and marked (photos 14 and 15). In the case of severe deficiency, plants are stunted and all the top leaves may be chlorotic. Iron deficiency is rare, except in some calcareous, high pH or clay soils where soil iron may be fixated, therefore becoming less available for the plant.
Magnesium deficiency
- Role in the plant: magnesium is an essential element for the process of photosynthesis (because it forms part of the chlorophyll molecule) and is also involved in protein production. Magnesium is therefore essential to plant growth, particularly during tuber bulking.
- Symptoms of deficiency: the bottom leaves first present mild interveinal, pale-green to yellow chlorosis (photos 16 and 17) which subsequently turn brown and finally become necrotic (photos 18 to 20). Such lesions are not unusual on potato plants, during the last part of the growing season, particularly on some cultivars. Lesions are typically located on the central part of the leaflets, except on the veins of the older leaves (as magnesium is highly mobile in the plant).
Manganese deficiency
- Role in the plant: manganese is important chiefly for the process of photosynthesis and as a metabolic activator.
- Symptoms of deficiency: the young upper potato leaves have a dull green colour and show interveinal chlorosis with many small necrotic dots, localised mainly along the veins (photos 21 and 22). The plants are smaller, with necrotic areas on leaf margins (photo 22).
Toxicity: excess of manganese, particularly in acidic soils, may cause the formation of pale green leaves and numerous necrotic spots on stems, petioles and can cover the entire leaf surface.
Sulphur deficiency
- Role in the plant: sulphur is a constituent of certain amino acids and proteins.
- Symptoms of deficiency: there is an overall chlorosis i.e. yellowing of the foliage (photos 23 to 25) which can be confused with nitrogen deficiency. Deficiencies are usually observed during the early stages of crop development but later, the upper leaves may become paler than the bottom ones and show necrotic spots in the central parts of the leaflets. Lack of sulphur encourages early maturing.
Salinity phytotoxicity
The potato is a highly salt susceptible crop and damage due to excessive salinity occurs in certain warm regions, where crops are irrigated with saline water and/or where high temperatures cause major evaporation and lead to the presence of high quantities of mineral salts on the soil’s surface. Excessive salinity, i.e. concentration of salts, in irrigation water or in the soil may be phytotoxic for some crops such as the potato and so reduces the plant’s growth and yield (photo 26).