Fertilisation

 

Potatoes have many uses, from fresh markets to processed fried products, starch, seed, and more. Each type of potato has specific nutrient requirements, which are affected by the location and climate. To achieve higher yields, the potato plant needs high amounts of nitrogen (N) and potassium (K), but other macro and micronutrients are also necessary. The quality of the tubers depends mostly on the supply of potassium (K), calcium (Ca), and magnesium (Mg), but other macronutrients like nitrogen (N), phosphorus (P), and sulphur (S), and micronutrients like iron (Fe), zinc (Zn), boron (B), and manganese (Mn) also play essential roles in plant metabolism and quality. The nutrient supply affects the starch content and cooking type, undesirable discolourations, internal disorders, tuber defects, and glycoalkaloid content of fresh-market potatoes. For processing into fried products, the nutrients’ contribution to high dry matter contents and low concentrations of reducing sugars and free amino acids must be considered above all. Important functions of macronutrients (black circle) and micronutrients (grey circle)

 

Macronutrients

Table 4.2: Recommended macronutrient rates per ha per growth phase.

Phase	Days from planting	kg/ha/phase
		N	P2O5	K2O	CaO	MgO
Planting	1	1	0	1	0	0
Vegetative growth	2-40	61	21	95	2	2
Tuber initiation and bulking	41-80	126	43	194	3	4
Maturation	81-130	118	41	182	3

 

A. Nitrogen

Nitrogen (N) is one of the most important macronutrients for plant growth, biomass production, and yield and quality formation. This is because N is a vital component of chlorophyll, proteins, amino acids, nucleic acids, and coenzymes. Different forms of N are available for plants, such as molecular N₂ (not available for potatoes), volatile ammonia (NH₃), nitrogen oxides (NO_x), amino acids, and mineral N in the forms of nitrate (NO₃^–) and ammonium (NH₄⁺). The most important forms of N for plant nutrition under agricultural conditions are NO₃^– and NH₄⁺. NO₃^– is the most common form of N in aerated soils, whereas NH₄⁺ is more prevalent in slightly acidic or anaerobic environments. Additionally, NO₃^– plays a crucial role in the modulation of root system architecture.

Potato yield largely depends on adequate N fertilisation, although excessive fertilisation (300 kg/ha) may have a negative impact on tuber dry weight (DW) and tuber yield. Additionally, N fertilisation positively impacts the yield and proportion of large tubers (70 mm) but has a negative impact on starch content.

 Nitrogen deficiency in potato plants may cause not only chlorosis but also growth retardation resulting in significantly reduced yields

B. Phosphorus

C. Potassium

Potassium (K) plays a vital role in potato cultivation, as it is the most crucial macronutrient required for the growth of potatoes. It has a significant impact on both the yield and quality of the crop. Potassium is found in the highest concentration in tubers and leaves and is absorbed by the plant roots through diffusion. The soil must provide potassium in an exchangeable form for it to be available for the plant. The central role of K in achieving the desired tuber and starch yields in potato plants is due to its roles in enzyme regulation, photosynthesis, and carbohydrate allocation within the plant.

Potassium also plays a significant role in enhancing crop yield, quality, and tuber size, while reducing the incidence of diseases. Soil that is deficient in potassium can further deteriorate due to various factors, such as low pH (acidic soils), leaching (sandy or light soils), drought conditions, high rainfall, heavy irrigation, heavy clay (illite) soils, low K reserves, and magnesium-rich soils.

The recommended rate of potassium application ranges between 60 and 360 kg K₂O/ha, and the K is broadcast applied before planting. When calculating fertiliser rates, the end-products, or final utilisation of the potatoes, should also be considered. For growing fresh-market potatoes, K rates should be applied that are about 10-20% above the recommendations for the particular site. For starch potatoes, K rates are recommended that are 30-40% below the recommended fertiliser rate. It is recommended to use fertilisers free of chloride for potatoes.

 

D. Calcium

Calcium plays a crucial role in the growth of potato plants, especially in stabilising cell walls and membranes, acting as a counteraction for organic and inorganic anions in the vacuole, and serving as a second messenger in the processes of intracellular signal transduction. The plant transports Ca through the xylem, which is closely related to transpiration rates. However, Ca is not mobile in the phloem, which means it cannot be transferred from older to younger leaves that have a low transpiration rate. As the potato tuber has low transpiration, it is considered one of the Ca-deficient organs.

Calcium also influences soil pH levels and is usually added in the form of lime. Potato plants can tolerate low soil pH, but adding lime can improve the uptake of K and Mg at pH values below 5.0. At pH levels in the low alkaline range, however, the addition of lime may lead to common scab (Streptomyces spp.) development, making it difficult to suggest an ideal pH level for potato production. Soil acidification may result in the deficiency of essential cations like Ca and Mg, due to antagonistic and inhibited uptake, caused, for instance, by Mn. Hence, liming with CaCO₃ or CaO in acidic soil conditions can enhance Ca availability, neutralise soil pH, and decrease the potential for Mn toxicity in the plant.

 

E. Magnesium

Magnesium (Mg) plays a critical role in plant metabolism. It is essential for various functions such as photosynthesis, carbohydrate partitioning, enzyme regulation, and protein synthesis. Plants absorb Mg through mass flow from the soil solution, and its uptake can be affected by pH and high concentrations of other cations like hydrogen, potassium (K), calcium (Ca), and manganese (Mn). Mg is also a central atom of chlorophyll, making it crucial for photosynthesis.

However, competition among ions during uptake can occur and lead to antagonistic interactions between cations, such as K⁺ and Mg²⁺. One of the most frequently observed phenomena is potassium-induced Mg deficiency. Despite the antagonistic effects, it is not necessary to avoid supplying K and Mg together to prevent such effects on uptake.

 The most common symptoms of magnesium deficiency are chlorosis and deformation of the leaves

 

Micronutrients

Iron, manganese, zinc, and boron are essential for the normal development and growth of potatoes. If one or more of these become deficient, visual deficiency symptoms will appear on the leaves. Deficiencies must be corrected early in the growing season to prevent any further yield losses. Micronutrients are not normally recommended in standard fertilising programmes since there are generally enough micronutrients available in the soil. In the case of plants showing deficiency symptoms, foliar sprays will be sufficient to rectify the deficiency.

Describing various micronutrient deficiencies in potatoes.

Nutrient deficiency	Description	Image
Iron (Fe)	Iron deficiency is marked by the pale green and yellow colour of the youngest leaves, while the veins remain darker green (interveinal chlorosis).
Manganese (Mn)	Deficiency includes interveinal chlorosis, development of dark lesions on leaves and leaf deformation often confused with late light (view image on the right).
Zinc (Zn)	Zinc deficiency may cause interveinal chlorosis and tip-burn in younger leaves. Whitish spots can develop within the brown necrotic tissue. Leaves may also be deformed and cupped upwards (see image on the right) and the shoot may have stunted growth.
Copper (Cu)	Copper deficiency causes permanent wilting of plants. Leaves may appear dried out without precursory signs of chlorosis.