For other quality parameters, requirements depend on the intended use. One of the best examples for these variable parameters is starch content: If the potato is intended for use by the potato starch industry, its starch content should be as high as possible. In ware potatoes, however, excessive starch content will lead to a loss of desirable waxiness. Especially on loess-loam soils, the variety-typical starch content for ware potatoes is often exceeded.
If on the other hand the starch content is too low, the potato’s taste will be impaired, as starch is an important flavour carrier. The optimum starch content may be achieved by applying potassium. Fertilisation with potassium sulphate will increase the tuber’s dry substance content, and will at the same time ensure a desirable starch-protein ratio (12:1 to 16:1), which will result in a hearty, pleasant taste. Particularly direct marketers should not neglect taste as a quality feature.
Potato quality requires balanced fertilisation
High tuber quality is based on a balanced supply of nutrients. In particular, unbalanced nitrogen fertilisation needs to be avoided. Important quality characteristics such as starch content and the content of other important components do not benefit from nitrogen, or only minimally so, as these characteristics are often variety-specific. Excessive or late N-applications, however, may delay tuber formation. This will also result in excessive vegetative or canopy growth, making potatoes more susceptible to leaf blight. This will in turn delay the natural ripening process and will produce potatoes of varying degrees of maturity. These tubers will also be more susceptible to damage during the harvesting process.
Magnesium, phosphorus, calcium, sulphur as well as trace nutrients boron and manganese influence many of the potatoes’ quality characteristics, as shown by the following chart. Sulphur in its sulphatic form may, for instance, inhibit scabbing. Potassium fertilisation is particularly important for potato quality.
Effect of nutrient supply on important quality characteristics in potatoes
|
Quality characteristics
|
N )1
|
P
|
K
|
Mg
|
Ca )5
|
S
|
Boron
|
Mn
|
|
Core characteristics
|
Tuber yield
Starch-content (dry-matter content)
Protein content
|
+++
−
++
|
++
+
++
|
++
+/−)2
0/+
|
++
+
0
|
+
+
0
|
+
+
+
|
+
0
+
|
+
0
+
|
|
Content of
|
Ascorbic acid
Citric acid
Reducing sugars
Sucrose
Nitrate
|
0
0
+
−−
−
|
+
0
0
++
+
|
+++
++)4
++
−
−/+)3
|
0
0
+
|
0
0
|
+
|
0
|
0
|
|
Resilience
against
|
Tuber injury
Blue/black spots IM
Raw // red/brown colouration
Cooking blackening
Hollow heart / brown centres
Glassiness
Internal rust spots / bitter pits
|
+/−)3
−
−−
−−
−−
−−
0
|
+
0
−
0
|
+
+++
+++
++)4
+
−
++
|
0
+
0
0
+
+
+
|
0
0
0
0
|
|
+
0
0
0
|
0
0
0
0
|
|
Other parameters
|
Skin firmness and maturity
Sorting
Tuber deformation
Storage qualities
Taste
|
−−−
−
−−
−
−
|
++
+++
0
++
0
|
0
0
0
+
++)4
|
0
0
0
+
0/+
|
0
0
++
++
0/+
|
+++
|
+
+
0
|
0
+
0
|
)1 negative effects of nitrogen seem limited to one-sided N-oversupply and late N-application ( such as by late applications of liquid manure or N-foliar-fertiliser)
)2 both a lack and excess of K impair starch storage in potatoes; positive results may be achieved mainly by sulphatic potassium
)3 the effect is greatly influenced by the tubers’ stage of maturity
)4 a positive effect is mainly achieved by using the sulphatic form of potassium
)5 existing influence may be triggered by Ca or by pH-effect
Increased nutrient supply will result in:
| + |
a tendency to positive effect |
− |
a tendency to negative effect |
| ++ |
positive effect |
−− |
negative effect |
| +++ |
significant positive effect |
−−− |
significant negative effect |
| 0 |
no influence |
|
|
Blank spaces: no information currently available
Sources: Kolbe, H. (1995 und 1997); van Loon (1991); Neubauer et. al. (1984), supplemented by Kudelja and Wirsing (1976), published by G. Pienz (1998); Birkmann (1990); agridea Datenblätter Ackerbau (2010); Orlovius (1996, 2006 and 2007)
The outstanding role of potassium
Potassium is the nutrient with the greatest influence on the inner and outer quality of potatoes. For most quality characteristics affected by potassium, the absolute amount i.e. potassium supply play a greater role than the nutrient form; this is the case for blue and black spots. There are however a few quality parameters (starch content, citric acid content, cooking blackening, taste), which are best, handled by applying potassium in its sulphatic form, such as Patentkali® (also refer to Potato Fertilisation – Potassium Suphate or Potassium chloride?).
A sufficient supply of potassium also reduces undesirable reducing sugars and the potatoes’ tendency to discolouration. This is mainly due to the fact that an increase of potassium raises the tubers’ content of citric acid and of vitamin C.
Peeled potatoes may turn grey after cooking (so-called cooking blackening). This may also occur in processed products (chips and crisps) following pre-baking, and is caused by a chemical reaction between an organic acid (chlorogenic acid) and iron. This is where potassium will help: it will on the one hand decrease the contents of undesirable chlorogenic acid, while at the same time increasing the content of citric acid, which bonds with iron.
Indirect influence on other quality characteristics
In addition, several nutrients such as nitrogen, potassium, magnesium, sulphur and calcium indirectly influence external quality characteristics. These are mainly scurf, wireworm damage, wet and dry rot as well as susceptibility to mechanically induced damage. Fertilisation will also directly influence mineral content – yet another, internal quality criterion. One of the reasons why potatoes are considered a valuable contribution to human nutrition is their naturally storage capacity for potassium, calcium, magnesium and phosphorus – provided, these minerals are supplied in sufficient amounts.
It is however necessary to take into consideration that the influence of nutrients on various quality characteristics also depends on other factors, such as mutual reactions between nutrients and between biotic and abiotic factors. Of these, the most influential may be considered weather conditions and variety.
Additional information
Potato Fertilisation – Potassium Sulphate or Potassium Chloride?
Nutrient deficiency symptoms in potatoes – the ABC of deficiency symptoms
Potato cultivation
Nutrient potassium
