Optimising Potato (Solanum tuberosum L.) Cultivation by Selection of Proper Soils

Growing potato demands considerable external inputs of pesticides due to its susceptibility to various pests and pathogens. Here we present an attempt to differentiate the Slovak rural landscape with respect to the possibility of effective potato cultivation and to characterise soil parameters of current potato cultivation areas with the aim to increase the sustainability of the potato production. The selection was based on soil climatic, production and economic parameters. By using the GIS tools and existing databases on soil characteristics in Slovakia, maps of soil suitability categories for potato cultivation were generated. In Slovakia, it was found that 12.3% of farmland is very suitable for potato cultivation and that as much as 43.1% is not suitable. Later the specified categories were characterised in detail and specified with respect to geographic, soil, climatic, production and economic parameters. Currently, most potato crops are cultivated on Eutric Cambisols (27%), Chernozems (20%) and Mollic Fluvisols (18%). Loamy soils (65%), soils without gravel (62%), deep soils (74%) and soil situated on plains (55%) are dominant in these regions. We suggest that potato cultivation should be concentrated on the most suitable areas, thereby increasing the economic profitability, improving the ecological stability of the country and supporting the sustainability of the agriculture.


Introduction
There is an urgent need for decreasing external inputs such as fungicides when cultivating potato. Originating from South America where after the Spanish colonization it was first spread to Europe but later globally. Currently (2017), potato cultivation takes about 19.3 million hectares, the world global production is 388 million tons and mean hectare yield is on the level 20.1 tons (FAOSTAT, 2018). The potato consumption is about 35 kg per capita per year. The trend is that the area decreases whilst the production per hectare increases [1]. With an intensified production the need for external inputs such as mainly fungicides increases resulting in a less sustainable production. In Slovakia, the situation follows the global trend. The development of potato production areas has steadily decreased ( Figure 1). Currently, potato cultivation is, taking up 8,000 hectares compared to 200,000 ha fifty years ago [2]. The decreasing trend is a result of changing consumer behaviour usages [3]. While in 1991, the average potato consumption per capita was 90.8 kg, now (2017) it is only 48.9 kg [4].
Simultaneously, the yields per hectare are gradually increasing following the global tendency, although with variations between individual years. Over the last 66 years, the lowest harvest (5.04 t ha-1) was recorded in 1964 and the highest (20.94 t ha-1) in 2011. The increased yields per ha have been followed also by a steadily increasing use of pesticides [5]. Currently, the average pesticides consumption in Slovakia is 1.08 kg per hectare, while the consumption in EU countries is 3.71 kg per ha. The highest pesticides consumption is reached in Benelux countries (11.5 kg ha -1 ).
The potato is versatile and can be grown in almost any soil type except saline and alkaline soils. Naturally loose soils, which offer little resistance for tuber enlargement, are preferred. Loamy and sandy loam soils rich in organic matter, with good drainage and aeration, are considered the most suitable ones [6]. Ideal soil porosity is about 50%, with a water capacity between 30 and 40% (vol.) and an air capacity of about 15% (vol.). The optimum pH value for potato cultivation ranges between 4.7 and 6.2, while acidic soil is preferred over alkaline soil. Soils originated from weathered granites, flysh sandstones, phyllites and andesites are optimal for potato growing.
Given this, our hypothesis is that it will be possible to grow potato in a sustainable way by properly matching soil and climatic conditions. Here we present an attempt to select optimal soil and regions for potato cultivation in Slovakia by identifying suitable soils by multicriterial GIS analysis using analytical overlapping of data representing soil properties [7].

Materials and Methods
An analysis of hectares yields, costs, revenues and profits of chosen farmers in the period 1990-2017 in Slovakia, were executed using data from 120 agricultural farms, which were cultivated the potato and were able to provide us the requested data. In the following, data obtained directly from farmers was assigned to a given point value reflecting the soil production potential of a specific subject (agricultural farm). Point value of soils of individual farms was calculated based as the average of the point values of the occurring soil units (on the base of soil and ecological evaluation) [8], as follows:  1950 1955 1960 1965 1970 1975 1980 1985 1990 1995  The values of the climatic region (TP) were determined according to the long-term average sum of the temperature in respective climatic regions (Tab. 1). Point values (PV) of production potential were analysed for their correlation in the form of nonlinearpolynomial regression in relation to real yields (Y), costs (C), sales (S) and revenues (R) (average value for 1990-2017 per one hectare). The following statistical parameters were determined by this way: PV↔C: y = -0.1435x2 + 23.122x + 1218.5 (r = 0.4147, n = 109) PV↔S: y = 0.014x2 + 13.209x + 679.67 (r = 0.5242, n = 110) PV↔R: y = 0.1619x2 + 15.33x + 883.29 (r = 0.7533, n = 117) where y = yield, cost, sales, revenues, x = soil point value, r = coefficient of correlation and n = number of paired values.
The regression equations were used to assign potential possible yields of potato for each soil unit (considering the point value) as well as potential economic parameters: costs, sales, profits or losses ( Table 2). Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 5 February 2020 doi:10.20944/preprints202002.0050.v1 Profit is given as the difference between revenues and costs and profitability rate is the share of profit and cost expressed in%.
The specification of soil suitability for potato cultivation (from very suitable to unsuitable) was determined by uniform dividing the point scale of the soil production potential (one hundredpoint scale) into four categories as follows: -unsuitable soils: 25 points and less -suitable soils: 26-50 points -less suitable soils: 51 -75 points -very suitable soils: more than 75 points.
Thereafter, a final map of spatial distribution of particular categories (Figure 2) was created by using ArcGIS software based on vector maps in scale of 1:5000, which graphically identify soil point diffusion (Soil Science and Conservation Research Institute database in Bratislava, Slovakia). Analytical overlay of this map layer with layers represented data of soil type, soil texture, climatic region or geomorphological parameters allowed create soil-ecological characteristic of individual categories (regions). Their characteristics are mentioned in the part Results and Discussion. Except these, the characteristics of potatoes cultivation suitability is added with the productive categories of cultivated lands [8] and integrated index of agricultural soils quality of Slovakia [7].  (Table 3) divides agricultural soils of Slovakia into types of their rational use (type O -typical arable lands, type OT -arable land or grasslands, type Ttypical permanent grassland, while type T does not occur in potato cultivation). When determining the soil quality index, the methodology according to Vilček and Koco (2018) [7] was used. According to its, the quality of soils is the result of the assessment of their production potential, environmental potential and potential threat (erosion, compaction, organic matter content).
Integrated indices are defined as follow: very high-quality soils -index 1, high-quality soilsindex 2, medium-quality soils -index 3, low-quality soils -index 4, very low-quality soils -index 5. Identification of potato cultivation areas in the years 2011-2015 was processed on the basis of information of farmers who demanded subsidies by means of the LPIS (Land Parcel Identification System). The LPIS is a system based on aerial or satellite photographs recording all agricultural parcels in the member states of the European Union. A unique number is given to each land parcel to provide a unique identification in space and time. We linked the statistical cultivation data with spatial data using this unique identification and selected all parcels where potato was cultivated during the 2011-2015 period. Description of soil properties in selected LPIS parcels where potato was cultivated was obtained using analytical overlay with relevant spatial databases and spatial identifications of various soil parameters. These databases for the whole Slovakia territory are managed by Soil Science and Conservation Research Institute in Bratislava; spatial analysis where performed using ArcGIS 10.3 software environment.

Results and Discussion
The above mentioned methodological processes and pedological data enabled us to create a regionalization of the suitability of soils (rural areas) for growing potatoes in Slovakia. Individual regions are characterized and specified on the basis of production, economic and soil-ecological parameters.
The spatial analysis of current potato cultivation areas was created as well.

Region of very suitable soils
This region covers about 12.3% of the agricultural soils in Slovakia. In this region the dominant soil types are Chernozems, Mollic Fluvisols, Haplic Luvisols and Fluvisols, which are moderately heavy, deep and without soil gravel. They can be found in the warm to very warm, dry to very dry climatic regions with continental climate. Highly productive to productive arable soils were included in this region (76-100 soil points), with a potato production potential greater than 21tons per hectare and a share of crop rotation on arable soil of up to 7%. According to the production soil categorisation, this region includes the six most productive soil categories (O1-O6). It is possible to obtain a profit of over €800 per hectare and a profitability rate higher than 37% for potato cultivation. This region is extremely suitable for early and semi-early potato varieties. Agricultural soils evaluated according to five scale index of soil quality [7] reach 1.79 points, which means they belong to the category of very high to high quality soils.

Region of suitable soils
This region covers about 32.7% of agricultural soils. The dominant soil types are Cambisols, Fluvisols, Haplic Luvisols, Dystric Planosols and Regosols, with a soil point value between 51 and 75 points. The soils are mostly moderately heavy to heavy, without soil gravel or slightly gravelly and deep, situated on plains and slight slopes. This area is represented by the climatic regions 05-10 (relatively warm and dry to very cold and moist). Potato production potential is 15-21 tons per hectare and its share of crop rotation on arable soil should be up to 9%. According to the Slovak production soil categorisation, this area includes very productive arable soils to less productive arable soils (O3-O6). It is possible to obtain a profit of €60-800 per hectare and a profitability rate of 7-37% for potato cultivation. Agricultural soils evaluated according to five scale index of quality [7] reach 2.14 points, e.g. they belong to the category of high quality soils.

Region of less suitable soils
This region covers about 12.9% of all agricultural soils, with the dominant soil types Chernozems, Molic Fluvisols and Haplic Luvisols, with a soil point value between 26 and 50 points. The soils are moderately heavy, deep, without gravel, and situated on plains. Less suitability for cultivation of potato is result of the predominance of the very warm and very dry climatic regions 00-01 that occupy 98% of the soils. There are moderately heavy, deep soils on variably steep slopes. Potato yields reach 12-15 tons per hectare. According to the Slovak production soil categorisation, the area includes the medium productive arable soils to poorly productive fields (O5-OT3). The profit is under €60 per hectare, with a potato cultivation profitability rate of 1%. Agricultural soils Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 5 February 2020 doi:10.20944/preprints202002.0050.v1 evaluated according to five scale index of quality [7] reach 2.83 points, which puts them into the category of medium quality soils.

Region of unsuitable soils
Soils that are not suitable for potato cultivation make up about 42.1% of agricultural soils in Slovakia. They occur mostly in the northern and central regions of the country, as well as on heavy to very heavy soils of the Eastern Slovak lowland. From a soil science perspective, there is a great variety of soil types, including light, extremely heavy, acid and loose soils with unfavourable physical and chemical characteristics. Cambizems, Rendzics, Fluvisols and Gleysols are the predominant soil types here. In terms of grain size, moderately heavy, strongly gravelly, shallow soils, occurring on steep and very steep hills, are predominant. The climatic regions 07-10 are dominant. Potato yields do not exceed 12 tons per hectare in the long term, so it should not occur in the crop rotation. These soils are mostly suitable for permanent grass overgrowth. In terms of economic values, potato cultivation will incur losses in these areas. Agricultural soils evaluated according to five scale index of quality [7] reach 3.65 points, it means they belong to the category of low quality soils. The spatial locations of the above mentioned regions are presented in Figure 2.

Distribution of potato cultivation at present time
Due to the biological, but also agronomic requirements of potato, the selection of suitable sites is based on climatic, geomorphological and soil conditions. In terms of climate regionalization of Slovakia, 26% of the potato cultivation areas are located in a very warm and very dry region. In contrast, almost 14% of the areas are located in a very cold and wet region.
When comparing the different categories of soil suitability for potato cultivation (potential) with real cultivation areas (Figure 3), 12% of the selected areas have very good conditions for potato cultivation, 33% are suitable, 13% less suitable, and up to 42% are not suitable for potato cultivation. The cultivation of potato on less suitable and unsuitable soils inevitably requires increases input, such as irrigation, erosion protection, fertilisation, etc., which is reflected in the economic outcome. An analysis of various soil parameters on the production capacity of potato under Slovak conditions [2] showed that the yield potential on slight slopes (to 7°) was 8.3% lower, while on moderately steep slopes (7-12°), it was 15.9% lower when compared with cultivation on plains. Soils with strongly eroded soils obtained 16.6% lower yields. DeFauw et al. [9] reviewed the effect of erosion on the yield of potato by using GIS tools in Maine (USA). They stated that close to 85 % of potato production soils are classified as either "potentially highly erodible" or "highly erodible" and they required the highest standards in soil conservation practices.
The potential for negative effects on soil quality and erosion at potato cultivation is significant also due to levels of low plant residues input to the soil following potatoes, and the effect of tillage on soil structure, particularly in sandy-textured soils. Similar results have been achieved also by Moulin et al. [10]. Zarzyńska and Pietraszko [11] report about the impact of unfavourable weather conditions results in greater losses in crop value.
For potatoes, therefore, it is necessary to choose structural soils with a good stock of organic matter without the threat of water erosion in favourable climatic regions. The choice of suitable soils brings not only economic, but also ecological benefits. If potato were cultivated only in regions very suitable for this crop (higher yield assumption), it would be possible to decrease the growing area by about 2000 hectares. Reducing pesticides consumption (by about 25%) would be another environmental benefit. A good choice of cultivation area of agricultural plants (not only potato) is an ecological stabilizing factor for agrarian landscape. Vilcek and Torma [12,13] defined the areas for winter wheat and sugar beet cultivation using similar methods and process. Based on their calculations, the correct location of winter wheat could save 153,000 hectares of land and save 165,000 tons of pesticides (saving 19.8%). In the case of sugar beet this would be 4,700 ha of land and 5.1 thousand tons of pesticides (saving 24.1%), respectively.

Conclusions
The natural and soil ecological conditions of Slovakia represent suitable conditions for potato cultivation. However, heterogeneous soil climatic factors do not allow efficient and long term sustainable cultivation of this crop in all regions. Based on the results of this study, decreased potato cultivation in Slovakia is related to inadequate selection of cultivation sites, resulting in low yields and poor economic profitability. To maintain and intensify potato production in Slovakia, it will be necessary to pay particular attention to the selection of appropriate cultivation areas. Current potato cultivation areas in Slovakia do not guarantee sustainability of this crop in the long term. We suggest that potato cultivation should be concentrated on the most suitable areas, thereby increasing the economic profitability, improving the ecological stability of the country and supporting the sustainability of the agriculture.