Current Status of Greenhouse Tomato Production in Aksu Area of Southern Xinjiang

Chunling WANG Weitang SONG

Abstract With the implementation of the "13th FiveYear Plan" to promote the border area and raise rural living standards， the Xinjiang Production and Construction Corps has paid more and more attention to the development of protected agriculture， which is needed for the comprehensive development and stability of the border groups. In this paper， the current status of greenhouse tomato production in Aksu of southern Xinjiang is analyzed and summarized， finding the following： the structure is not reasonable with poor performance; the soilless cultivation mode is gradually developed， and there are innovative forms; the fertilization structure is not reasonable and needs improvement; the technical level is low， and it needs to be upgraded. Therefore， to solve the various problems that exist at present， some efforts are needed achieve a better production model.

Key words Greenhouse tomato production; Southern Xinjiang; Development

Aksu Prefecture is located in southwestern Xinjiang Uygur Autonomous Region， Peopleюs Republic of China， and Tarim Basin is on the alluvial fan of the Aksu River in the northwestern edge of Aksu Prefecture， making it have a typical warm temperate continental arid climate. The characteristics of rare precipitation， large evaporation， and dry climate make the soil salinization area in this area wide and serious. Soil salinization has become an important factor affecting the sustainable development of agricultural industry in the prefecture， and has become an increasingly prominent ecological environment[1]. The saline soil area accounts for about 55% of the total wasteland area in the prefecture[2]. Some researchers have studied the engineering properties of saline soil in different areas of southern Xinjiang， and found that the degree of mineralization of groundwater was 2 035-5 842 mg/L on the right side of the road sampled from the S207 road in Aksu Prefecture， and 8 060-15 596 mg/L on the left[3]. Zhou et al.[4]studied and evaluated the distribution of medium salinity groundwater in the plain areas of Tarim Basin， and found that the groundwater was mostly medium salinity water with a salinity of 2-5 g/L. When irrigated with the groundwater with a high degree of mineralization， the crops can have the growth affected to some extent[5-8]. Due to poor soil characteristics and production needs， the protected agriculture has developed rapidly in Aksu Prefecture in recent years， especially the production of processed tomato which is developed as the red industry in Xinjiang. By 2012， the planting area of greenhouse tomato in the region had reached more than 3 300 hm2， and tomato had become one of the main vegetable varieties in the local solar greenhouse in winter[9]. The greenhouse tomato production in Aksu is affected by environmental characteristics and production conditions， which has certain specialities and obvious regional characteristics. There are differences in the greenhouse types， planting techniques， variety selection and production and sales with the tomato production in the mainland.

Greenhouse Application Status

Referring to the greenhouse planting in Shandong， Liaoning and other regions， most of the greenhouse types in Aksu give little consideration to the special climate characteristics of the local areas during the construction process， and therefore many problems have arisen during the process of using[10-12]. Extremely low temperatures are likely to occur in winter， and the heatretaining capacity of greenhouses is poor， which makes it difficult to achieve the cultivation during winter without heating. It is common to have strong winds in winter and spring in southern Xinjiang， and the frequency of heavy snow in winter is relatively high， but the greenhouses have poor performances in resisting disasters. Liu et al.[13]studied the lighttemperature characteristics of 3 different types of greenhouses in Aksu， and found that the temperature and transmittance of straw bale greenhouses， which had been widely used， was the lowest with the temperature preservation ratio of 0.91 and minimum temperature at night of only 7.3 °C， which was far lower than the room temperature of greenhouses with brickcement and mud walls. Most of the greenhouses have problems such as insufficient front lighting angle， long rear roof， and unreasonable height of the rear wall[14]. In addition， there is no specific greenhouse construction code for the region. The major structure parameters are different for the greenhouses built by different entities， and the using effects are also different. At present， there are few studies on the construction of greenhouses based on the climatic characteristics and production status， and some only study and evaluate limited types of greenhouses[15-16].

Among the protected agricultural production in Aksu area， tomato is the vegetable variety with the largest production proportion. Tomato is a plant preferring warmth， and the optimal growth temperature is about 25-28 in the day and about 13-15 at night[17-18]. During the winter production in the solar greenhouses in the area， it is difficult to ensure the temperature requirements of tomato growth without heating. Studies have shown that tomato is fond of light. When the light conditions meet the requirements， the number of fruits， total yield and single fruit quality of tomato can increase by 48.91%， 94.86% and 30.86%， respectively， and when the light is insufficient， the growth of tomato will be slowed， and the yield and quality will be reduced[19-22]. Located on the edge of the desert， Aksu area can see frequent occurrences of fine sand flying up in the air in winters and springs， and the severe decrease in the light transmittance in greenhouses is not conductive to the photosynthesis of tomato， which can affect the growth and yield formation of tomatoes and reduce economic benefits.

Tomato Cultivation Modes

Unfavorable conditions such as soil salinization and high degree of mineralization of irrigation water contribute to various problems aroused in the cultivation and production of tomato. After several years of continuous production， the increasing soil salinization slows down the growth of tomato， worsens the pests and diseases， leading to yield decrease. Producers will take measures such as increasing the application amount of fertilizers and pesticides to improve the status quo， but the actual effect is not ideal. On the other hand， it is prone to have tomato cracking， quality decline， and nitrate content increase[24]. In addition， excessive fertilization and soil nutrient imbalance can limit tomato yield[25]. In response to this situation， local farmers have taken other measures during the planting process， which have alleviated the existing adverse consequences to some extent. For example， ditchers are dug along the inner sides or outer sides of the greenhouses to discharge alkali （also for against freezing）. The specific method is as follows： inside the greenhouse， dig a ditch of 30-50 cm deep， 50 cm wide with the same length of the greenhouse at the south end inside the greenhouse. the inside ditch effectively alleviates the phenomenon of alkali and salt returning caused by the great water irrigation and fertilization， and it is also in favor of the moving of excess water along the ridges southwards. The size of the outer ditch is determined according to the types and sizes of greenhouses. In general， the outer ditch is wider and deeper than the ordinary coldproofing ditch. For example， if the rear wall of the rammedearth greenhouse is 150 m in length and 12 m in span， 1 ditch is designed between the front and back greenhouse with the width of about 12 m， length of 150 m and depth of 1.0-1.5 m. In addition to discharging the alkali， the 2 ditches inside and outside the greenhouse can also improve the temperatureretaining effect， but the improvement is not clear when compared with only 1 ditch.

As the drawbacks of soil cultivation gradually emerge， growers in Aksu area gradually transform the tomato planting mode to soilless cultivation. The selection of substrates has become an important task for growers and researchers. The transportation cost of conventional cultivation substrates from the inland is too high and the benefits are reduced. Most growers choose sand as the soilless culture substrates. In Aksu area， sand can be used as a substrate in situ， with low cost and good venting and water permeability. In order to facilitate the operation and reduce the labor intensity， the growers usually mix the sand with the treated slag （cleansed） according to a volume ratio of （2-3）≥1， which is  used as a substrate for tomato soilless cultivation. In order to extend the service life， some growers will also add different proportions of organic fertilizer， which can effectively improve the situation that inorganic matrix can not provide nutrients.

Fertilization

In the process of tomato cultivation， the selection and supply of nutrient solution and fertilizer . In addition to the environment and management means， the effect of nutrient solution is also affected by the water quality， which is the most direct influencing factor. The irrigation water with high mineralization degree in southern Xinjiang makes it impossible to fully exert the efficacies of the fertilizer and nutrient solution， so the crop plants grow slowly， which seriously affects the yield and quality. Zhang et al.[26]analyzed the quality of water for agricultural irrigation in typical areas in southern Xinjiang， founding that there were huge amounts of Na+， Ca2+ as well as HCO3-， Cl- in the well water for irrigation in Korla and Aksu. Crop growth will be seriously affect if using such water sources for a long time.. In view of the characteristics of this water quality， corresponding studies have been carried out on the salt stress on tomatoes[27-30]， which find that the addition of NaCl to the nutrient solution decreases the net photosynthetic rate and transpiration rate of tomato seedlings， and reduces the yield per plant of tomato to some extent. Investigation and measurement make it clear that the degree of mineralization in underground irrigation water sources was high in some tomato planting regions in Alar City， Aksu Prefecture，  and some cation contents are high （Table 1）.

As shown in Table 1， the total salt content of irrigation water was high in the greenhouse in Alar. Apart from Na+， Ca2+， the content of Mg2+ is also high， and the content of Cl- is 4.6 times that of tap water. In addition， the high pH of the water in the area is not conducive to the absorption of nutrients by plants.

Tomato is a calciumloving crop， which requires a large amount of calcium throughout the growth period. The calcium has long transport distance in tomato plants， and poor mobility in the phloem， so it is prone to have calcium deficiency[31-32]. Calcium deficiency at flowering stage could lead to premature senescence of tomato and heavy occurrences of umbilical rot in the fruit[33]. However， the occurrence of umbilical rot could increase with the increase of calcium concentration if the concentration of calcium in the nutrient solution exceeds a certain range[34]. In addition， it also may be related to other ions in the nutrient solution. Due to the antagonism of the cation， the ammonium nitrogen， potassium and magnesium in the nutrient solution reduce the calcium absorption of the root system[35-37]. Tomatoes in southern Xinjiang also suffer from calcium deficiency and umbilical rot， which may be caused by the positive ion antagonism of high concentration of Na+ and Ca2+ in nutrient solution， leading to calcium deficiency in tomato. Therefore， large amounts of Na+ and Ca2+ could restrict and affect the growth and yield formation of tomatoes. There are some studies on the effects of Ca2+ and Na+ stress on tomato roots and photosynthesis， but there are few studies on the effects of the 2 factors on tomato growth and yield formation. At present， the fertilizers selected by growers in Aksu are commercially available fertilizers for tomato cultivation in most areas， and there are few special fertilizers developed specifically to the local water quality characteristics. Thus， it is very necessary and urgent to find a set of tomato nutrient solution and fertilizer formula suitable for the water quality in Aksu area in southern Xinjiang.

Production Management Level

The main planting mode of greenhouse tomato is scattered family cultivation in Aksu area， which have the characteristics of small planting area， low production technology level and lagged management concept. In recent years， departments of all levels have strongly supported and encouraged farmers to establish farmer cooperative organizations in all aspects. Although there are a certain number of cooperatives registered， the scale of operations is relatively small and the driving ability is relatively weak[38]. Some growers are from outside Xinjiang， such as Henan， Shandong， Hebei and other regions. With certain planting experience， these people rent greenhouses for production， and bring the existing planting techniques in the mainland to the local area. In the production process， these growers could make timely judgments and find the corresponding solutions to some conventional production problems. Thus， the greenhouses under the management of these growers have good tomato growth vigor， high quality and yield and relatively better benefits. On the other hand， there are some greenhouses planted by the local residents. Most of these growers have not received specialized skills training and learning， and lack planting experience. There are many problems in the greenhouse managed by these growers. For example， it is common to have excessive application of fertilizers and pesticides during the conventional management process， and the pests and diseases cannot be noticed and treated in time. Such circumstances lead to low greenhouse tomato yield and benefits.

In addition to the input of basic production materials such as fertilizers， pesticides and mulching films， the low management levels are also reflected in the low mechanization of greenhouse tomato production in Aksu area. The different sizes of solar greenhouses， different cultivation modes and different management means make it less likely to adopt machines for production. At present， in addition to the soil preparation， mechanization has also being developed in the processing links such as ditching， ridging， mulching， transporting and harvesting in some of the greenhouse tomato production areas in mainland[39-40]. In addition to the above reasons， the unwillingness of the producers to purchase agricultural machinery also contributes to the low utilization rate of machines. Most of the production is completed by humans， including the picking， harvesting and transporting， which have high labor intensities. The labor costs are relatively lower than the mainland， and the adaptability of machines is not high enough， which make the producers prefer manual operations. Therefore， in order to improve production efficiency and reduce the labor intensity of workers， further exploration and research are needed for the mechanization of tomato production in the region.

Conclusion

Due to the social， economic and technical limits， the greenhouse tomato production in Aksu is still at the beginning stage. Efforts are needed in many ways to improve the current situation. For example， the standardized construction of greenhouses is necessary， the environmental regulation capacity within the greenhouses should be strengthened; nutrient solution formula and fertilizer formula should be developed to suit the characteristics of local water quality and soil properties， to reduce the investment in such aspect; training courses should be offered to enhance the technical level of growers; the construction of agricultural cooperative organizations should be stepped up， so that production and sales can be systemized. All these require the joint efforts of relevant units， managers and frontline producers， so as to increase the yield and income of farmers.

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