Currently, China has become the world’s most important producer and exporter of tomato products, the third largest producer and the first exporter after the United States and the European Union. According to incomplete statistics, the area of continuous glass greenhouses in China has reached 630 hm2, and the area of glass greenhouses is still expanding. Equipped with modern equipment, the greenhouse can adjust the temperature, humidity, CO2 concentration, light and nutrient solution of crops in the greenhouse to create environmental conditions suitable for plant growth and development, thus realizing continuous crop production on an annual basis.
The production of tomatoes in continuous glass intelligent greenhouses is usually done in a soilless way, and water and fertilizer management is an important part of it. Based on a high standard water and fertilizer integration hardware system, appropriate nutrient concentrations and formulations are regulated and suitable irrigation strategies are developed for the crop growth stage to achieve optimal growth and high yield.
At present, China’s smart greenhouses are mainly traditional glass greenhouses, which are usually planted in September and seedlings are pulled in July, with a production cycle of 10 months and a harvesting period of 8 months. This paper will summarize the hardware configuration, irrigation management, formula adjustment, liquid return management, and nutrient solution management of tomatoes at different growth stages, using a tomato production base in China as an example, to provide technical references for annual tomato production in glass greenhouses.
Equipment Configuration
The main hardware of the irrigation equipment in the glass greenhouse includes fertilizer application machine, clear water tank, liquid return tank and disinfection tank, UV disinfection machine, mother liquor tank, RO reverse osmosis water production, pressure compensator and drip arrow, and substrate scale.
The substrate scale is an intelligent device that collects, stores and transmits data in real time, including inlet and outlet volume, dry-down (substrate water loss rate), EC, pH, substrate weight, and substrate water content, providing growers with more information about the substrate and root system for accurate irrigation. Currently, the main irrigation water in China includes RO water, underground well water and rainwater. Rainwater is widely used because it is freely available and contains less mineral salts and lower EC, so matching rainwater ponds are needed for rainwater collection.
Irrigation Management
The precise supply of water and fertilizer is achieved through the use of precision irrigation management systems in factory production. Generally speaking, the main technical points involved in the precise supply of water and fertilizer are irrigation stage, single irrigation volume / total irrigation volume, irrigation frequency and radiation accumulation, irrigation start time, etc.
1 Irrigation stage
The irrigation stage is divided into 3 stages: after sunrise – return of liquid, stable return of liquid, and before sunset, according to the change of EC and substrate water content WC. The purpose of the first stage of irrigation is to flush the nutrient elements in the substrate, to promote the saturation of the substrate WC, the end of which must ensure the emergence of back liquid, which is mainly for the second stage of high transpiration under high radiation conditions to prepare for high photosynthetic rate.
The second stage of high frequency irrigation things, the purpose is to meet the plant in high radiation conditions high transpiration rate of water demand. The third stage generally ends at 2~3 h (sunny day) and 3~5 h (cloudy day) before sunset, and the end time of the last irrigation and irrigation amount is mainly used to regulate dry-down to the balance between plant nutrient growth and reproductive growth before.
2 Irrigation volume
The irrigation volume is divided into single irrigation volume and total irrigation volume of the day, which are directly related to the amount of radiation accumulation. In the case of the tomato site in this paper, the greenhouse light transmittance is about 50% and the crop cut-off rate is calculated to be 75%, so that the irrigation radiation ratio is not less than 2.24 cc/m2/J for an outdoor light accumulation of 1000 J and to maintain a suitable inter-root EC. The general water requirement of tomato plants is 1.5~2.0 L per plant per day during the establishment period. The Dutch classical irrigation model gives 3 cc/m2 of irrigation for every 1 J/cm2 of radiation accumulated. However, the calculated value of this model is highly dependent on temperature and humidity, and maintaining a suitable absolute humidity HD (3-7) or water vapor pressure difference VPD (0.8-1.2) is essential to maintain normal transpiration.
The amount of single irrigation depends on the irrigation stage and the substrate and plant status. In general, each substrate strip is irrigated at a single irrigation rate of 2.5% to 5% (total volume of substrate strip irrigated / volume of substrate strip after soaking), which aims to fully flush and replace the mineral elements in the substrate, and either more or less will result in partial flushing of some areas of the substrate, leading to a gradient of nutrients.
2.5% of the irrigated substrate ratio implies low single irrigation but frequent irrigation, i.e. low radiation accumulation triggering irrigation, while 5% implies low frequency and high irrigation with high radiation accumulation triggering values. The frequency of irrigation is related to the type of substrate, coconut bran has better water retention and buffering properties than rock wool, so the frequency of irrigation is lower than rock wool in the case of coconut bran selection, i.e., a larger irrigation substrate ratio is chosen to facilitate the entry of oxygen into coconut bran after irrigation.
Different irrigation substrate ratios were selected to determine the amount and frequency of single irrigation, and if the plants favored nutritional growth, the irrigation substrate ratio could be appropriately increased, the irrigation interval expanded, and the irrigation frequency reduced. In addition, the single irrigation volume is differentiated by different stages of the day.
Taking the first stage of the day as an example, if the dry-down measurement is 10% and the substrate mass per unit area is 8.0 L/m2, in the first stage with 24 h average temperature of 22°C and 75% average daytime humidity, if the light accumulation is 400 J and the preliminary calculation of transpiration is 2 mL/J, together with 10% dry- down, if the substrate water content WC is to be restored to the saturation case, the total amount of irrigation in the first stage is 800 mL + 8 L × 10% × 1000, that is, about 1600 mL, and the irrigation radiation ratio is about 4.
3 Irrigation frequency and radiation accumulation
Irrigation frequency that is the number of irrigation in 1 day and the amount of irrigation each time, the number of irrigation and the total amount of irrigation, radiation accumulation, minimum / maximum interval time related. In general, under the sunny days, the radiation accumulation amount triggered irrigation (that is, when a certain amount of radiation accumulation triggers an irrigation) to this paper tomato production base for example, the first stage of radiation accumulation amount triggered irrigation value of 120 J, the second stage of 90 J, the third stage is 150 J, the specific number of irrigation and the amount of radiation accumulation related to the day.
In the case of cloudy days, as the radiation accumulation cannot reach the set value, it will lead to a long irrigation interval, and in order to maintain a suitable substrate WC, it is necessary to set a minimum interval and a maximum interval, the purpose of the minimum interval is to avoid two irrigation interval is too short, commonly used in the summer high radiation season, generally 15-30 min, while the maximum interval is generally used in cloudy days, generally 120-150 min.
4 Irrigation start time
Irrigation start time is the first irrigation start time and the last irrigation end time, which is related to the amount of radiation accumulation, humidity, cloudy and sunny weather. Generally, the first irrigation is carried out 2 h after sunrise and ends at about 10:00. It should be noted that the first irrigation is done 1.0-1.5 h after sunrise in hot weather, while the first irrigation is usually done 3-4 h after sunrise in cloudy weather. The last irrigation was carried out as early as 5 h before sunset and as late as 2 h before sunset. Irrigation ended earlier on cloudy days and was delayed on sunny days.
The purpose of irrigation is to meet the crop’s water and fertilizer uptake triggered by the root system due to transpiration. If the air humidity is high ( RH>85%, or HD<2), the space for water vapor that can be accommodated by the air at the moment becomes smaller, and when the timely radiation accumulation exceeds the irrigation setting, the transpiration of the plant is still relatively weak, and at this time, if irrigation is too early, the water balance of the plant will be broken, which is manifested by the thin and long flowering spike stalk ( (Figure 3), the fruit set and the stalk break, resulting in serious fruit drop, so timely ventilation and dehumidification after sunrise.
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