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9th IPI-ISSAS Regional Workshop
December 5-8, 1999, Haikou, Hainan, PRC.
Recirculating nutrient solutions in greenhouse production
Hillel Magen, Coordination China,
International Potash Institute
c/o DSW, P.O. Box 75, Beer Sheva, Israel, 8410
Contents
- Abstract
- Introduction
- Description of the system
- Saving of water and nutrients
- Nutrient management
- Salinity buildup
- Renewing the nutrients in recirculated systems
- Conclusions
- References
Abstract
Environmental laws and regulations support recirculation of DW from greenhouse facilities. The value of water and nutrients in the DW is of significant value and recirculation system may save up to 40% of water and nutrients. The initial and the setting of critical limits of EC, Na and Cl in the solution dictate the frequency of discharge from the system. The system constantly monitors the nutrient solution for EC, pH and nutrient concentrations. Nutrients are applied according to factors such as plant requirements, transpiration, and required content of elements in tissue and level of nutrients in the solution. A frequent level of analysis and monitoring of these factors is required to maintain the stability of the system, to save water and nutrients and to achieve maximum yield.
Introduction
Recirculation of nutrient solutions is increasing steadily in many countries with intensive greenhouse culture. There are few driving forces for the development of these sophisticated systems: 1) restricting environmental laws and regulations aiming to reduce nutrient leaching to the environment; 2) significant savings of water and nutrients; and 3) better control of nutrient supply.
Recent European Community directives emphasize the need for growers to reduce their nitrate emission. For example, the greenhouse industry in the Netherlands is targeted in 1995 80% of the glasshouse vegetable production based on closed systems and 100% by the year 2000 (Hand & Fussel, 1995). Several other countries are reducing the amount of discharged drainage water (DW) by regulations and/or by subsidizing water recirculation (Bolusky and Regelbrugge, 1992).
Water is a scarce natural resource, but is at a constant total quantity. The sphere of mankind activity is constantly reducing the fraction of fresh water in favor of useless water (Tognoni et al., 1998). According to this report, agriculture uses only 60% of the globally available fresh water. Total protected cultivation is 200,000 ha and the average biomass production is approximately 50kg/m2/year. Since water content in this biomass is 90%, the total theoretical usage of water is approximately 100 million m3/year which is only 0.0037% of total irrigation water in the world. Global usage of irrigation water is 2.7 million *10 6 m3/ year, but the actual greenhouse is estimated at ~5,000* 106m3/year, much more than the theoretical value. Calculation of possible savings through the use of recirculation systems shows that a saving share of 30% in quantity, will directly save $150 million / year (for price of water of $0.10/m3).
The technology involved in such operation requires heavy investments (Bell & Marchant, 1998) that are possible due to new enforcement of environmental laws and regulations (Tognoni et al., 1998) and to significant savings of water and nutrients.
This paper describes recirculation systems in greenhouse, their potential savings and the different approaches applied to the nutrient management in such systems.
Description of the system
Recirculation is a closed system of water, nutrients and plants, in which the nutrient solution that drains from plant's beds is re-introduced to the plants. The preferred water source for such system is that with the lowest salinity (expressed as electrical conductivity, EC) and with minimal content of ions, especially those absorbed in minimal quantity by plants such as Na and Cl. This will allow more circulation and reduce the drainage out of the system. Online EC, pH and nutrient concentration measurement devices are installed at various points in the system to monitor and alter inputs of water, fertilizers and acids. A typical recirculation system is presented in figure 1.
| Figure 1: Recirculated greenhouse solution system | ||
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The use of rainwater as a diluter when solution's EC builds up is essential. The price of setting up a collection system and storage is high and depends on the precipitation pattern. If rain is concentrated within a short period, volumes of storage become too large for economic return. A typical analysis of irrigation and rainwater is presented in table 1.
The change from soil-grown crops to soiless culture growing has not resulted in a disappearance of soil-borne pathogens (Van Os, 1998). In addition, contaminants such as sprays and other chemicals used in greenhouse production, threat the health of plants. The installation of a disinfection unit or treatment to control level of pathogens and other
contaminants is therefore an integral part of the system. Disinfections methods of irrigation water are presented in table 2. While heat treatment is the most frequent method used in Europe (Van Os, 1998) this may differ in other places. For example, big volume sand and biological filters are used in Israel.
Saving of water and nutrients
The cost of recirculating systems is high. Installation of 1 hectare can cost $18,000, but the annual saving from water and nutrients can reach about $4,000 each year (Bell & Marchant, 1998). In Israel, the price of water for farmers is $0.3-0.2/m3, thus the value of saving of 40% in water consumption is in the order of $3,000/ha/year. When available water is limited, recirculation is the only way to supply sufficient water for greenhouse production. The luxury feeding usually used in intensive systems and the relatively high leaching fraction leads to high concentration of nutrients in the drainage water (MacAvoy, 1994). The method employed with soiless culture demands approximately 30 - 50% leaching fraction. This fraction is simply spilled and diverted out of the growth area. The combination of high leaching fraction (LF) and the high nutrient concentration in the DW (or LF) leads to significant amount of wasted nutrients. Typical nutrient concentrations in DW in tomatoes and roses grown in greenhouse are presented in table 3.
| Table 3: Typical average summer run-off nutrient analysis (Bell & Marchant, 1998) | ||||||||
|---|---|---|---|---|---|---|---|---|
| Crop | pH | EC | N-NO 3 | P | K | Ca | Mg | Na |
| dS/m | ppm | |||||||
| Tomatoes | 6.1 | 2.75 | 175 | 22 | 350 | 220 | 75 | 100 |
| Roses | 6.5 | 2.34 | 260 | 25 | 250 | 150 | 42 | 135 |
An example of potential saving of potassium from recirculation is presented. Assuming that LF is being recirculated instead of sending LF out of the system:
| LF = 20%, | |
| 220 days of irrigation with 5 mm/day = 11,000 m3/ha/year, | |
| the total amount of LF: 11,000 * 0.2 = 2200m3/year, | |
| 2200m3 * 350 ppm (K) = 770 kg K/ha/year. |
Nutrient management
Bar-Yosef (1999) describes the main management topics related to recirculated systems as follows:
- Salinity buildup,
- Root pathogens
- The presence of root exudates and growth substrates dissolution,
- Al3+ toxicity (released from substrate),
- Adjusting nutrient and oxygen demand.
The topic of salinity buildup will be described in details.
Salinity buildup
Since irrigation water contains an initial amount of soluble ions, mainly such that are not needed in plant nutrition (Na, Cl), the initial EC of nutrient solution should be considered. Buildup of EC is due to accumulation of ions that are not absorbed by plants. The following example (table 4) shows the concentrations of nutrients in solution in tomato crop grown in greenhouse after 3 weeks of applying the treatments (Raviv et el., 1995). In this experiment, the influence of rainwater and recirculation (RCRLN) were tested. Data for the nutrient solution are presented in the upper part of the table, and the respective analysis of DW is at the bottom part of the table.
The treatment with no recirculation (tap, commercial) had a slight reduced level of EC in the nutrient solution as compared to the recirculating treatments (2.05 compared to 2.12 and 2.13 dS/m, respectively), and reduced levels of Cl, Na and Ca entrations (6.4 compared to 7.4 and 7.8 meq/l, respectively), but no clear differences and even opposite results for other ions. Salinity in DW has increased by approximately 50% for all treatments. In this experiment, a build up of NO3 in DW is also observed, inclinig an excess application of this element. It is also concluded that under these conditions, neither of the treatments proved significantly superior.
Renewing the nutrients in recirculated systems
Management of recirculated systems is complicated because of the combination of two systems: 1) water consumption and 2) nutrient absorption (Ben Asher, personal communication). If water uptake is higher than that of nutrients, EC will build up, and vice versa. Replenishing nutrients should be calculated as per single ion; otherwise a quick excess of ions is built, leading to spillage of water.
Various factors influence the application of nutrients. A major factor is the consumption of nutrients by plants, along the growing and yielding periods are known from other field or laboratory experiments, as described in figure 2 (data from Bar-Yosef, 1999).
| Figure 2: Periodical N, P & K consumption rate (kg ha-1 day-1 ) of greenhouse tomatoes (Bar-Yosef, 1999) | ||
 |
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| Days (DAP) |
Bugbee (1996) suggests using both nutrient and water requirement and combining these via the transpiration to dry-mass growth ratio: The plant needs to transpire 300 kg of water to form 1 kg of dry matter (Bugbee, 1996). This ratio of 300:1 is acceptable under normal conditions. At low humidity, the ratio increases to 400 but under high CO2 levels is reduced to 200 or even less. For example, if the desired K concentration in the plant is 4% (40 g kg-1), at a 300:1 ratio (transpiration to dry matter), 300 liters must have 40 g of K, and the consequent concentration in the nutrient solution is 3.4mM.
Moreover, essential nutrients can be classified to 3 major categories, based on rate of depletion from the solution (Bugbee, 1996). Group "1" elements are actively absorbed by roots and removed from the solution in a short time (few hours). The second group (2) elements have intermediate uptake rates and are usually removed from solution slightly faster than that of water via transpiration. Ca and B (group 3) are passively absorbed from solution and often accumulate in solution. For example, phosphorus and potassium (group 1) are rapidly absorbed by plant, therefore very low concentrations (µM) in solution may occur. This is a positive indication of the plant's ability to absorb nutrients, and there is no need to immediately add more of these elements. P concentration of only 0.5mM is sufficient after the first vigor growth period (Bugbee, 1996)
Recirculated systems are totally based on the concentration of nutrients in solution, without the buffer supply that soil provides.
Two examples of beneficial effect by nutrients usually not considered as plant's nutrient are given: 1) Silicon is not known as a plant nutrient in solutions, but there are evidences for its role in avoiding toxicity of metals and increased protection against insects (Bugbee, 1996); 2) Chloride, usually regarded as a undesired element, but at concentrations of 8-13 mM increases Ca uptake, decrease blossom-end rot but increases gold specs in tomato fruit (Kreij, 1995).
Conclusions
Recirculating systems have a significant potential for water and nutrient savings. Environmental laws and regulations pose a new situation to the farmer: Instead of delivering water and nutrients through root zone, as done with traditional irrigation, collection and management towards the re-use of these solutions is applied.
Proper management and saving efficiency is achieved when divert factors such as pH, EC and nutrient concentrations in the solution are controlled along with correct irrigation regime.
References
Bar-Yosef, B. 1999. Advances in Fertigation. In: Advances in Agronomy (D.L. Sparks ed.). V-65, pp 2-65.
Bell, F. R. and A. Marchant. 1998. The development and implementation of run-off management plans for the collection, containment and disposal of nutrient rich run-off from glasshouse crops, grown in artificial substrates in Guernsey. Proc. IS Water Quality & Quantity in Greenhouse Horticulture. (Ed): Rafael Munoz Carpena. Acta Hort. 458, ISHS. pp 395-399.
Bolusky, B. & C. Regelbrugge. 1992. Political perspective on water quality impact. Hort. Technology, 2:80-82.
Bugbee, B. 1996. Nutrient management in recirculating hydroponic culture. Crop Physiology Laboratory, Utah State Univ. Logan, UT 84322-4820.
De Kreij, C. 1995. Latest insights into water and nutrient control in soiless cultivation. Acta Hort. 408: Soiless Cultivation Technology. pp 47-61.
Hand, D. J. & M. Fussel. 1995. The effect of reduced nitrate input on tomato yield and fruit quality. Acta Horticulture 401. Growing Media & Plant Nutrition. Pp 319-325.
MacAvoy, R.J. 1994. Nitrate nitrogen movement through the soil profile beneath a containerized greenhouse crop irrigated with two leaching fractions and two wetting agent levels. J. Amer. Soc. Hort. Sci. 119;446-451
Raviv, M., Reuveni, R., Medina, Sh. Shamir, Y., Duvdevani, O., Shor, O., and R. Schayar. 1991. Changes in tuff during prolonged cultivation which affect rose productivity. Acta Hort. 294. Pp. 99-104.
Raviv, M., Reuveni, R., Krasnovsky, A. and Sh. Medina. 1995. Recirculation of rose drainage water under semi-arid conditions. Acta Hort. 401. Growing Media & plant nutrition. Pp. 427-433.
Tognoni, F., Pardossi, A. and G. Serra. 1998. Water pollution and the greenhouse environmental costs. Proc. IS Water Quality & Quantity in Greenhouse Horticulture. (Ed): Rafael Munoz Carpena. Acta Hort. 458, ISHS. pp 385-394.
Van Os, E. A. 1998. Closed soiless growing systems in the Netherlands: The finishing touch. Proc. IS Water Quality & Quantity in Greenhouse Horticulture. (Ed): Rafael Munoz Carpena. Acta Hort. 458, ISHS. pp 279-291.