Presented at the IPI-PRII-KKV Workshop on:
Recent Trends in Nutrition Management in Horticultural crops
11-12 February 1999. Dapoli, Maharashtra, INDIA
Recent Techniques in Fertigation of Horticultural Crops in Israel
Patricia Imas, International
Potash Institute Coordinator India.
c/o Dead Sea Works Ltd., POB 75 Beer Sheva 84100 Israel.
- Advantages of fertigation
- Chemical and biological guidelines for a sound fertigation
- Fertilizers solubility
- Interaction between the fertilizers and irrigation water
- Soil pH
- Physiological effects: antagonism and synergism
- Practices of fertigation
- Fertilizer preparation
- Fertilizer injection methods
- Fertigation management in greenhouse crops
- Example: recommendation of a fertigation program by the Israeli Extension Service
Israel is a small country with a total land area of 21,000 km2, from which 20% is arable land. More than half of Israel has an arid to semi-arid climate. Approximately half of the cultivated area (200,000 hectares) has to be irrigated due to lack of rainfall and other water resources. Approximately 80% of the irrigated land in Israel uses the fertigation method, combining the application of water and fertilizers through the drip irrigation system.
The Israeli production of vegetables, flowers, ornamental plants and spices in greenhouses has been experiencing accelerated growth in recent years, reaching 3,000 hectares today. Most of the greenhouses are computerized, allowing automatic control of water, fertilizers and climate systems.
The direct delivery of fertilizers through drip irrigation demands the use of soluble fertilizers and pumping and injection systems for introducing the fertilizers directly into the irrigation system. Many Israeli companies specialize in manufacturing fertigation systems and in producing fertilizers and mixtures for their application through the drip irrigation system.
Fertigation allows an accurate and uniform application of nutrients to the wetted area, where the active roots are concentrated. Therefore, it is possible to adequate the nutrients quantity and concentration to their demand through the growing season of the crop. Consequently, recommendations have been developed for the most suitable fertilizer formulation (including the basic nutrients NPK and microelements) according to the type of soil, physiological stage, climate and other factors. Special attention should be given to the pH and NO3/NH4 ratio, nutrient mobility in soil and salinity conditions.
Planning the irrigation system and nutrient supply to the crops according to their physiological stage of development, and consideration of the soil and climate characteristics, result in high yields and high quality crops with minimum pollution.
Israel is a small country with a total area of 21.000 km2, from which 20% is arable land. More than half of the area of Israel has an arid to semi-arid climate Near half of the cultivable area (200.000 hectares) must be irrigated due to the lack of rain and other water resources. Approximately 80% of the irrigated area in Israel uses the method of "fertigation", that combines the application of irrigation water with fertilizers. This practice contributes to the achievement of higher yields and better quality by increasing remarkably the efficiency of the fertilizer application. Greenhouse crops in Israel are fertilized exclusively through the irrigation system. The Israeli production of vegetables, ornamental flowers, plants and spices under greenhouses has experienced an accelerated growth in the last years, with more than 3.000 hectares of greenhouses nowadays Most of these greenhouses are computerized, allowing the automatic control of the irrigation, the fertilization and the climate. Hydroponics in Israel reaches a total area of 700 Has, being the main crops tomatoes, cucumbers, strawberries and flowers (roses, crisantemum, gerbera and gypsophylla). The most common growing medium is tuff (volcanic stone) which is a reactive substrate with high power of adsorption and high indigenous phosphorus content. Inert substrates as rockwool and vermiculite are also used. At the moment most of the greenhouses have an open system. The aim is to change them by closed systems in which the farmer must collect the leached solution and reuse it thus avoiding contamination
Israel is an unequaled example of the use of fertilizers by fertigation. In 1996, the Israeli farmer used an average of 115 kg N/Ha, 46 kg P2O5/Ha and 57.5 kg K2O/Ha. Over 50% of the N and P2O5, and 65% of the K2O is applied by fertigation (Tarchitzky and Magen, 1997).
The fertigation allows to apply the nutrients exactly and uniformly only to the wetted root volume, where the active roots are concentrated. This remarkably increases the efficiency in the application of the fertilizer, which allows reducing the amount of applied fertilizer. This not only reduces the production costs but also lessens the potential of groundwater pollution caused by the fertilizer leaching. Fertigation allows to adapt the amount and concentration of the applied nutrients in order to meet the actual nutritional requirement of the crop throughout the growing season. In order to make a correct planning of the nutrients supply to the crop according to its physiological stage, we must know the optimal daily nutrient consumption rate during the growing cycle that results in maximum yield and production quality These functions are specific for each crop and climate, and were determined in different experiments for the main crops in Israel like tomatoes, cucumbers, melons maize, etc. (Table 1). The optimal curve of consumption of nutrients defines the minimal application rate of a certain nutrient that is required to maintain a constant nutrient concentration in the soil solution. These data constitute the base of the recommendations given by the Israeli Soil Extension Service for the farmers regarding the fertigation regime for the different crops.
Other advantages of the fertigation are: (1) the saving of energy and labor, (2) the flexibility of the moment of the application (nutrients can be applied to the soil when crop or soil conditions would otherwise prohibit entry into the field with conventional equipment), (3) convenient use of compound and ready-mix nutrient solutions containing also small concentrations of micronutrients which are otherwise very difficult to apply accurately to the soil, and (4) the supply of nutrients can be more carefully regulated and monitored. When fertigation is applied through the drip irrigation system, crop foliage can be kept dry thus avoiding leaf burn and delaying the development of plant pathogens.
Drip and microirrigation have a characteristic not shared by other irrigation methods: fertigation is not optional, but is actually necessary. Fertigation provides the only good way to apply fertilizers physically to the crop root zone. On high value drip irrigated crops, such as lettuce, tomatoes, and peppers, the level of fertigation management for achieving high yields and crop qualities exceeds to what is found with other irrigation methods and crops.
Effective fertigation requires an understanding of plant growth behavior including nutrient requirements and rooting patterns, soil chemistry such as solubility and mobility of the nutrients, fertilizers chemistry (mixing compatibility, precipitation, clogging and corrosion) and water quality factors including pH, salt and sodium hazards, and toxic ions.
An essential pre-requisite for the solid fertilizer use in fertigation is its complete dissolution in the irrigation water. Examples of highly soluble fertilizers appropriate for their use in fertigation are: ammonium nitrate, potassium chloride, potassium nitrate, urea, ammonium monophosphate and potassium monophosphate.
The solubility of fertilizers depends on the temperature. The fertilizer solutions stored during the summer form precipitates when the temperatures decrease in the autumn, due to the diminution of the solubility with low temperatures. Therefore it is recommended to dilute the solutions stored at the end of the summer. Fertilizer solutions of smaller degree specially formulated by the manufacturers are used during the winter.
|Table 2: Fertilizers solubility and temperatures (g/100 g water) (Wolf et al., 1985).|
Water quality: Many water sources in Israel have high contents of calcium, magnesium and bicarbonates (hard waters), the reaction of the water is alkaline with pH values between 7.2 and 8.5. The interaction of these waters with fertilizers can cause diverse problems, such as formation of precipitates in the fertilization tank and clogging of the drippers and filters. In waters with high calcium content andbicarbonates, use of sulphate fertilizers causes the precipitation of CaSO4 obtruding drippers and filters. The use of urea induces the precipitation of CaCO3 because the urea increases pH.
The main problem concerns phosphorus application: the presence of high concentrations of calcium and magnesium and high pH values lead to the precipitation of calcium and magnesium phosphates. Recycled waters are particularly susceptible to precipitation due to its high bicarbonate and organic matter content. The resultant precipitates are deposited on pipe walls and in orifices of drippers and can completely plug the irrigation system. At the same time, P supply to the roots is impaired. When choosing P fertilizers for fertigation with high calcium and magnesium concentrations, acid P fertilizers (phosphoric acid or monoammonium phosphate) are recommended.
Clogging: This is specially critical for drip systems that must be kept free from suspended solids and microorganisms that plug the small orifices in the emitters. In the case of clogging of the drip system by bicarbonate precipitation the use of fertilizers with acid reaction partially corrects this problem. However, acid fertilizers cause corrosion of the metallic components of the irrigation system and damage the cement and asbest pipes. Therefore, the periodic injection of acid in the fertigation system is recommended in order to dissolve the precipitates and to unclog the drippers. The following acids can be used phosphoric, nitric, sulfuric and chlorhydric. In Israel, HCl is widely used due to its low cost. Acid injection through the system will also remove bacteria, algae and slime. The irrigation and injection system should be carefully washed after the injection of acid.
Fertigation under saline conditions: Crops vary widely in their tolerance to plants, reference tables are available defining individual crop sensitivity to total soluble salts and individual toxic ions (Maas and Hoffman, 1977). When brackish waters are used for irrigation, we must bear in mind that fertilizers are salts and therefore they contribute to the increase of the EC of the irrigation water. Nonetheless, calculation of the contribution of chloride from KCl to the overall load of chloride from irrigation water shows its relative by low share (Tarchitzky and Magen, 1997).
When irrigation water has an EC > 2 dS/m (with high salinization hazard), and crop is sensitive to salinity, we must decrease the amount of accompanying ions added with the N or K. For example, in avocado - a very sensitive crop to chloride - KNO3 is preferred on KCl to avoid Cl accumulation in the soil solution This practice diminishes leaf burning caused by Cl excess. Also in greenhouse crops grown in containers with a very restricted root volume we must choose fertilizers with low salt index. Sodium fertilizers as NaNO3 or NaH2PO4 are unsuitable due to the adverse effect of sodium on the hydraulic conductivity and the performance of the plant.
A correct irrigation management under saline conditions includes water application over the evaporation needs of the crop, so that there is excess water to pass through and beyond the root zone and to carry away salts with it. This leaching prevents excessive salt accumulation in the root zone and is referred to as leaching requirement (Rhoades and Loveday, 1990).
Fertilizers compatibility: when preparing fertilizer solutions for fertigation, some fertilizers must not be mixed together. For example, the mixture of (NH4)2SO4 and KCl in the tank considerably reduce the solubility of the mixture due to the K2SO4 formation. Other forbidden mixtures are:
- Calcium nitrate with any phosphates or sulfates
- Magnesium sulfate with di- or mono- ammonium phosphate
- Phosphoric acid with iron, zinc, copper and manganese sulfates
The use of two fertilization tanks allows to separate the fertilizers that interact and cause precipitation, placing in one tank the calcium magnesium and microelements, and in the other tank the phosphorus and the sulfate.
pH values for optimal availability of all the nutrients is in the rank of 6-6.5. The main factor affecting pH in the rhizosphere is NH4/NO3 ratio in the irrigation water specially in sandy soils and inert substrates with low buffer capacity such as rockwool Rhizospheric pH determines the phosphorus availability since it affects the processes of precipitation/solubilization and adsorption/desorption of phosphates pH also influences the availability of micronutrients (Fe, Zn, Mn) and the toxicity of some of them (Al, Mn).
The nitrogen form absorbed by the plant affects the production of carboxylates and the cation-anion balance in the plant. When NH4 absorption is predominant, the plant absorbs more cations than anions, H+ are excreted by the roots and rhizosphere pH decreases. Fluctuations of pH of the ground around the roots of the order of 1.5 units of pH due to ammonium or nitric nutrition have been reported in the literature (Barber, 1984). According to Ganmore-Neumann and Kafkafi (1980, 1983), NH4 is an undesirable source of nitrogen for tomato and strawberries when the temperature in the root zone is greater than 30°C, due to its adverse effect on root growth and pant development. The pattern of cationic uptake due to an ammonium nutrition decreases the uptake of other cations like Ca2+, Mg2+ and K+.
When NO3- anions are absorbed, the plant takes up more anions than cations and the excess of anions is palliated by a greater synthesis of carboxylates. During the carboxylation process dicarboxylic acids (citric, malic, etc.) and OH- are produced.
Both the carboxylates and the hydroxyls can be exuded by the roots to the soil The exuded OH- increase the pH of the rhizosphere The organic acids exuded by the roots increase the availability of phosphorus since the carboxylates are specifically adsorbed to iron oxides and clays of the ground, releasing therefore adsorbed phosphorus to the soil solution. The carboxylates can also increase to the availability of iron and phosphorus by chelation: for example, citrate forms a chelate with calcium, thus releasing phosphorus that is under the calcium phosphate form (Imas et al., 1997).
According to this, NO3 nutrition is recommended due to the greater organic acid synthesis and enhanced cations uptake, whereas the ammonium nutrition is detrimental However, nutrition with 100% nitrates would increase rhizospheric pH up to undesirable values - values of more than 8 have been registered - and this would decrease the availability of P and micronutrients by precipitation. Therefore it is recommended to use a nitrogen mixture with 80% of nitrates and 20% of ammonium to regulate pH.
When two or more ions are present in external medium, antagonistic and synergetic effects can be observed. Synergism means the increase of the absorption of an ion due to the presence of another ion; antagonism refers to the competition between two ions. There is a competitive antagonistic effect between NO3 and Cl anions: the presence of Cl ion reduces the absorption of NO3 and vice versa (Imas, 1991; Kafkafi, 1982). Therefore, under saline conditions, the damage by salinity can be reduced fertilizing with NO3. The nitrate ions will be more absorbed replacing the chloride ions.
To capitalize on fertigation benefits, particular care should be taken in selecting fertilizers and injection equipment as well in the management and maintenance of the system.
In Israel, application of fertilizers is executed by various methods (Sneh, 1995):
- Stock solution preparation: farmers mix solid fertilizers as ammonium sulfate, urea, potassium chloride and nitrate, and liquid phosphoric acid to prepare a "tailor made" stock solution. The stock solution is then injected into the irrigation system, at rates of 2-10 L/m3, depending on the desired concentrations of N, P and K Clear NK, PK and NPK fertilizer solutions with at least 9-10% nutrients (N, P2O5, K2O) based on cheap solid fertilizers (urea, phosphoric acid and KCl) can be easily prepared on the farm site with limited facilities under "grass roots" field conditions, with minimal mixing (Lupin et al., 1996)
- Compound solid fertilizer mixtures: manufactured for use in fertigation, with different ratios between the three major elements. The first mixture used in fertigation was 20-20-20 and was produced in the mid-sixties. Some compositions contain microelements in the form of chelates.
- Compound liquid fertilizer solutions: due to solubility, the total nutrient concentration is much lower (5-3-8; 6-6-6; 9-2-8, etc.). Specified for use in greenhouses. Some compositions contain microelements in the form of chelates.
Generally two fertilizer tanks that contain the concentrated fertilizer solutions are used to separate those fertilizers that can interact. A possible combination is: a tank "A" containing calcium nitrate, potassium nitrate magnesium nitrate and microelements, whereas tank "B" contains ammonium sulfate, phosphoric acid and nitric acid; in this way P and Ca/Mg are in different tanks to avoid their precipitation. A third tank "C" contains an acid solution to control the pH of the fertilizer solution and to wash the irrigation system to avoid drippers clogging.
There are two types of fertigation, the type of fertigation chosen depends on the crop grown, the soil type and the farm management system.
- Quantitative: is the application of the plant nutrients in predetermined concentrations to the irrigation system. The fertilizer is applied in a pulse after a certain water sheet without fertilizer using a fertilizer tank. The advantages of this method are the low cost and the low required maintenance. The disadvantages are: the system is affected by water pressure changes; the concentration of the fertilizer varies during its application and it does not adapt to work with automation.
- Proportional: the nutrients are is applied in a constant and proportional ratio to the water sheet, so that the irrigation water takes a fixed concentration of the applied fertilizer. In this case the fertilizers are applied by direct injection through fertilizer pumps. The advantages are: precise control of the dosification and the injection moment, is not affected by the water pressure changes, and it can be easily automated. The disadvantages are: high cost and maintenance and complicated operation.
Modern fertigation equipment should be able to regulate:
- quantity applied
- duration of applications
- proportion of fertilizers
- starting and finishing time
It is important to select an injection method that best suits the irrigation system and the crop to be grown. Incorrect selection of the equipment can damage parts of the irrigation equipment, affect the efficient operation of the irrigation system and reduce the efficiency of the nutrients. Each fertilizer injector is designed for a specified pressure and flow range.
The majority of injectors available today can generally incorporate automatic operation by fitting pulse transmitters that convert injector pulses into electric signals. These signals then control injection of preset quantities or proportions relative to flow rate of the irrigation system Injection rates can also be controlled by flow regulators, chemically resistant ball valves or by electronic or hydraulic control units and computers.
Suitable antisiphoning valves or non-return valves should be installed to prevent backflow or siphoning of water and fertilizer solution into fertilizer tanks, irrigation supply and household supply.
The three methods of injection are:
Pressure differential (by-pass tank)
A pressure differential tank system is based on the principle of a pressure differential created by a valve, pressure regulation, elbows or pipe friction in the mainline. The pressure difference forces the water to enter through a by-pass pipe into a pressure tank which contains the fertilizer, and to go out again, carrying a varying amount of dissolved fertilizer.
The application of nutrients is quantitative and inaccurate, therefore is adapted for perennial crops like citrus, fruit trees and/or crops grown on heavy soil.
- Very simple to operate, the stock solution does have not to be pre-mixed
- Easy to install and requires very little maintenance
- Easy to change fertilizers
- Ideal for dry formulations
- No electricity or fuel is needed
- Concentration of solution decreases as fertilizer dissolves
- Accuracy of application is limited
- Requires pressure loss in main irrigation line or a booster pump
- Proportional fertigation is not possible
- Limited capacity
- Not adapted for automation
- Inlet valve
Vacuum injection (Venturi)
This method uses a venturi device to cause a reduced pressure (vacuum) that sucks the fertilizer solution into the line.
- Very simple to operate, no moving parts
- Easy to install and to maintain
- Suitable for very low injection rates
- Injection can be controlled with a metering valve
- Suitable for both proportional and quantitative fertilization
- Requires pressure loss in main irrigation line or a booster pump
- Quantitative fertigation is difficult
- Automation is difficult.
Pumps are used to inject the fertilizer solution from a supply tank into the line. Injection energy is provided by electric motors, hydraulic motors (diaphragm and piston).
- Very accurate, for proportional fertigation
- No pressure loss in the line
- Easily adapted for automation
- Complicated design, including a number of moving parts, so wear and breakdown are more likely
- Plants: the determination of the nutrients content and dry matter in the whole plant is tedious, destructive and needs laboratory facilities. Therefore we monitor plant nutrient status in the diagnostic organ, whose concentrations are correlated with the total nutrients content in plant and is a good indicator of the nutrition state of the crop. In Israel it was developed calibrated methods of monitoring in diagnostic organs for roses and different fruit trees.
- Soil: soil sampling and the determination of the nutrient concentrations in the extracts is a difficult and tedious method. Instead, the soil solution can directly sampled by porous ceramic cups permanently inserted in the soil at a certain depth. The solution is collected periodically and sent to the lab for analyzing the different nutrient concentrations. This method is easy, cheap and widely used by the Israeli farmers
- Field quick test kits: allows a quick determination of pH and approximate content of nitrates, potassium and chlorides in the soil solution and in the plant sap without sending the samples to the lab (usually by colorimetric strips).
The growth of vegetables and flowers in greenhouses built on sandy dunes and/or with inert substrates requires a special and precise control of the fertigation, because the CEC of these growing media are very low and therefore they do not provide nutrients. The only source of nutrients is through the fertigation system. Growing plants in containers allows the collection of the leaching water and its comparison with the irrigation water. The measurement of pH, EC and nutrients concentration in the leached solution indicates if fertilizers are being applied in excess or deficiency, and therefore allows the consecutive correction of the fertigation regime. It is recommended to collect the leached solution from the containers and the solution that leaves the drippers, and to compare both solutions on a daily basis. In Israel there are automatic computerized devices that measure pH and EC of both solutions and automatically corrects the next irrigation solution according to optimal values entered beforehand.
Electric Conductivity: A higher value of EC in the leached solution that in the applied solution indicates that the plant absorbs more nutrients than water, therefore we must apply greater amount of water to the plant. On the other hand, if the difference between the EC of the leached solution and the incoming solution is more than 0.4-0.5dS/ m, we must apply a leaching irrigation in order to wash the excess of salts.
Chlorides: An impaired management of the irrigation regime may lead to an unwanted accumulation of Cl ions present in the irrigation water. If the Cl concentration in the leachate is higher than the Cl concentration in the incoming solution and surpasses 50mg/L, it indicates a chloride accumulation in the root zone. Then it is recommended to apply an irrigation without fertilizers to leach the chlorides.
pH: the optimal pH value of the irrigation solution must be around 6 and the pH of the leaching solution should not exceed 8.5. A more alkaline pH in the leaching water indicates that pH in the root zone reaches a value that causes phosphorus precipitation and decreases micronutrients availability. When pH in the leachate is higher than 8.5, we must adjust the NH4/NO3 ratio of the irrigation solution by increasing slightly theNH4 proportion. When pH in the irrigation solution is higher than 6, we must inject acid to the solution (from tank C) to lower the pH.
For each crop there are many fertilizer programs. Fertigation allows changing the program during the growing season, adjusting it to suit fruit flower, shoot and root development. A specific fertigation program is developed on the basis of leaf and soil analysis and tailored to suit the actual crop requirements.
The following is the recommendation of the fertigation program for tomato offered by the Extension Service of the Ministry of Agriculture of Israel. It is observed that the recommended doses of each nutrient are different according to the physiological stage of the crop. The recommendations are different for tomato crop grown in open field or in greenhouse. Regarding the soil type, the doses recommended for inert substrates are precise and expressed as concentration in the irrigation water (proportional dosification). On the other hand, in heavy clay soils recommendations are expressed for quantitative dosification (in kg/ha); and recommended doses for phosphorus and potassium are not provided since in this type of soils these elements are adsorbed by clays and therefore is very difficult to determine their concentrations in the soil solution.
a.- Tomato in open field
|Plant density||11.000-12.500 plants/Ha|
|Expected yield||100 ton/Ha (for processing)|
|PLANTING - FLOWERING||25||1||1||1||1.6||1.6||1.6|
|FLOWERING - FRUIT SET||20||1||0.5||1.5||2.1||1.0||3.1|
|FRUIT SET- FRUIT RIPENING||25||1||0.3||2||2.8||0.6||5.6|
|Physiological Stage||Fertilizers *||kg/ha/day **|
|FLOWERING - FRUIT SET||14-7-21||15|
|FRUIT SET- FRUIT RIPENING||14-3-28||20|
** Plants are irrigated every 3-5 days in heavy soils, and every 2-3 days in light soils. To calculate the fertilizer dose at each irrigation, multiply the daily amount of fertilizer by the days interval between irrigation cycles
b.- Tomato in greenhouse
Substrate - soilless culture
|Physiological stage||Concentration in the irrigation solution (dripper)|
|Planting and establishment||120-150||40-50||180-220||100-120||40-50|
|Ripening and harvest||180-200||40-50||270-300||100-120||50-80|
|Physiological stage||Concentration in the irrigation solution (dripper)|
|Planting and establishment||120-150||40-50||180-220|
|Ripening and harvest||180-200||40-50||270-300|
|Ripening and harvest||4.0-4.5||?||4-5.5 ?|
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