Feb 14, 2017 - fertilizers such as potassium sulphate (K2SO4) and mono ... set at ±0.4 meq L-1 when inert substrates such as gravel or rockwool are used.
safeguarded and used with care.
Nutrient solution Nutrient solution Nutrient solution management management management Nutrient solution management Nutrient solution management Nutrient solution management
Lecturers from the department of Lecturers from the department of Agronomy at the University of Agronomy at the University of Stellenbosch have interacted with their Stellenbosch have interacted with their Belgian colleagues in Louvain since Belgian colleagues in Louvain since 1993. Several of these Belgian scientists 1993. South Several of these scientists visited Africa and Belgian also acted as visited South Africa and also acted guest speakers at local hydroponic as guest speakers at localsupport hydroponic symposia. Due to priceless resymposia. Due to priceless support ceived from Stan Deckers, a soil scientist refrom Stan Deckers,the a soil scientist atceived 'Soil Service of Belgium' authors at 'Soil Service this of Belgium' wish to dedicate book to the him,authors as wish to expert dedicate book ofto plant him, as respected in this the field nutrition. theseinpictures, is respected On expert the fieldStan of plant standing next to South African rivers, nutrition. On these pictures, Stan is carrying life-giving should rivers, be standing next towater Souththat African safeguarded and used with care. carrying life-giving water that should be
Nic Combrink Nic Combrink Nic Combrink
Untitled-1 1
2/14/2017 1:03:15 PM
Nutrient solution management
Nic Combrink
Improved edition (February, 2017) published by Combrink Family Trust (IT 1143/2000)
www.greenhousehydroponics.co.za
ISBN 978-0-620-51564-1
i
Contents 1 Introduction
1
2 Acidity and alkalinity
1
pH Total alkalinity
1 1
3 Electrical conductivity (EC)
2
4 Feeding water quality 4a Chemical composition
4 4
Feeding water EC Macronutrients in feeding water Micronutrients in feeding water Micronutrient phytotoxicity Feeding water pH
4 4 5 5 5
4b Harmful micro-organisms in water
6
5 Removing unwanted ions from water Iron and manganese Sodium, chloride and other ions Ions associated with alkalinity
6 Basic hydroponic chemistry 6a Defining some terms Mole Molecular mass Equivalent mass Molar Normality Transforming mg L-1 to meq L-1 6b Essential nutrients 6c Non-essential Na and Cl 7 Basics to start with nutrient solutions Nutrient solution recipes Ammonium as pH regulator Ammonium as N-source ‘Ready mix’ nutrient solutions Predicting EC EC and nutrient uptake
ii
6 6 7 7
8 8 8 8 8 9 9 9 10 12 12 12 13 13 13 14 14
8 Planning a macronutrient solution 8a Adjust alkalinity 8b Using acids or bases as ion sources 8c Ammonium-containing macronutrient solution
15 16 21 22
9 Planning a micronutrient solution
24
10 Solubility of salts
25
11 Mixing macro- and micronutrient solutions 11a Using a big reservoir at the desired EC 11b Using two or more stock solution tanks
27 27 27
12 Using nutrient solutions in experiments
31
13 Recipes for specific crops 13a Drain-to-waste systems 13b Closed systems 13c Leaf vs. root zone solution analyses
32 32 34 37
14 Procedure for adjusting nutrient levels 14a Sampling 14b Check laboratory 14c Root zone interpretation principles 14d Managing pH
37 37 38 38 40
15 Procedure to change input solutions 15a Example 1: Drain-to-waste 15b Example 2: Closed system
41 41 44
16 Sterilization of feeding water and solutions 16a Harmfull waterborne organisms 16b Sterilization of feeding water 16c Sterilization of re-cycled solutions
46 47 48 48
Chlorination Hydrogen peroxide Ozone UVc Irradiation Filtration Heat
48 48 48 49 50 51
17 References
51
iii
Preface
This book was written as study guide for students in soil-less culture. Procedures to adjust the alkalinity of different types of feeding water and to top it up with limiting nutrients to serve as nutrient solutions for the production of different greenhouse crops are discussed. The procedure used in the Netherlands and in Belgium, where the composition of a root zone solution serves as guide to correct nutrient imbalances during production, was adapted for local conditions (Combrink, 2005). The author tried to present this procedure in a user-friendly way. This practice is useful for 'drain-to-waste' production systems but it is essential where nutrient solutions are recirculated in closed systems. Most of the information shared in this book is the result of a long association between lecturers of the department of Agronomy (Univ. Stell.) and their Belgian colleagues in Louvain. Due to the priceless support received from Stan Deckers (Bodemkundige Dienst België), the author wishes to dedicate this book to him, a book that should stimulate the re-use of nutrient solutions to protect our water streams from nutrient pollution. This is the reason why his photographs on the back page were taken next to South African rivers. The first edition of this book was a manual for a workshop, launched by Dr Nic Combrink at Infruitec (ARC) in Stellenbosch on the 18th of October, 2011. Due to feedback from valued readers and colleagues, several changes were made. This improved edition will be used during a Netafim symposium on the 23rd of February, 2017.
Dr. Nic Combrink iv
1
INTRODUCTION
Plant growers are dependent on the process of photosynthesis, driven in green leaves by energy from the sun, building carbohydrates with carbon dioxide (CO2) and water (H2O). Thirteen essential plant nutrients affect this process by ensuring that the leaves are green, filled with chlorophyll and that the integrity of membranes and strength of cell walls are maintained. Plant roots absorb these nutrients from fertile soil, but it can also be taken up from a well-balanced nutrient solution. Arnon and Hoagland (1940) developed some of the first well-balanced nutrient solutions, enabling soil-less crop production. Apart from the introduction of chelated iron, modern nutrient solutions do not differ much from the original ones. In this booklet, guidelines will be given to qualify the readers to 'build' and manage their own nutrient solutions. Before this can be done, it is important to ensure that terms such as pH, salt concentration and some basic chemical principles are understood.
2
Acidity and alkalinity
pH is a value taken to represent the acidity or alkalinity of an aqueous solution with values ranging from 1 to 14. At a reading of 7, the pH is considered to be neutral. At this point equal concentrations of hydrogen ions (H+) and hydroxide ions (OH-) occur. Values lower than 7 indicate acidity and solutions with pH values higher than 7 are alkaline. The pH scale is logarithmic, meaning that a solution at pH 6 is ten times more acidic than at pH 7 and at pH 8 it is ten times more basic than at pH 7. This is why the pH value of water will not decrease linearly when acids are added. Total alkalinity is the aggregate concentration of bases such as carbonates, bicarbonates and hydroxides. The concentration of these ions (CO3=, HCO3- and OH-) is determined with a titration, as a measure of total alkalinity. It can be expressed as HCO3-, but some laboratories prefer to express it as CaCO3 or as CO3=. Total alkalinity can be neutralized by adding an equivalent amount of acid (H+), as explained on pages 16 to 18. By manipulating total alkalinity, the pH is also controlled in nutrient solutions. Most soils are well-buffered which means that their pH values do not change easily. This is not the case for soil-less conditions. Special precautions are needed to control the pH and consequently the chemical and physical form and availability of nutrients in solution. Some nutrients precipitate as insoluble salts in high pH solutions. Most crops (excluding a few acid-loving plants) are grown in nutrient solutions at an optimum pH
2
of about 5.8. Compared to pH values of soil, the optimum pH of hydroponic solutions may seem to be low. Having a look at the dissociation pattern of phosphate in solution (Table 1), it is clear that the optimum pH for nutrient solutions should be slightly acidic and that neutral to alkaline solutions should be avoid to ensure that enough soluble phosphate is present. In addition, micronutrients such as Fe and Zn precipitate, forming insoluble salts in nutrient solutions with pH values higher than 7. Table 1 Phosphate’s dissociation pattern. (Steiner, 1984). H2PO4(Forming a soluble Ca salt) pH = 5.0 pH = 6.0 pH = 6.5 pH = 7.0 pH = 8.0
100 % 90 % 78 % 50 % 15 %
HPO4= (Forming an insoluble Ca salt) 0% 10 % 22 % 50 % 85 %
At pH values lower than 5, problems may arise on crops that are sensitive to Cadeficiencies (peppers and tomatoes). This is due to an associated high H+ concentration that may suppress the absorption of Ca2+ and other positively charged ions (cations). A high ammonium (NH4+) level will aggravate this problem since it may also supress the uptake of other cations. Most soil-less crops are grown in solutions with pH values ranging from 5.3 to 6.3. The acid-loving blueberry is grown at pH 4.5 to 4.8 and is also adapted to grow at relatively high NH4+ levels. Disas grow well at high NH4+ levels and should also be considered as an acid-loving crop (Pienaar, 2005).
3
Electrical conductivity (EC)
EC is a well-known term amongst soil-less growers. The EC of water is used as an indication of its salt content since higher concentrations of charged particles (ions) increase the electrical conductivity of water and nutrient solutions. When salts are added to water, the crystals dissolve or ionise. Using table salt (sodium chloride; NaCl) as an example, one sodium cation (Na+) and one chloride anion (Cl-) will be released per dissolved NaCl unit. These charged ions in the water enable the flow of an electrical current. The higher the concentration of ions (charge) per unit volume of water, the better the conductivity and the higher the EC will be. These high EC solutions are better buffered against pH change than low EC water, where small applications of H+ or OHmay drastically lower or increase the pH. At a very high EC, the water contains a lot of dissolved salts, restricting the osmotic movement of water from the solution into roots. Take note that non-polar molecules (sugar or urea) dissolve in water without the release of charged particles. Thus, these
3
substances (not to be used in nutrient solutions) dissolve but do not increase the EC of a solution, but affect the osmolarity of solutions and restrict the uptake of water by roots. The use of different EC units may confuse growers. Siemens (S) is the SI unit for conductivity. Since the conductivity values of nutrient solutions are relatively small, a smaller unit such as milli Siemens (mS) is used. The unit ‘mho’ is equivalent to S and mmho to mS. Some laboratories use the electrical resistance of soil as indication of its salt content. Due to an inverse relationship between conductivity and resistance, higher resistance values (measured in ohm) are indicative of low salt contents.
A lower
resistance implies that more salts are in solution and the conductivity will thus be high. The direct relationship between conductivity and salt content is easy to understand and an extremely important managerial tool for soil-less growers. The distance between the two electrodes, used to measure the conductivity of a solution, also affects the reading. The reading in mS m-1 will be 100 times bigger than a reading in mS cm-1. Most growers use mS cm-1 as unit to test the EC of standard nutrient solutions. Depending on the crop and the season, these values may vary between 0.7 and 3.0 mS cm-1. The EC of good quality feeding water may be very low, explaining why most laboratories prefer to express the conductivity of irrigation water in mS m-1. Relation of mS cm-1 to other EC units:
1 mS cm-1
= = = =
1dS m-1 1 mmho cm-1 100 mS m-1 1000 µS cm-1
Since temperature affects EC readings, EC meters are calibrated at 25 oC. The electrical conductivity of a solution increases with an increase in temperature. The EC readings, as shown in Table 2, can be expected when a standard 2 mS cm-1 solution is measured at different temperatures. EC meters such as those used by growers are not temperature compensated. These EC measurements can be corrected by subtracting 2% from the measured value for every 1oC that the solution temperature exceeds 25oC. o
o
solutions colder than 25 C, 2% can be added to the reading per 1 C deviation. Table 2 Temperature affects the measured electrical conductivity (EC) of a 2 mS cm-1 nutrient solution. Solution temperature (oC) 15 20 Standard 25 30
Measured EC (mS cm-1) 1.62 1.80 2.00 2.20
With
4
4
Feeding water quality
The term ‘feeding water’ is used to describe an untreated water source that is available to prepare nutrient solutions. Different factors can be used to define feeding water quality but (a) chemical composition and (b) the presence of potential dangerous microorganisms are of great importance.
4a Chemical composition Feeding water EC.
The concentration of ions, measured as EC, can be used as
indication of potential quality for feeding water. Water with a low EC can normally be used to grow any crop. High EC feeding water, usually high in sodium (Na+) and chloride (Cl-) can only be used to grow saline-tolerant crops. These include amaranths, Swiss chard (Sedibe, Combrink and Reinten, 2005), melon (Combrink et. al., 1995) and cherry tomatoes. Examples of crops that are extremely sensitive to saline conditions are disas (Pienaar, 2005), anthurium, cymbidium and roses (De Krij et al., 1999). Most of the remaining greenhouse crops vary between moderately sensitive to moderately tolerant, as can also be deducted from the EC levels that are recommended for the different crops in Table 17 (Page 33). It should be kept in mind that the absorption of water is restricted at increased root zone salinity levels. The water in the lake of Galilee is widely used in Israel, even though it has an EC of ±1.0 mS cm-1. The EC of water in the Vaal dam varies from 0.5 to 1.0 mS cm-1 but the EC in the lower Vaal River may be higher. Compared to this, water in Stellenbosch has an excellent quality with an EC of less than 0.1 mS cm-1, as is also found in other rivers from unpolluted, high rainfall mountainous areas. However, there is no guarantee that water with a low EC can safely be used for soil-less crop production, since micronutrients may still be present at phytotoxic (plant-killing) levels, even in low-EC feeding water. Macronutrients in feeding water.
Water should be chemically analysed in order to
determine the levels of dissolved salts in feeding water. In low rainfall areas, high levels of salts may be found. Apart from Na+ and Cl-, high levels of essential nutrients such as calcium (Ca2+) magnesium (Mg2+) and sulphate (SO42-) may also be present in high-EC feeding water. The other essential ions are usually found at lower concentrations, depending on the area and the water source. +
-
The higher the ratio of useful ions
compared to Na and Cl , the better the potential of high-EC feeding water.
When
useful ions are present at high concentrations, they should be taken into account when a nutrient solution is planned.
5
Micronutrients in feeding water. The risk of finding toxic levels of micronutrients in feeding water increases with increased EC levels. Micronutrients are usually present at such low concentrations that it will not affect the EC.
Even at relatively high
concentrations, micronutrients do not affect EC readings. This means that low EC feeding water may contain micronutrients at phytotoxic levels. With concentrations exceeding prescribed micronutrient levels (Table 17), the water should be avoided or handled with care. As with macronutrients, the micronutrient levels in feeding water should also be taken into account when planning a nutrient solution. Nutrient levels in the water are simply topped up to the levels prescribed for the crops (Table 17). Micronutrient phytotoxicity.
Strawberries need relatively low B levels due to
physiological problems with B at >0.32 mg L-1. Tomatoes can tolerate B at levels up to 1.1 mg L-1, almost three times higher than its recommended rate (Deckers, 2002). De Kreij, et al., (1999) recommend that Zn be used at 0.33 mg L-1 for substrate-grown tomatoes but at only twice this concentration, toxicity can be expected (Deckers 2002). High Zn-levels are usually found in water gathered from galvanised roof surfaces or fed through galvanised pipes. Copper-sulphate is a well-known chemical, used to kill algae in swimming pools. Thus, the potential phytotoxic effect of high Cu-levels is wellknown. Most crops need Cu at 0.05 mg L-1. According to Steiner (1984), Cu may be phytotoxic when this concentration is doubled to 0.1 mg L-1. Copper pipes should thus not be used to feed water to hydroponic units. Manganese toxicity problems may develop on lettuce (open or loose tulip shaped heads) where seedlings are raised on sphagnum peat, due to high levels of Mn in this European substrate (Deckers, 2004). Both iron (Fe) and manganese (Mn) may cause production problems by blocking irrigation drippers. Feeding water pH. Water pH cannot be used as sole parameter for pH adjustments. This must be done by adjusting total alkalinity; the aggregate concentration of bases such as carbonates, bicarbonates and hydroxides. =
Laboratories can measure the total
-
concentration of these bases (CO3 , HCO3 and OH-) with a titration. Since some hydroponic fertilizers may be slightly acidic, a small total alkalinity should be present in feeding water to prevent the pH from dropping too low when adding fertilizers. In mountainous, high rainfall areas, low-EC water may have a zero alkalinity or it may even be acidic (corrosive).
In order to lower the corrosiveness of this water,
municipalities allow it to flow through huge reservoirs, filled with limestone pebbles (almost insoluble). This treatment removes the acidity and slightly lifts the alkalinity of the water from zero to levels suitable to be used as feeding water for soil-less crop production. In some cases the pH of water from these lime pebble tanks may seem to be
6
high, but this is due to poorly buffered water with low ion concentrations. The pH of this water may be extremely variable and its EC can be changed easily. Apart from the limestone pebble treatment, the alkalinity of feeding water can be increased by using soluble hydroxides such as KOH or Ca(OH)2. However, the dosage of these products should be accurately calculated and applied to lift the total alkalinity to safe levels, as will be discussed later (Table 9, page 18). Water with a high alkalinity needs to be treated with acid to lower its alkalinity, as will be discussed on pages 19-20.
4b Harmful micro-organisms in water The levels of plant pathogens in water from boreholes are usually very low. When using water from rivers, the incidence of plant pathogenic organisms is much higher. Due to informal settlements and urbanization, water sources that were relatively free from plant pathogens a few years ago, might have deteriorated, needing sterilization before using it for soil-less crop production. Apart from chlorination, used by municipalities, other sterilization options are also available, as discussed in chapter 17 (Pages 46-51).
5
Removing unwanted ions from water
Iron and manganese. High levels of soluble (ferrous) iron may be found in wells or boreholes from the mountainous areas of the Cape and along the Drakensberg. This iron is in a reduced state (Fe+2) and when oxidized, ferric iron Fe+3 precipitates as an insoluble, red substance. Using this water for sprinkler irrigation, the red pigment can be seen on leaves and on garden walls. Manganese (Mn) is also soluble in its reduced state and precipitates as insoluble MnO2 when oxidized. When using water with high Fe or Mn concentrations for drip irrigation, the ions are oxidized and these insoluble salts block the drippers. Apart from oxidization due to aeration, ferric and manganese bacteria are chemotropic and also contribute to oxidize Fe and Mn. These ferric and manganese bacteria cause the oxidized residues to accumulate among the bacterial waste, creating a slimy residue, also blocking the drippers. Water quality can be defined as good, medium or poor, according to its Fe and Mn concentrations. Good quality is regarded as safe to use, with no blocking of drippers with Fe at 0.3 mg L-1 and pre-treatment to lower the concentration of these ions is needed when using drippers. By aerating the water, Fe and Mn can be oxidized. Small oxidized particles can then be removed with sand
7
filters. The oxidizing process is, however, extremely slow in acidic water, necessitating increasing the pH of the water to decrease the oxidation times, as illustrated in Table 3.
Table 3 The effect of pH on the aerobic oxidation times for iron (Fe) and manganese (Mn) (http://www.netafim.com). pH level
Aerobic oxidation time for Fe (minutes)
Aerobic oxidation time for Mn (minutes)
180 < 60 < 10 < 6.6 1000 >1000 >1000 < 200 < 45
Apart from oxidation by aeration, the oxidizing process may also be accelerated with UV tubes, the addition of chlorine gas (Cl2), ozone (O3) or H2O2. Growers who make use of drip irrigation should remove as much Fe as possible in the feeding water and it should then be replaced with the correct concentration of chelated Fe (Deckers, 2002). When using an ebb-and-flow irrigation system or production methods without drippers, removal of Fe is not critically important. Sodium, chloride and other ions. Should sodium (Na+) and chloride (Cl-) levels exceed the maximum limits as shown in Table 19 (page 35), it should not be used as feeding water. However, it may be diluted with rain water or the ions can be removed with an expensive water purification system such as reverse osmosis. Should water purification companies claim that they can remove these ions from feeding water, samples should be obtained and the water should be chemically analysed to measure the Na+ and Cl- levels before and after passing through the device. The system of reverse osmosis removes all ions, thus also macroand micronutrients that may be present at phytotoxic levels. This method of water purification creates a large percentage of saline wastewater which should be wellmanaged to prevent pollution of soil and rivers. Ions associated with Alkalinity The alkalinity level in saline feeding water is usually high, due to high levels of one or more of the following: CO32-, HCO3- and OH-. These ions can easily be removed and replaced by nitrate or phosphate (or even sulphate), simply by using nitric- or phosphoric acid (or in rare cases sulphuric acid) to lower the alkalinity, as illustrated
8
later (Tables 10 & 11). Since carbon dioxide gas will be released when the alkalinity is lowered with acid: H+ + HCO3- = H2O + ↑CO2, the gas should be allowed to escape from an open mixing tank, when water with a high alklinity (>0.8 meq L-1) is treated.
6
Basic hydroponic chemistry
6a)
Defining some terms:
Mole: ‘Mole’ is Avogadro’s number with the value of 6.02 x 1023. The atomic mass, expressed in grams, contains Avogadro’s number of atoms. For example, 12 g of carbon (C) and 23 g of sodium (Na) contain the same number of atoms = 6.02 x 1023 = 1 mole. The atomic masses of some elements are shown in Table 4. Molecular mass: One mole of a substance has a mass in grams, numerically equal to the molecular mass. With KCl as an example, K (39.1) and Cl (35.5) are added to reach 74.6, the molecular mass of KCl. Thus, 74.6 g of KCl contains Avogadro’s number of KCl units: 1 mole (6 x1023) K and 1 mole Cl atoms. Table 4 Some chemical formulas, ions of some elements and some complex ions with their respective atomic and molecular masses. __
Element Symbol
Ion
Atomic mass
Complex ion
Hydrogen Boron Carbon Nitrogen Oxygen Sodium Magnesium Silicon Phosphorus Sulphur Chloride Potassium Calcium Manganese Iron Copper Zinc Molybdenum
H+
1.0 10.8 12.0 14.0 16.0 23.0 24.3 28.1 31.0 32.1 35.5 39.1 40.1 54.9 55.8 63.5 65.4 95.9
Bicarbonate Carbonate z Ammonium z Nitrate z Phosphate z Sulphate
H B C N O Na Mg Si P S Cl K Ca Mn Fe Cu Zn Mo
z
Na+ Mg2+ z z
Cl – K+ Ca2+
Chemical Molecular formula mass HCO3 – CO3 – NH4 + NO3 – H2PO4 – SO4 2-
61 60 18 62 97 96
___
z
Associated in a complex ion
9
Equivalent mass (mass per charge): The atomic mass of K is 39.1. When losing an electron it becomes a single charged cation K+. Its mass (and equivalent mass) remains 39.1 (the mass of an electron is close to zero, so it can be ignored). The atomic mass of Ca is about 40, but when ionised, it loses 2 electrons, becoming a double charged cation (Ca2+). By dividing atomic mass by the charge of its ion, the equivalent mass is calculated. Calcium’s atomic mass is 40, but its mass per single charge (equivalent mass) is 20. When dealing with a molecule, with a number of atoms, the molecular mass is calculated by adding the mass of all the atoms. Its equivalent mass is the molecular mass, divided by the highest charge of the ions that will be released when it is dissolved in water. This equivalent mass will then contain mole positive and mole negative charges. As an example: The mass of 1 mole CaCl2 is 111 g (40+35.5+35.5) containing 2 mole positive (Ca2+) and 2 mole negative (Cl- & Cl-) charges.
The
equivalent mass of this salt is calculated by dividing its mole mass by two. Thus 111g divided by two = 55.5 g of CaCl2. Since low concentrations are used in nutrient solutions for soil-less crop production, a 1000 times smaller unit is used; milliequivalent (meq). Molar (M): By dissolving 1 mole KCl (74.6 g) into one litre of distilled water, the molarity of the solution will be 1 M KCl. In this case 6 x1023 positively charged ions (K+) and 6 x1023 negatively charged ions (Cl-) would be present in the water. Due to these charged particles, the solution will be able to conduct an electrical current. By dissolving only 2% of 74.6 = 1.49 g KCl per litre distilled water, the molarity of the solution will be 2% of 1 M = 0.02 M. This solution can be used as standard EC test solution to calibrate your EC-meter; it should have an EC of 2.77 mS cm-1 at 25oC. Normality (N): By dissolving the equivalent mass of a substance into one litre of distilled water (1 eq per litre or 1 eq L-1) the solution's normality will be one (1 N). The normality of acids or liquid fertilizers can be calculated with the following formula: N = (SG x C) / Eq mass N SG C Eq mass
= = = =
Normality Specific gravity of the solution Concentration as g kg-1 or (% x 10) Equivalent mass of the dissolved chemical
Transforming mg L-1 to meq L-1 It is possible to predict the EC of water or of nutrient solutions if the concentrations of all the dissolved salts are known. The EC can be predicted fairly accurately where ion concentrations are expressed as meq L-1, rather than mg L-1. See: ‘Predicting the EC of
10
a solution.’ (Page 14).
Most laboratories use mg L-1 (ppm) as unit for ion
concentrations in their reports. These values should be changed to meq L-1 by dividing it by the equivalent mass of the ion or complex ion. (See Table 5). Table 5
Concentrations expressed as mg L-1 by labs must be divided by the equivalent mass of the substance to transform it to meq L-1.
Substance
Eq. mass
Ammoinium - N Ammonium (NH4 +) Bicarbonate (HCO3–) Calcium (Ca2+ ) Carbonate (CO32-) Chloride (Cl– ) Magnesium (Mg2+)
14 18 61 20 30 35.5 12.1
Substance Nitrate - N Nitrate (NO3-) Phosphate –P Phosphate (H2PO4-) Potassium (K+) Sodium (Na+) Sulphate (SO4 2-)
Eq. mass 14 62 31 97 39 23 48
With 30 mg L-1 of Ca in feeding water the meq L-1 concentration can be calculated by dividing it by the eq. mass of Ca: 30/20 = 1.5 meq L-1. Say that the laboratory expressed nitrate (NO3-) as 124 mg L-1, or 124 ppm NO3-. This value should then be divided by the molecule’s equivalent mass (62) to express NO3- as 124/62 = 2 meq L-1. This same solution may be analysed by another laboratory, stating that the NO3--N concentration is 28 mg L-1. This is an indication that nitrate is expressed as mineral N and that it should be divided by the equivalent mass of N (14) to calculate that the nitrate-N concentration is 28/14 = 2 meq L-1.
6b
Essential nutrients
Micronutrients.
Extremely low concentrations of Fe, Mn, Zn, Cu, B and Mo are
needed by plants (Table 18). Chloride is also considered to be an essential micronutrient, but it is usually present at macronutrient quantities.
In European soil-less production
environments, concentrations of micronutrients are expressed as μmol.L-1, a unit 1000 times smaller than mmol L-1.
In South Africa, micronutrient concentrations are
expressed as mg L-1 (mg kg-1 or ppm). Macronutrients. Plants need six macronutrients: N, P, K, Ca, Mg and S. These are still seen as the only essential macronutrients, although Si and other minerals may positively affect the growth of some plants under specific conditions. Roots absorb macronutrients as ions. Four positively charged ions (cations), NH4+, K+, Ca2+, Mg2+ and three negatively charged ions (anions), NO3-, H2PO4-, and SO42- are involved. Only
11
N can be absorbed as a positively charged cation (NH4+) as well as a negatively charged anion (NO3-). Several chemicals can be used to release these ions into feeding water. The molecular mass as well as the equivalent mass of most of these water soluble fertilizers are shown in Table 6. Liquids are listed in Table 7. Table 6 Chemicals in crystalline form that may be used as macronutrient sources Chemical name
Chemical formula
Molecular Equivalent mass mass
Ammonium phosphate (MAP) NH4H2PO4 Ammonium sulphate (NH4)2SO4 Calcium carbonate CaCO3 * Calcium nitrate crystals Ca(NO3) 2.4H2O ** Calcium nitrate crystals Ca(NO3) 2.0.2(NH4NO3.10H2O) Calcium sulphate CaSO4.2H2O Magnesium nitrate Mg(NO3) 2.6H2O Magnesium sulphate MgSO4.7H2O Potassium carbonate K2CO3 Potassium chloride KCl Potassium hydroxide KOH Potassium nitrate KNO3 Potassium phosphate (MKP) KH2PO4 Potassium sulphate K2SO4 Sodium chloride NaCl
115.0 132.1 100.0 236.1 216.1 172.1 256.3 246.4 138.2 74.6 56.1 101.1 136.1 174.3 58.4
115.0 66.0 50.0 118.0 108.0 86.0 128.2 123.2 69.1 74.6 56.1 101.1 136.1 87.1 58.4
* Contains zero ammonium ** Contains ammonium
Table 7 Liquid fertilizers and acids that may be used as sources of macronutrients Chemical name
Chemical formula
Ammonium nitrate NH4NO3 Ammonium nitrate NH4NO3 Calcium nitrate Ca(NO3) 2.0.2(NH4NO3.10H2O) Magnesium nitrate Mg(NO3) 2.6H2O Nitric acid HNO3 (59%) Phosphoric acid H3PO4 (80%) Phosphoric acid H3PO4 (85%)
Eq. mass 80 80 108 128 63 98 98
X
Concentration (g kg-1) = % x 10
Y
Normality N = (SG x Concentration) / Eq. mass (page 9).
X
Concentration (g kg-1) 543 543 640 695 590 800 850
Specific gravity (g ml-1) 1.26 1.26 1.48 1.32 1.36 1.63 1.69
Y
Normality (N)
8.5 8.5 8.7 6.2 12.7 13.3 14.7
12
6c
Non-essential Na and Cl
Sodium is not essential for crop growth but most South African water sources contain some sodium. Most crop plants are salt excluders with limited uptake of Na+ by roots. Some plants do not transport sodium ions from the roots to the shoots (Epstein and Bloom, 2004).
The plant’s ability to tolerate high Na+ levels may be somewhat better at high
Ca2+ levels. This was demonstrated with saline-sensitive beans that could be grown in the presence of extremely high NaCl levels, but only with addition of relatively high Ca-levels (La-Haye and Epstein, 1971). Chloride is an essential micro nutrient but is usually present at macronutrient quantities. Please do not confuse the chloride ion (Cl -) with chlorine gas (Cl2), used by municipalities and growers as an oxidant to sterilize water. Some saline water sources contain high levels of chloride that may limit the uptake of nitrate (Kafkafi, Valoras and Letey, 1982). Fortunately the opposite is also true. Weigel et al., (1973) used feeding water with a high chloride level and by increasing the nitrate level in a nutrient solution from 1.25 to 7.0 meq L-1, found that chloride concentrations in soybean leaves dropped. See Table 19 for Na+ and Cl- absorption rates and the maximum levels tolerated by different crops (page 35).
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Basics to start with nutrient solutions
Nutrient solution recipes. By combining and evaluating the information available in the literature, Steiner (1968) made a huge contribution when he summarised basic nutritional principals for soil-less plant production. He proposed a well-balanced ‘Universal nutrient solution’ by dealing with cations- and anions separately, describing safe ratios for the cations, K+ : Ca2+ : Mg2+ as well as for anions, NO3- : H2PO4- : SO42-. He used meq L-1 as unit and expressed the ideal cation ratio as 35:45:20 % and chose 60:5:35 % as anion ratio. Some deviations from these ratios are allowed but he also defined the outer limits beyond which deficiencies or toxicities may develop (Steiner, 1968). He suggested that these ratios be used on all crops, but that saline- and draught tolerant crops need more concentrated solutions whereas water-loving crops need to be grown on low EC solutions. Before trying to compile nutrient solutions, the role of NH4+ should be clarified first since serious problems may develop when ammonium is excluded, as in Steiner’s solution.
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Ammonium (NH4+) as pH regulator. Steiner used NO3- as sole nitrogen source to grow standard greenhouse crops, even though the pH of this nutrient solution tended to increase as it moved through the root zone. Modern soil-less growers of standard greenhouse crops use some ammonium to regulate the pH in the root zone. To understand this, it should be remembered that a neutral charge is maintained in the root tissue. When a cation or anion is absorbed, the root secretes H+ or OH- respectively. Due to the fact that NH4+ is a very small ion, it is easily absorbed by roots, secreting H+ and acidifying the root zone. Using only nitrate, OH may accumulate, increasing the root zone pH. Where organic substrates such as saw dust or coir are used, bicarbonate is released during its decomposition, also increasing the alkalinity and lifting the substrate pH. An increased ammonium level may then be useful to counteract this. However, too much NH4+ and a low root zone pH may reduce the uptake of Ca and other cations.
Thus, ammonium should be carefully
managed, especially on crops sensitive to Ca deficiency disorders such as blossom-end-rot and tipburn (Combrink, 2005).
Kafkaffi (2000) claims that damage by ammonium
increases at high root zone temperatures, a problem in Israel as well as in South Africa. Ammonium (NH4+) as N-source.
Acid-loving crops need NH4+ as source of N.
Blueberries, azaleas, orchids and disas are acid- or ammonium-loving crops. In a pot trial, where ammonium levels were increased from 10% to as much as 50% of the total Napplication, the growth of disas increased linearly (Pienaar, 2005). Plants that evolved on well-aerated and normal pH soil (good nitrification), are adapted to use nitrate. Rice (waterlogged conditions), disas and blueberries (cold and acidic soil) are adapted to use ammonium-N due to the absence of nitrification under their natural conditions. The optimum ratio of ammonium to nitrate for most summer crops is ±1:10. More ammonium and lower nitrate levels should be used for acid loving crops (Disa ±1:1, Blueberry ±3:1). ‘Ready mix’ nutrient solutions. Publications by Arnon and Hoagland (1940), Steiner (1984) and Dutch research workers (de Kreij et al., 1999; Straver, et al., 1999) helped local fertilizer companies to develop ‘ready mix’ nutrient solutions. All of these ‘ready mixes’ contain some ammonium but their cation (K+ : Ca2+ : Mg2+) and anion (NO3- : H2PO4- : SO42-) ratios fall within the limits of Steiner’s ‘Universal nutrient solution.’ These products allow growers to avoid the tedious job of weighing and mixing fertilizers from several different bags, when compiling their own mixes. But, 'ready mixes' can only be used where the feeding water contains almost no dissolved salts (EC
