PL S 217 Course Notes

Chapter 8
Irrigation Systems, Fertilizers and Nutrient Solutions

INTRODUCTION

Plants can tolerate a wide range of watering and nutritional conditions…
However…for a commercial operation, the bottom line is profit which means optimizing plant growth and yield.

Optimum watering and mineral nutrition are critical for optimum plant growth.

Optimum watering and nutritional conditions can vary

• For different plant species
• For the same plant species at different times of its life cycle
• For the same plant species at different times of the year
• For the same plant species under different environmental conditions

This chapter describes:

• Properties of the nutrient solution
• The physical systems required to deliver the nutrient solution to the plants
• How to calculate how much of each compound to use

DEFINITIONS

Irrigation = The supplying of water to dry land using ditches, pipes, streams, etc.

Fertilizer = Inorganic "salts" containing the essential macro and micro elements necessary for plant growth (see Chapter 7). Also organic compounds that contain such elements (i.e., manure, fish emulsion, bat guano, etc.) that, when added to the soil or water, increase it's "fertility".

Fertigation = The use of fertilizers (usually inorganic for commercial greenhouse hydroponics and smaller systems, though some hobbyists use organic mixtures), in the appropriate combination, concentration and pH, for every irrigation cycle.

Nutrient solution recipe = A list of inorganic compounds, and their final concentrations in ppm ("parts per million" or "milligram per liter") or mMol (millimole), etc. This can also include actual amounts of the compounds needed to achieve the prescribed concentrations, given specific tank volumes, dilution factors, etc.

NUTRIENT DELIVERY SYSTEMS

Simple systems:
Non-recirculating/air gap system or the raft systems (see Chapter 5) where the roots hang down directly into the nutrient solution.
Basic wick system (see Chapter 5) in which the nutrient solution is drawn up by an absorbent wick into an aggregate where the roots grow.

Complex systems:
The flood and drain, top feeder, NFT or Aeroponic systems (see Chapter 5) all of which require pumps to move the nutrient solution from a reservoir or series of tanks to the plants via PVC, poly and drip tubing, emitters, etc.

See attached: SYSTEM DESIGN: INJECTOR SYSTEM/BAG CULTURE

NUTRIENT SOLUTIONS

The importance of good quantity/quality water for hydroponic plant production:
Any hydroponic nutrient solution begins with the "source water".
A grower can obtain source water from

• City water supply
• Private wells
• Water harvesting (channeling rain water into catchments)

The source water must have the appropriate quantity and quality.

Quantity: There must be sufficient water available for plants and for cooling.
Ex: For tomatoes in greenhouse hydroponics:

~4 liters/plant/day or if 2.5 plants/m2, then 10 liters/m2/day.
If evaporative cooling is used, especially in desert areas, water needs may be doubled!

Quality: Factors to consider include pH, EC (salt levels) and contaminants:

pH: The p(otential of) H(ydrogen): Acid or base character of the water.
pH = - log [H+] (neg. log of the H+ conc.) Scale = 0-14

Ex: If [H] = 10-7, then pH = 7 (Neutral)
If [H] = 10-4, then pH = 4 (Acidic)
If [H] = 10-9, then pH = 9 (Basic)

Ways to test the pH: Litmus paper (color change) pH meter (analog or digital)- meas. [H+] For most plants: pH 5-7. For tomatoes: 5.8 - 6.3

• Above pH 7 may cause problems with nutrient uptake.
• Below pH 5 may cause abnormal absorption of certain ions resulting in deficiencies or toxicities.

EC (Electrical conductivity): a measure of the total salts in water.
Pure water (no salts) does not conduct electricity: EC = 0.

The higher the salt levels, the higher the EC.
Measured in: TDS (total dissolved solids)
mS/cm (milli-Siemens per centimeter)
For tomatoes: EC = 2.5 - 3 mS/cm

Elevated salt levels:
Certain geographic areas have high salt levels in the water.

High boron, fluoride, chloride, sulfates and sodium:
-Can cause poor plant growth.
-May influence soluble salt levels in the water.

High iron, especially in "hard water" (having high Ca and Mg):
-Can cause rusty spots on leaves with overhead irrigation.

High salt levels can also cause rapid salt buildup on cooling pads.
-May need to bleed off and replace pad water regularly.

Heavy metal contaminants:
Certain geographic areas have high levels in the soil and/or water.

High lead, cadmium, aluminum, silver, etc.:
-May be excluded or absorbed on a limited basis by plants.
-May be absorbed and stored (but not toxic to the plants)
   Ex: Vegetables grown in Colorado mining areas contain excess lead and cadmium!
-May be toxic to the plants.

The QUALITY of the water MUST BE ASSESSED by an ANALYSIS
A sample of the source water is sent to a lab for analysis.
Ex: CropKing has a service whereby you send them a sample of your source water, they send it to a lab and get back the results, then they send you specific instructions on how to make up your nutrient solution including any adjustments for pH variations or contaminants

 

MINERAL ELEMENTS OR NUTRIENTS: 16 elements required for plant growth (see Chapter 7)
Elements from air and/or water: C, O, H
Elements from the soil/nutrient solution:

Macros: N, P, K, Ca, Mg, S
Micros: Fe, Mn, B, Zn, Cu, Mo, Cl

The 13 essential mineral elements can be obtained in the following compounds:

• MgSO4*7 H2O (Magnesium Sulfate)
• H3BO3 (Boric Acid)
• KH2PO4 (Monopotassium Phosphate)
• MnCl2*4H2O (Manganous Chloride)
• KNO3 (Potassium Nitrate)
• CuCl2*2H2O (Cupric Chloride)
• K2SO4 (Potassium Sulfate)
• MoO3 (Molybdenum trioxide)
• Ca(NO3)2 (Calcium Nitrate)
• ZnSO4*7H2O (Zinc Sulfate)
• Fe 330 - Sequestrene (chelated iron)

In solution these compounds dissociate into ionic forms (see Resh or a chem. book):
Ex: MgSO4 dissociates into the cation Mg++ and the anion SO4=
Ex: KNO3 dissociates into the cation K+ and the anion NO3-
Ex: CuCl2*2H2O dissociates into the cation Cu++, the anions 2Cl- plus 2 H2O

Nutrient interactions:
Plants maintain a balance between the cations (positively charged ions) and anions
(negatively charged ions) in their cells and tissues.
NOTE: In a chemical equation the cations are listed first, then the anions.

Plants also maintain a constant sum of cations in their cells and tissues.
Therefore, if one cation is increased, it may decrease the uptake of others.
Ex: Increasing Mg++ can cause decreases in Ca++ and calcium deficiencies.
Ex: Increasing NH4+ (to increase acidity) can cause decreases in Ca++ uptake.

Interactions between anions are not as common.
Ex: Increasing Cl- can decrease NO3- uptake and visa versa.

Nutrient uptake rates and mobilities:
Plant roots take up mineral nutrients at different rates.
Ex: NO3-, K+ and Cl- are taken up quickly; Ca+2 and SO4-2 are taken up slowly.
This results in unequal removal of nutrients from the solution.

Once in the plant different ions have different mobilities within the plant.
Ex: Mobile ions include N, K, P (PO4-2), Mg and Cl.
Deficiency symptoms for these ions usually appear in the old growth.

Slightly mobile ions include S (SO4-2), Mn and Mo.
Deficiency symptoms usually appear in the middle and old growth.

Immobile ions include Ca, B, Zn, Fe and Cu.
Deficiency symptoms for these ions usually appear in the new growth.

 

Recommended nutrient levels (ppm) according to plant species (Agrodynamics):

However, several crops can grow perfectly fine on the same nutrient solution.
U of A CEAC GH, with all three crops: N=189, P=39, K=341, Mg=48, Ca=170

Plant growth as a function of nutrient concentration in plant tissue:

Plant nutritionists, in the mid-1900's, discovered that there is a critical nutrient concentration, below which plant growth is reduced or terminated.

Above the critical nutrient concentration is the adequate zone where growth is 100% of maximum.

At high nutrient concentrations, plant growth is again reduced. This is the toxic zone.

Open (drain to waste) verses Closed (recirculating) systems:
In an open system the nutrient solution is only used once on the crop plants.
In a closed system the nutrient solution is used then recycled.

• The solution is analyzed for pH and individual nutrient concentrations.
• The solution is then adjusted using acid/base, water and/or nutrients to the appropriate pH and nutrient concentration levels.
• The solution is also sterilized to control the spread of water-borne pathogens.
• This can include UV, ozone or other treatments.
• The solution is then returned to the plants.

NUTRIENT SOLUTION CALCULATIONS

Mineral nutrients are available in several forms:

Pre-mixed liquid concentrates that are then diluted with water.
-"A" and "B" formulas that when mixed have all essential elements.

Pre-mixed powder concentrates that are then diluted with water.
-Many are a teaspoon per gallon mixes - fairly simple.

NOTE: DO NOT USE Miracle Gro - This is meant for soil culture and does not have all the essential elements.

Many commercial growers buy the individual compounds and mix the nutrient solution themselves. See above under Mineral elements or nutrients for a list of the compounds required.

Macroelements (or macronutrients) are usually purchased in 50 lb bags.
These are called horticulture grade. These need to be in a soluble form.

Microelements (needed in much smaller amounts) can be purchased as pre-mixed powders: specific for hydroponics.
Individual compounds: at least horticultural grade, but can be technical or reagent grade and need to be soluble.

PRECAUTIONS:
Note above the "A" and "B" formulas…
There is a reason…
Usually, the calcium containing compounds are kept separate from the phosphate and sulfate compounds.

Why? In high concentration the calcium will combine with the phosphates and sulfates to form insoluble precipitates.

THEREFORE:
A typical nutrient solution will be divided into 3 tanks:

1. Calcium/iron tank (iron gives it color)
2. Macro/Micro tank (all other macro and micro elements)
3. Acid tank (kept separate so pH can be adjusted individually)

A grower will start with a nutrient solution recipe:

The choice of recipes is up to the grower (many variations exist).
Choose a recipe that has been successful:

• For the plant you want to grow.
• For the regional location and environmental conditions.
• For the time of year you wish to grow.
  Our recipe: Used by Sunco, Ltd. for tomatoes in Las Vegas!
  See table below.

IF a grower notices deficiency/toxicity symptoms,
THEN adjustments to the recipe can be made to compensate.

Most recipes will vary according to stage of plant growth (Sunco recipe below).

• Ex: 0 - 6 Week recipe: Higher nitrogen, calcium and magnesium for good structure/vegetative growth.
• 6 - 12 Week recipe: Lower nitrogen and higher potassium to enhance flower (generative) production
• 12 + Week recipe: To maintain balance - vegetative/generative

Nutrient	WEEK 0-6 (ppm)	WEEK 6-12 (ppm)	WEEK 12 + (ppm)
N	224	189	189
P	47	47	39
K	281	351	341
Ca	212	190	170
Mg	65	60	48
Fe	2.00	2.00	2.00
Mn	0.55	0.55	0.55
Zn	0.33	0.33	0.33
Cu	0.05	0.05	0.05
B	0.28	0.28	0.28
Mo	0.05	0.05	0.05

 

NOTE: Sulfur (a macronutrient) and chloride (a micronutrient) concentrations are not given in this recipe. That does not mean that sulfur and chloride are not present. Usually sulfur is added with magnesium and chloride is added with the manganese and copper. Enough will be added with these other elements (see calculations below).

NOTE: Two significant changes to this type of standard recipe have recently been made by Bonita Nurseries in Willcox, AZ to improve growth of the plant and quality of fruit.

To avoid over-vegetative growth during hot fall weather, begin with no nitrogen in the nutrient solution when seedlings are put on the bags then increase to full strength.

Chlorides can be added during fruiting in macronutrient levels (150-200 ppm) to improve fruit quality and taste. Note, significant adjustments must be made to the recipe.

CALCULATING NUTRIENT SOLUTIONS (how much to add of what…):

Important factors:
The irrigation water contains ~29 ppm Ca (CAC water analysis)
1 ppm = 1 mg/l
1 gallon = 3.785 liters
2.2 pounds = 1 kg

The nutrient calculations depend on several things:

• What is the final concentration desired, in ppm, of a particular element?
• Does the source water already contain any essential elements (from water analysis)? If so, less of that nutrient will be needed.
• You know the final concentration in ppm desired for a particular element,
  BUT that element is part of a compound. SO, what is the percentage of the element in the compound?
• If you use concentrated nutrient solution stock tanks and injectors:
• What is the size of the tanks?
• What dilution factors are the injectors set for?
  NOTE: Do not round off until the end of your calculation!

From the Sunco Recipe, 12+ weeks (see above):
Always start with Calcium (it starts a "cascade" of calculations)

• Final concentration of calcium desired = 170 ppm
  The irrigation water already contains = 29 ppm
• Therefore, amount of calcium needed = 141 ppm Ca
• BUT, we don't add the element Ca, we add the compound calcium nitrate:
• The % of calcium in Ca(NO3)2 (from bag) = 19 %
• Therefore, to find the ppm required for the compound calcium nitrate:
• 141 ppm Ca / 0.19 = 742.105 ppm or 742.105 mg/l
• The nutrient tank is = 50 gallons
• BUT ppm is mg/LITER not gallons, so 50 gallons x 3.785 liters/gal = 189.25 liters

Therefore, the amount of calcium nitrate required is

742.105 mg/l x 189.25 liters = 140,443.37 mg

HOWEVER, the solution also has to go through injectors set at 1:200 dil.

Therefore, the FINAL amount of calcium nitrate required to obtain a final
calcium concentration of 141 ppm is

140,443.37 mg x 200 = 28,088,674 mg

• IF your scale is in kilograms (kg=106 mg)
  Then 28,088,674 mg / 1,000,000 mg/kg = 28.088674 kg calcium
  nitrate for 141 ppm Ca
• IF your scale is in pounds (lb)
  Then 28.088674 kg x 2.2 lb/kg = 61.795 lb calcium nitrate

OKAY… So you've added the appropriate amount of calcium nitrate to get 141
ppm of Ca…

BUT, how much nitrogen did you add? NEED TO WORK BACKWARDS!

• The final amount of calcium nitrate = 28,088,674 mg
• Divide by the dilution factor (200) = 140,443.37 mg
• Divide by 189.25 L in a 50 gal tank = 742.105 mg/L
• The amount of nitrogen in calcium nitrate = 15.5%

Therefore, 742.105 mg/L x 0.155 = 115 mg/l or 115 ppm N from calcium nitrate

• HOWEVER, the total N that is needed from the recipe (week 12+) = 189 ppm
• The difference is 189ppm - 115ppm = 74 ppm
• This 74 ppm of N will come from potassium nitrate - KNO3
• Instead of getting the % of nitrogen from the bag…
  Calculate the % of nitrogen in potassium nitrate using molecular weights:
• MWt KNO3 = K(39.1) + N(14) + 3O(3X16=48) = 101.1
• AWt N (14) / MWt KNO3 (101.1) = 0.1385 or 13.85% N

To find the ppm required for the compound potassium nitrate

• 74 ppm / 0.1385 = 534.3 ppm or 534.3 mg/l
• Take into account the tank size (50 gallons or 189.25 liters)
• 534.3 mg/l x 189.25 l = 101,116.275 mg
• Take into account the dilution factor (1:200)
• 101,116.275 x 200 = 20,223,255 mg

OR 20,223,255 mg / 106 mg/kg = 20.223255 kg of KNO3 for 74 ppm N

BUT, how much potassium did you add when you added 20 kg of KNO3?
YOU HAVE TO WORK BACKWARDS, AGAIN!

• Convert back to mg:
• 20.223255 kg x 106 mg/kg = 20,223,255 mg
• Dilution factor: 20,223,255 / 200 = 101,116.275 mg
• Tank size: 101,116.275 mg / 189.25 l = 534.3g/l
• % K in KNO3: AWt K (39.1) / MWt KNO3 (101.1)
  = 0.3867 or 38.67% K
• 0.3867 x 534.3 mg/l = 206.6 mg/l or 206.6 ppm K
  added with 20.2 Kg KNO3
• HOWEVER, the total K needed from the recipe is 341 ppm.
• The difference is 341 - 206.6 = 134.4 ppm K still needed
• To get the needed K use KH2PO4. HOWEVER, this is the only source for P. Figure the P first. Need 39 ppm P
• Figure the % P in KH2PO4 using molecular weights:
  MWt KH2PO4 = K (39.1) + 2H (2x1+2) + P (31) + 4O (4x16+64) = 136.1
  AWt P (31) / MWt KH2PO4 (136.1) = 0.2278 or 22.78% P
• ppm needed of KH2PO4 = 39 ppm P / 0.2278
  = 171.2 ppm or mg/l KH2PO4
• Tank size: 171.2 mg/l x 189.25 l = 32,399.6 mg KH2PO4
• Dilution factor: 32,399.6 x 200 = 6,479,920 mg KH2PO4
• Conversion: 6,479,920 mg / 106 mg/Kg = 6.47992 Kg KH2PO4

To figure the amount of K added from 6.47992 Kg KH2PO4, WORK BACKWARDS

• Dilution factor: 6,479,920 mg KH2PO4 / 200 = 32,399.6 mg KH2PO4
• Tank size: 32,399.6 mg KH2PO4 / 189.25 l = 171.2 mg/l KH2PO4
• %K in KH2PO4 = AWt K (39.1) / MWt KH2PO4 (136) = 0.2875 or 28.75 % K
• 171.2 mg/l KH2PO4 x 0.2875 = 49.2 mg/l or ppm of K from KH2PO4

Total K so far = K from KNO3 (206.6ppm) + K from KH2PO4 (49.2ppm)= 255.8 ppm K

HOWEVER, total K needed from recipe = 341 ppm
341 ppm K - 255.8 ppm K = 85.2 ppm K still needed. Use K2SO4.

• Figure % K in K2SO4 by using molecular weights.
  MWt K2SO4 = 2K (2x39.1=78.2) + S (32.1) + 4O (4x16=64) = 174.3
  AWt K (78.2) / MWt K2SO4 (174.3) = 0.4487 or 44.87% K
• ppm needed of K2SO4 = 85.2 ppm K / 0.4487 = 189.9 ppm or mg/l K2SO4
• Tank size:189.9 mg/l K2SO4 x 189.25 l = 35,938.575 mg K2SO4
• Dilution factor: 35,938.575 mg x 200 = 7,187,715 mg K2SO4 = 7.187715 Kg K2SO4 to get 85.2 ppm K

Final total of K = K from KNO3 (206.6 ppm) + K from KH2PO4 (49.2 ppm)
+ K from K2SO4 (85.2 ppm) = 341 ppm K

NOTE: S is also added in K2SO4. How much? WORK BACKWARDS

• Dilution factor: 7,187,715 mg K2SO4 / 200 = 35,938.575 mg K2SO4
• Tank size: 35,938.575 mg K2SO4 / 189.25 l = 189.9 mg/l or ppm K2SO4
• % S in K2SO4 = AWt S (32.1) / MWt K2SO4 (174.3) = 0.184 or 18.4% 189.9 ppm K2SO4 x 0.184 = 34.9 ppm of S from K2SO4
• Finally, calculate the amount of MgSO4 * 7H2O needed to give Mg = 48 ppm.
• From the bag, the % Mg in MgSO4 * 7H2O = 9.8% ppm needed of MgSO4 * 7H2O = 48 ppm Mg / 0.098
  = 489.8 ppm or mg/l MgSO4 * 7H2O
• Tank size: 489.8 mg/l MgSO4 * 7H2O x 189.25 l = 92,694.65 mg MgSO4 * 7H2O
• Dilution factor: 92,694.65 mg MgSO4 * 7H2O x 200 = 18,538,930 mg MgSO4 * 7H2O
• Conversion: 18,538,930 mg MgSO4 * 7H2O / 106 = 18.538930 Kg MgSO4 * 7H2O needed to supply 48 ppm Mg
• But, how much S is added? WORK BACKWARDS (ppm of S not specified)
  Added 18,538,930 mg MgSO4 * 7H2O
• Dilution factor: 18,538,930 mg MgSO4 * 7H2O / 200 = 92,694.65 mg MgSO4 * 7H2O
• Tank size: 92,694.65 mg MgSO4 * 7H2O / 189.25 l = 489.8 mg/l or ppm MgSO4 * 7H2O
• From the bag the % S in MgSO4 * 7H2O = 12.9% 489.8 ppm MgSO4 * 7H2O x 0.129 = 63.2 ppm S from 18.538930 Kg MgSO4 * 7H2O

The final amount of S added
= 63.2 ppm from MgSO4 * 7H2O + 34.9 ppm of S from K2SO4
= 98.1 ppm S

Calculations for the microelements are done the same. Always take into account the desired concentration (ppm), the percentage of the element in the compound, the tank size and the dilution factor from the injectors.

REFERENCE MATERIAL:

1. Hydroponic Vegetable Production. 1985. M.H. Jensen and W.L. Collins.
Horticultural Reviews, Vol 7: 483-558. ISBN 0-87055-492-1

2. Protected Agriculture. A Global Review. 1995. M.H. Jensen and A.J. Malter.
The International Bank for Reconstruction and Development/The World Bank.
1818 H St., NW, Washington, DC 20433. ISBN 0-8213-2930-8

3. Tailoring Nutrient Solutions to Meet the Demands of Your Plants. 1992. M. Schon. In: Proceedings of the 13th Annual Hydroponic Society of America Conference on Hydroponics. pp 1-7.

 

ceac : cea basics : pls 217 course notes (chpt 1-10) : Chapter 8