Fertilizers

Jump to Subsections:
The Elements of Life
Choosing the Right Fertilizer Solution
Conventions in Fertilizer Labelling
Units of Measure
Recipes for Hydroponic Fertilizer Solutions

The Elements of Life

All organisms need an variety of elements to grow and thrive.  The primary element used by most life forms on earth is carbon, and plants get theirs by pulling it out of the air in the form of carbon dioxide (CO₂).  Plants grown in the ground get other necessary elements directly from the soil, but in the Bucket Hydroponics system, these elements are mixed into the hydroponic fluid in the form of inorganic salts.

Macronutrients

• Nitrogen (N)
• Phosphorus (P)
• Potassium (K)

The amounts of the macronutrients are the most important part of any fertilizer blend, as they determine the overall growth characteristics of the plant.  Whenever you see 3 numbers on the front of a bag of fertilizer, these refer to the amounts of N-P-K.  Note that many fertilizer blends will supply nitrogen in up to 3 forms:  nitrate, ammonium, and urea.  In hydroponics, plants should be getting the majority of their nitrogen in the nitrate form, while the ammonium and urea forms should be kept to a minimum.

Secondary nutrients

• Calcium (Ca)
• Magnesium (Mg)
• Sulfur (S)

Micronutrients

• Chlorine (Cl)
• Iron (Fe)
• Boron (B)
• Manganese (Mn)
• Zinc (Zn)
• Copper (Cu)
• Molybdenum (Mo)
• Nickel (Ni)
• Cobalt (Co)
• Silicon (Si)

Choosing the Right Fertilizer Solution

How much of each nutrient should we provide in the hydroponic solution?  For general growing, I have relied upon 4 main sources of information in determining how much of each nutrient to provide:

• • Dr. Bernard Kratky, pioneer of non-recirculating hydroponics in containers
  • Dr. Howard Resh, hydroponics expert
  • the widely-known “Lucas Formula”
  • Bobby Smith, also known as MHPgardener on YouTube

Here are the recommended concentrations for each nutrient (in parts-per-million or ppm):

	N	P	K	Ca	Mg	S	Cl	Fe	B	Mn	Zn	Cu	Mo	Ni	Co	Si
Kratky	112	31	142	90	28			2.0	1.0	1.0	0.2	0.2	0.1
Resh	180	50	210	180	40			3.0	0.5	0.5	0.1	0.1	0.1
Lucas	106	211	190	106	63	42		2.1	0.2	1.1	0.4	0.2	0.02
MHP	103	41	168	100	29	34		2.1	1.1	1.1	0.3	0.3	0.05

The MHP recipe is similar to the recipe that I outlined in the Step-By-Step Guide.  I have found that it has worked pretty well for me in growing a wide variety of plants.  If you would like more information about the MHP recipe, check out Bobby’s 10-minute instructional video on YouTube.

Keep in mind that there is no single “ideal” recipe for all hydroponics.  Different plants will have different requirements, and what works well for one type of plant may not work well for another.  Even considering the life cycle of a single plant, a high amount of nitrogen may be best during the early vegetative growth stages, but then a higher amount of phosphorus may be more beneficial during the flowering/fruiting stages.

You should also be aware that the type of water you are using can influence the final concentration of certain nutrients.  As discussed on the Water Quality page, groundwater can contain significant amounts of calcium and magnesium, so you may want to compensate by decreasing the amount of added fertilizer.

Conventions in Fertilizer Labelling

MasterBlend 4-18-38 in U.S.A.

In various countries, the N-P-K values found on the front of a package of fertilizer can have different meanings.  Let us consider the common fertilizer MasterBlend 4-18-38, and how it would be labelled using the various conventions.

In the U.S.A., the first number on the label refers to the percentage of elemental nitrogen (N) by weight.  For MasterBlend 4-18-38, there is 4% Total Nitrogen, composed of both nitrate and ammoniacal nitrogen.  The second number refers to 18% Available Phosphate (P2O5).  To obtain the percentage of elemental phosphorus (P), we need to perform a conversion.  Considering that P2O5 is composed of 43.6% phosphorus and 56.4% oxygen by weight, the percentage of elemental phosphorus (P) by weight in MasterBlend 4-18-38 is therefore 18% * 43.6% = 7.848%.  The third number refers to 38% Soluble Potash (K2O).  Considering that K2O is composed of 83% potassium and 17% oxygen by weight, the percentage of elemental potassium (K) by weight is therefore 38% * 83% = 31.54%.

In the U.K., fertilizers are labelled slightly differently.  The first 3 numbers are the same as the American N-P-K convention, but then the elemental mass percentages follow in parentheses.  Thus, MasterBlend would be labelled as 4-18-38 (4-8-32), rounded to the nearest whole numbers.

In Australia, fertilizers are explicitly labelled with the elemental mass percentages, plus a fourth number denoting the percentage of sulfur (S).  Thus, MasterBlend would be labelled 4-8-32-0.

If you are interested in quickly converting your fertilizer nutrient mass percentages between oxides (such as P2O5) and elementals (such as P), please visit the online conversion calculator from Yara.

Units of Measure

Fertilizers come in either a dry powdered form, or a liquid form in which the inorganic salts are already dissolved in water.  It can be a bit confusing when comparing the amounts of nutrients in different types of fertilizers.  To make it simpler, I normalize everything by considering each nutrient in terms of parts-per-million (ppm) in the final hydroponic solution.  Since there are several different units of measure commonly used, the following relationships are helpful in calculating the final ppm of each nutrient:

• 1 ppm  = 1 mg/L = 1 milligram / liter = 0.001 g/L
• 1 % = 10 g/L = 10 grams / liter
• 1 % = 10,000 ppm
• 1 gallon (gal) = 3.78541 L (liters)

As an example, let’s say you want to dissolve 2 grams of MasterBlend 4-18-38 in 1 gallon of water.  MasterBlend 4-18-38 dry fertilizer contains 4% Total Nitrogen (N), so if you weigh out 2 grams of the fertilizer, it will contain:

2 grams x 4% = 0.08 grams N

If you then dissolve the 2 grams of the MasterBlend into 1 gallon of water, the final concentration of N will be:

0.08 g / 1 gal = 0.08 g / 3.78541 L = 0.021 g/L = 0.0021% = 21 ppm N

Let’s extend this example and calculate how much P is in the same solution.  MasterBlend 4-18-38 contains 18% Available Phosphate (P2O5).  Remember, the elemental mass percentage of P in P2O5 is 43.6%, so if you weigh out 2 grams of the fertilizer, it will contain:

2 grams x 18% x 43.6% = 0.157 grams P

So the final concentration of P is then:

0.157 g / 1 gal = 0.157 g / 3.78541 L = 0.041 g/L = 0.0041% = 41 ppm P

With liquid fertilizers, the situation is a bit less complicated, since you are simply making a dilution of the concentrated fertilizer with water.  For example, let’s look at Dyna-Gro GROW 7-9-5, which contains 7% Total Nitrogen (N).  Using the relationship 1% = 10,000 ppm, GROW 7-9-5 contains 70,000 ppm N.  If we take 1 mL of GROW and dilute it into 1 gallon of water, this will result in a 3785-fold dilution, which results in the following final concentration:

70,000 ppm / 3785 = 18 ppm N

Recipes for Hydroponic Fertilizer Solutions

Seedling Blend, using GROW 7-9-5:
1 gallon water (3.785 L)
1.2 mL GROW 7-9-5 (Dyna-Gro)
1.2 mL pH Down (General Hydroponics)
pH = 6.0
Nutrient concentrations (in ppm):

N	P	K	Ca	Mg	S	Cl	Fe	B	Mn	Zn	Cu	Mo	Ni	Co	Si
22	12	13	6	1.6	0.2	0.3	0.3	0.1	0.2	0.2	0.2	.003	.0003	.006

Regular-Strength Blend, using MasterBlend 4-18-38:
5 gallons water (18.93 L)
10 g MasterBlend 4-18-38
10 g calcium nitrate
5 g magnesium sulfate (epsom salt)
1 mL Pro-TeKt (Dyna-Gro)
5 mL pH Down (General Hydroponics)
pH = 6.0
Nutrient concentrations (in ppm):

N	P	K	Ca	Mg	S	Cl	Fe	B	Mn	Zn	Cu	Mo	Ni	Co	Si
103	41	168	100	28	34		2.1	1.1	1.1	0.3	0.3	0.05			1.9

Extra-Strength Blend, using MasterBlend 4-18-38:
5 gallons water (18.93 L)
12 g MasterBlend 4-18-38
12 g calcium nitrate
6 g magnesium sulfate (epsom salt)
1 mL Pro-TeKt (Dyna-Gro)
5 mL pH Down (General Hydroponics)
pH = 6.0
Nutrient concentrations (in ppm):

N	P	K	Ca	Mg	S	Cl	Fe	B	Mn	Zn	Cu	Mo	Ni	Co	Si
124	50	202	120	34	41		2.5	1.3	1.3	0.3	0.3	0.06			1.9

Bloom Blend, using MasterBlend 4-18-38:
5 gallons water (18.93 L)
12 g MasterBlend 4-18-38
6 g calcium nitrate
6 g magnesium sulfate (epsom salt)
1 mL Pro-TeKt (Dyna-Gro)
5 mL pH Down (General Hydroponics)
pH = 6.0
Nutrient concentrations (in ppm):

N	P	K	Ca	Mg	S	Cl	Fe	B	Mn	Zn	Cu	Mo	Ni	Co	Si
74	50	202	60	34	41		2.5	1.3	1.3	0.3	0.3	0.06			1.9

Jungle Juice Blend:
5 gallons water (18.93 L)
25 mL Jungle Juice Grow (Advanced Nutrients)
25 mL Jungle Juice Bloom (Advanced Nutrients)
25 mL Jungle Juice Micro (Advanced Nutrients)
2.5 g calcium nitrate
1 mL Pro-TeKt (Dyna-Gro)
5 mL pH Down (General Hydroponics)
pH = 6.0
Nutrient concentrations (in ppm):

N	P	K	Ca	Mg	S	Cl	Fe	B	Mn	Zn	Cu	Mo	Ni	Co	Si
113	35	122	91	26	13		1.3	0.1	0.7	0.2	0.1	0.01		.007	1.9

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