It’s not that hard to do once you understand some fertilizer basics.
By Deidre Hughes
Mixing your own fertilizer solutions might seem like a daunting task at first. But once you understand some fertilizer basics you’ll realize it’s not that hard to do. One of the biggest benefits of mixing your own fertilizer solutions is the amount of money you’ll save. Another benefit of mixing your own solutions from dry fertilizers is that it requires less storage space than pre-mixed fertilizers which are often in liquid form.
Be sure the fertilizers you purchase are soluble and are compatible for use in hydroponic production systems. Hort Americas offers a variety of fertilizers that can be used with different systems. These fertilizers can be purchased on the Hort Americas’ website. Use code FERTILIZERS5 to receive a 5% discount on your online order!
Where to begin mixing your own fertilizer solutions depends on the type of hydroponic system you are using. For the most part, smaller water reservoirs can be treated the same. However, larger deep water culture systems may require a few more components which will be discussed later in this article.
You need to be sure you have calibrated pH and EC meters before you start mixing your own fertilizer solutions. It is important to continue to calibrate your pH meters weekly to ensure correct readings. Calibration solutions can be purchased through Hort Americas.
What is EC?
EC stands for electrical conductivity. It is the reading that indicates the level of total dissolved salts (both good and bad) in your water. You should test your source water to ensure it is not above 0.5. If there is a high level of dissolved salts in the water, consider using a filtration system or finding a different source of water.
If you are growing leafy greens, lettuce or herbs in a nutrient film technique (NFT) system, a GrowRack or other small ebb-and-flood table, prepare a stock solution that is a concentrated mix of fertilizers. This makes it easy to amend the fertilizer solution when you are topping off the reservoir with water. A concentrated fertilizer solution can be made following these steps:
- Obtain two clean 5-gallon buckets with lids.
- Fill each bucket with 4 gallons of water, carefully measuring the water with a flow meter. If warm water is available this will help the nutrients dissolve more quickly. However, warm water is not necessary to prepare fertilizer solutions.
- Label one bucket Stock A, and the other Stock B.
- Carefully measure 1,432 grams of calcium nitrate and add it to the Stock A bucket. Mix the fertilizer solution thoroughly using a drill with a paint or concrete stirring attachment.
- Carefully measure 824 grams of Hort Americas 9-7-37 Hydroponic Fertilizer and 668 grams of magnesium sulfate and add them to the Stock B bucket. Mix the same way as the Stock A solution.
- Place a 4- to 5-inch round air stone in each bucket and attach a two-outlet air pump to keep the solutions agitated.
- Cover the buckets with lids and place them in a location out of direct sunlight to prevent algae growth.
If the water pH is above 6.5 adjust it down to about 6.5 using pH Down liquid. The pH will be fine if it is kept between 5.5 and 6.5. If the pH is too high or too low it can cause some nutrients to be unavailable to your plants. You can have the best fertilizer regimen in the world, but if you don’t keep the nutrient solution pH where it needs to be it will not matter.
Once the water pH is adjusted you can begin adding the fertilizer stock solutions. Always add equal parts of Stock A and Stock B. Add a small amount of the stock solutions at a time until an EC between 1.5 and 2.0 is reached. For example, a 40-gallon water reservoir should receive about 350 milliliters (ml) each of the Stock A and Stock B solutions to raise the EC by about 0.3.
Starting with 40 gallons of city water which had a starting EC of 0.4, a pH of 7.7 and temperature of 77°F, adding 350 ml each of Stock A and Stock B the EC was raised to 0.7. After adding another 350 ml each of Stock A and Stock B the EC was raised to 1.0. The pH was not adjusted prior to changing the EC to provide an example of how nutrients can affect the pH. The ending pH was 7.2 when the fertilizer solution was raised to 1.0 EC. This is why it is suggested to bring the pH down to no less than 6.5 before adding nutrients. Microgreens should be grown on the lower end of the EC recommendations, and as low as 1.0 EC.
The best advice for mixing your own fertilizer solutions is to take your time. It’s easier to keep adding a small amount of each stock solution, circulate the solution through the system to mix well, test again and add more stock solution if needed. This is better than having to dump your reservoir and start all over because you’ve added too much stock solution.
This also applies to adjusting the pH–always take your time. Keep good records of how much of each stock solution, gallons of water and pH Down or pH Up liquid you are adding. This will provide a point of reference when mixing future solutions that will save you time. It can also show you what may not be working for your crop.
See, it’s pretty easy! You can make it as complicated as you want, but for the hobbyist or small grower, it just doesn’t have to be.
Growing in a deep water pond production system
Growing in a large recirculating system means trying to balance all of the nutrients so that they don’t become deficient or toxic.
Smaller reservoirs evaporate more quickly so that fresh water is added more often. Smaller reservoirs are easy to dump regularly if needed. But with deep water culture ponds, dumping 5,000 or more gallons regularly isn’t practical and is a waste of water, time and money.
Before building a pond the first step should always be to send a source water sample to a lab for analysis to ensure the water is suitable for hydroponic production. A water analysis lab can be found using an internet search. The lab will provide a form and instructions for submitting a water sample. A water sample analysis generally costs $30-$50.
You will not be creating a stock solution for this amount of water, so you want to be able to account for any nutrients already present in your water source such as calcium. You also need to pay attention to the sodium, chloride and alkalinity.
Alkalinity can act as a buffer to stabilize the pH, but when it is very high it can require more acid to keep the pH in range. Acid will have to be used regularly as the pH will constantly want to creep up. Ideally the alkalinity should be no more than 150 parts per million (ppm). Sodium and chloride should not exceed 100 ppm combined.
Below is an example of a source water lab analysis:
Once you have determined that your water is suitable for hydroponic production, it is time to create your fertilizer recipe. There are many online resources for fertilizer recipes for different crops. For this example we will use the range of observed rates from the Hort Americas demonstration greenhouse in Dallas, Texas.
Listed above are the final ppm rates we are trying to achieve while taking into account what is already in the source water.
Starting with a 10- x 20-foot by 18-inch deep pond that is filled with about 2,244 gallons of water, the ppm of nutrients from the source water is already in the pond. Now it needs to be determined what to add to reach the acceptable nutrient ppm rates for the crop.
Starting with Hort Americas 9-7-37 leafy greens hydroponic fertilizer blend as the base for the recipe makes things easier as it contains all of the micronutrients. Look at the guaranteed analysis for any fertilizer you choose to use. This will always be your guide for your fertilizer solution calculations.
This is the fertilizer label analysis (each number is total percentage by weight):
Before using these numbers P2O5 and K2O must be converted to their elemental forms.
The formulas for the conversions are:
To convert K2O to K multiply by 0.83.
To convert P2O5 to P multiply by 0.44.
37% x 0.83= 30.71% K
7% x 0.44= 3.08% P
We will use milligrams per liter (mg/L) and will later convert to pounds per gallon (lb/gal). 1 mg/L is equal to 1 ppm.
To begin calculating how much fertilizer is needed, first determine the final ppm you’d like to achieve. Potassium (K) is the element that is the highest percentage of the fertilizer blend, which is 30.71% as you can see in the table below. A target of 180 ppm K has been chosen.
The formula used is: target ppm/% of the element in the fertilizer blend. In this case it would be 180 ppm/0.3071= 586 mg of Hort Americas 9-7-37 needed per liter of water.
Next, let’s determine the impact on the other elements in the blend if this amount of potassium is used. To do this, multiply the milligrams by the percent for each remaining element and then add the source water ppm for total ppm.
After completing the calculations the micronutrient manganese (Mn) is high, but it’s not high enough to be concerned about. The next step is to use other fertilizer products to ensure N, P, Ca and Mg are at the levels where they need to be.
These three products can be used:
- Calcium nitrate (15.5 % N, 19% Ca)
- Magnesium sulfate (9.6% Mg, 13% S)
- Monopotassium phosphate (22.7% P, 28.7% K)
There are 53.97 ppm total N from the source water. An additional 140 ppm N are added for a total of 193.97 ppm.
Next let’s raise phosphorus to an acceptable level of 70 ppm with monopotassium phosphate. There are already 18.4 ppm of phosphorus available. Be careful here, because If too much potassium is added, it can inhibit the uptake of magnesium and calcium.
Lastly magnesium sulfate is added. Aiming for a level of about 60 ppm Mg total, 55 ppm magnesium sulfate needs to be added to the already existing 4.31 ppm Mg.
Now compare the final ppm to the acceptable range we were aiming for:
If the numbers are within range the mg/L need to be converted to pounds required for the pond.
Here are some basic conversions:
1 gallon = 3.785 liters
1 ounce = 28.35 grams
1 gram = 0.03527 ounces
1 pound = 454 grams
1 pound = 0.45 kilograms
1 pound = 453,592 milligrams
1 kilogram = 2.205 lb
1 kilogram = 1,000 grams
1 gram = 1,000 milligrams
1 ppm = 1 mg/L
586 mg/L Hort Americas 9-7-37
903 mg/L calcium nitrate
573 mg/L magnesium sulfate
220 mg/L monopotassium phosphate
The 2,244 gallons of water needs to be converted to total liters.
Total gallons x 3.785= total liters
2,244 gal x 3.785 = 8,493.54 L
To convert from mg/L to pounds needed for the reservoir use this formula:
(total liters x mg/L needed)/1,000/1,000 x 2.205
(8,493.54 L x 586 mg/L)/1,000/1,000 x 2.205
(4,977,214.44)/1,000/1,000 x 2.205= 10.97 lbs Hort Americas 9-7-37 needed
Repeat the conversion from mg/L for each fertilizer product.
You will need a scale, a large measuring bowl and a 5-gallon bucket. Fill the bucket with water to about ⅓ full. Weigh the first fertilizer and add it to the water in the bucket. Mix all the fertilizers separately. Always add the fertilizer to the water, NOT to an empty bucket. Fill the rest of the bucket with water and mix well with a drill and stirrer attachment. Once the fertilizer is completely dissolved, it can be added to the pond. Repeat this procedure for each fertilizer ingredient.
Send a water sample to a testing lab once every month or two. At that time you can work through the steps again to find out what nutrients need to be added back into the pond. There are many products available in different combinations to choose from to fit your fertilizer needs.
*Note: This was an example of how to mix fertilizers using a wide range of observed rates and is not a guideline to follow specifically for final ppm. Please research recommended rates for your specific crops.