Taking Care of Plant Nutrition in Your High Tunnel – Fertilizer and Nutrient Solution Mixing Tips

Developing and mixing your nutrient solution is one of several very important steps in the production process. Mistakes are easily made when fertilizers are mixed. Not only does this have a cost implication, but also it can have severe consequences on production and therefore revenue. In the previous article Taking Care of Plant Nutrition in Your High Tunnel-Water Soluble Fertilizer Calculations, we have looked at basic calculations that is important when interpreting the fertilizer formulation in the bag and how to use that information to apply specific concentrations of mineral nutrients. In this issue, we will discuss what you need to consider when developing a nutrient program, and focus on key fertilizer characteristics and nutrient solution mixing tips.

Developing a nutrient program

A nutrient solution is just as good as the quality of the ingredients and the time spend on formulation, calculation and mixing. Fertilizer should be of a high quality and purchased from a reputable source. Make sure that you meet the specific nutrient needs of your crop. Most small greenhouse operations use 25 to 50 gallon stock tanks. Replenishment of these tanks will depend on the time of the year, the crop and the substrate used. In Nutrient Film Technique systems replacement is more regular since routine flushing of the sump tank is needed. Most production systems use two stock tanks. The reason being, certain fertilizer sources when mixed in concentrated form will form insoluble precipitates (refer to nutrient solution mixing tips).

Most N, P and K compound fertilizers are mixed in proportions that will meet the specific plant demand, and are developed for use with calcium-rich water sources. These blends may also contain other minerals and include micronutrients. Therefore, according to crop needs you might be required to add more calcium (in the form of calcium nitrate) or magnesium (in the form of magnesium sulfate or magnesium nitrate) to the nutrient solution.

Accurate calculations of the amounts of fertilizer to be added to reach the desired concentrations of individual nutrients are essential to get the most out of your fertilizer program. Premixed fertilizer might be perfect for beginner growers, but experienced growers will find that they can be more efficient and knowledgeable if they calculate their own formulations (similar to calculations we did in the previous article). To improve nutrient management, it is very helpful to know what minerals are in your start water, mixed nutrient solution, and substrate. By monitoring pH and EC, you can assure that nutrients are available in ample amounts. EC measurements are only helpful in checking total salt concentration. Make use of an analytical laboratory if you are interested to measure and know the concentration of individual elements. Remember to calibrate your meters regularly. Bad information leads to bad decisions.

Key Fertilizer Characteristics and Nutrient Solution Mixing Tips

  • Use high-quality ingredients from a reputable source for your nutrient solution.
  • Use a fertilizer specifically developed for fertigation.
  • Check maximum solubility.
  • If using a compound fertilizer, make sure the blend has a tag that shows the analysis of the fertilizer, the sources used and company’s name.
  • Do accurate calculations and use accurate scales to weigh each material.
  • The pH of the stock solutions need to be lower than 5 to make sure all the fertilizers would be completely dissolved.
  • Chelates are sensitive to low pH levels in the tanks. At a pH of 3.5 or lower, the chelate structure will break down (especially true for EDDHA and HBED chelates). EDTA, DTPA and HEDTA chelates are stable at pH 1.5-6.0, 1.5-7.0, and 2.5-7.0, respectively. EDDHA and HBED chelates are stable at pH 3.5-9.0 and 3.5-12.0, respectively.
  • Protect chelates from exposure to daylight. Chelates are sensitive to light, hydrogen peroxide, UV and ozone. UV and ozone will break down the chelate structures to some extent, and therefore replacing chelates after disinfection is recommended.
  • Make sure compatible fertilizer are mixed in the same tank. Insoluble precipitates will form when mixed in concentrated form. Fertilizer sources containing calcium, phosphorus and sulfate should not be mixed together. Insoluble precipitate formation of calcium phosphate (mixing calcium nitrate and phosphorus containing materials) and calcium sulfate (mixing calcium nitrate and magnesium sulfate or potassium sulfate) will occur.
  • Two-tank system: mix calcium, iron and half of the total potassium nitrate in one tank and the rest of the macro- and micronutrients in the other tank.
  • Lukewarm to hot water will speed up the time for dissolution of the fertilizer.
  • While small batches of fertilizer is added to the stock, the total stock tanks is brought to the desired level and stirred.
  • Stir while mixing the fertilizer, manually or electrical. Manual stirring is satisfactory for smaller batches, but large batches will require an electrical agitator.
  • It will take a few hours for the solution to become clear, but a sludge will often form in the bottom of the tank. This sludge is from certain additives in some fertilizers that are added to prevent caking and dust. Since these materials are not soluble, they will settle in the bottom of the tank and will need to be rinsed out periodically.
  • Keep solution in a dark environment.

This is the fourth article in a 7 part series that look at high tunnel nutrient management. In the next issue, we will concentrate on ‘Fertilizer Injection’.

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