The nuances of crop fertilization

Blog Exclusives from Urban Ag News

Originally published in Issue 5

Regardless of the crop being grown or the irrigation system being used, growers face the same issues related to water quality, nutrient delivery and nutrient uptake.

One size doesn’t fit all when it comes to fertilizing plants. Regardless of the crop being grown, whether ornamental or vegetable, different species have a nuance in what they require when it comes to fertilization, said George Murray, tree crop and horticulture specialist at Brandt Consolidated, Inc.

“Fertilization requirements can depend on the cultivar, on the growing conditions, on the substrate and the pH,” Murray said. “Once you start looking at specific crops and the different stages of the life cycle of a crop, you’re going to have to look at making changes to the nutrient solution.

“For leafy greens you can use the same fertilization solution through the whole production cycle from day one. There is no reproductive cycle that the crop is going through. The crop may start out at a lighter concentration when the plants are seedlings and then increase the rate when the plants are put out into the production system. For crops like tomatoes and cucumbers, a grower will switch to different nutrient solutions as plants move through the different stages of a crop’s life.”

Simple vs. sophisticated

Murray said the way growers fertilize their crops can vary from simple delivery equipment to sophisticated automated systems.

“Some growers use very simple fertilizer delivery systems and others have very high tech systems that are computerized and automated,” he said. “Bottom line, as long as you are able to control your parameters such as the parts per million and the nutritional output, basically the plant isn’t going to know the difference. What will make a difference is the labor you are putting into it, the input costs, and the costs it takes to maintain the equipment or system. The input costs and the costs it takes to maintain the equipment or system. The plant doesn’t know how the fertilizer is delivered and what kind of technology is used.”

George Murray said as long as growers are able to control parameters such as the parts per million and the nutritional output, plants aren’t going to know the difference in the type of fertilizer delivery system.

Murray said those growers who are using automated systems know the parameters that they need to be at and depend on the technology they’ve installed to replicate those set points.

“A grower has to be at a certain size in order to cover the overhead expenses, those fixed costs, for that technology,” he said. “For the grower who prefers to grow by “feel,” there are probably going to be times during the production cycle where there is over fertilization as well as under fertilization. These growers are also not going to know what their true costs are, both in materials and labor. Consequently, these growers may not make as much money as they could.”

Murray said that technology can also have its limits.

“When trying to determine what is a causing a problem with a crop, in some cases, the grower who is growing by “feel” might have a better understanding of the science behind what problems to look for,” he said. “The grower who is producing on a large scale with computer inputs may be too far removed from a crop to understand what is going on from a nutritional or disease standpoint. It’s really a case by case situation.”

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Impact of water quality

Murray said the biggest problem growers have regardless of their irrigation system or level of greenhouse technology is water quality.

“The water pH dictates so much of what ends up in the plant and in what quantity,” he said. “With a lot of the fertilizer mixes when there is a high water pH, the micronutrients are not going to be available to the crop. It is really important since growers are using a lot of nitrogen-based fertilizers which tend to raise the pH of the rhizosphere, which is the area around the plant roots. What are the nutritional sources as well and how do they react separately and to the environment? Sulfate-based micronutrients are not protected as well as EDTA-based micronutrients and will be less available to plants when the pH starts to climb.

Water pH dictates so much of what ends up in plants and in what quantity. A high water pH can limit the micronutrients that are going to be available to a crop. Changes to the water pH are going to occur more quickly in hydroponic situations. Symptoms of those changes also will appear more quickly.

“Growers looking at installing a closed loop irrigation system need to first look at the quality of their water source. Water quality is going to affect what nutrients the plants are going to take up and how much they are going to take up. Sometimes nutrients are not in a format that is readily available to the plants.”

George Murray said growers need to know:

  • What nutrient levels are in the water. • How much of the nutrients will have to be added.
  • What nutrients are available to the plants that are in the water.
  • How much of a nutrient is going to have to be added in.

“Growers who are using more of a traditional watering system like drip irrigation with a soilless growing medium, there is some buffering of the pH in that medium,” he said. “Changes to the pH are going to occur a lot more quickly in hydroponic situations and symptoms of those changes also will appear more quickly. A closed loop system is kind of a double-edged sword. There aren’t concerns with a poor substrate structure and those types of issues. But at the same time there aren’t any buffers or safeguards either.”

Maintaining a nutrient balance

Murray said once a grower has resolved any issues with water quality, then the nutrition-related factors the grower would be dealing are similar regardless of the crop grown or the type of irrigation system used.

“The amount of calcium in water tends to be relatively high,” he said. “Phosphorus interacts with calcium, magnesium and iron in the soil. A concern is what happens to that interaction in a closed loop system without a substrate. Up until now most of the research that has been done with the phosphorus cycle has been done with phosphorus in the soil. What needs to be determined is how does phosphorus move in a closed loop system.

Murray said growers need to be sure that they maintain a balance of micronutrients and macronutrients.

Growers looking at installing a closed loop irrigation system need to first look at the quality of their water source. Water quality is going to affect what nutrients the plants are going to take up and how much they are going to take up.

“There are certain sites on the roots where certain micronutrients and macronutrients are taken up,” he said. “Zinc, copper and nickel have the same uptake sites in the roots. If you have a higher percentage of one micronutrient and a lower percentage of another, say copper and zinc, if they are out of balance then deficiencies of one or the other occurs relatively quickly. From a closed system perspective, even though the soil may have been eliminated, the mechanics of the plant haven’t been eliminated. The plants are still taking up nutrients through the roots in the same way. Although a closed loop system may be more efficient and sustainable, growers need to understand that they have not eliminated the hurdles of the plant genetics, and more specifically how certain micronutrients move through a plant’s vascular system.”

Even with a closed loop system, Murray said there are still opportunities for foliar applications of nutrients when crops are flowering and fruiting.

“Boron is important in cell wall formation,” he said. “Boron can be in the water solution, but it can be immobile in the plants. Once boron is taken up by a plant, is it going to be where it needs to be when it needs to be there? Calcium deposition is also important in the formation of cell walls, especially in tomatoes. Just because there is calcium in the water doesn’t mean that it is going to get into tomato plants at the right time and be in the right place.”


For more: George Murray, Brandt Consolidated, Inc.; (812) 701-4076;;

David Kuack is a freelance technical writer in Fort Worth, Texas;

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