Tips to Improve Plant Health With Fertilizer
Healthy roots are the foundation of healthy cannabis. Growers can prevent overfeeding or underfeeding plants by measuring the salinity of the rootzone of their crops.
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Tips to Improve Plant Health With Fertilizer

How to troubleshoot nutrient imbalances and determine when and how much to feed plants.


Many factors affect plant growth and development when it comes to cultivation, such as nutrition, soil, lighting, water, temperature and humidity.

There are certain steps growers can take when monitoring plants' nutrition, pH balance and water intake to get better results, says Ian Bateman, who works in professional technical services at Hawthorne Gardening Company's Horticulture Division.

For optimal growth, plants must have all 14 plant essential nutrients in appropriate quantities to support balanced healthy growth; however, growers should not think of nutrients in isolation, Bateman says. Instead, he encourages growers to consider nutrients as interrelated "building materials" that work hand-in-hand with other inputs that help run the “machinery” of a plant. 

If a plant lacks essential nutrients, growers can utilize fertilizer to restore what's missing, but over- or underfeeding can cause damage or even kill plants, he says.

Here, Bateman describes how to use fertilizer to improve plant health, as well as determining when and what to feed them.

Flex Fertilizer Levels

Using fertilizer can help restore and balance plants' essential nutrients; however, growers should tailor the type of fertilizer, the quantity and the application process based on the plant's needs.

For example, the Flora Series  consists of three fertilizers that contain the “meat and potatoes” of what plants need: FloraMicro, FloraGro, and FloraBloom. When used in conjunction at targeted phases during the plant’s lifecycle, they provide plants with all 14 essential nutrients, Bateman says.

"What makes it unique is how we partition the nutrients into those three parts in a very strategic way," Bateman says. "For example, the FloraMicro contains calcium, nitrogen and all the micronutrients. So, those are the basic nutrients that plants are always going to need. And then, we have FloraGro, which contains primary and secondary nutrients to support plants' vegetative part of their life cycle. Finally, there's FloraBloom; it doesn't have any nitrogen, but it contains all the other primary and secondary nutrients, and it's higher in things like phosphorus and potassium. So, that's best for fruiting and flowering plants.”

The ideal fertilizer levels depend on the type of crop and how cultivators are growing them. For example, when growing high value crops like cannabis or hemp, utilizing available feed charts can help growers zero in on a strong starting point, meaning growers can spend less time having to fix nutrient disorders, and instead simply adjust their fertilizers to the right concentration based on their crop demand.    

Leverage General Hydroponics Feed Charts

Feed charts provide recommendations and guidelines designed to help guide farmers to grow a healthy crop based on plants that are considered light, medium and aggressive feeders, he says, after discovering through research that the appropriate feed concentrations can vary. Feed charts also include recommendations based on pot size, planting density and irrigation frequency so growers can further refine their nutrient concentrations for best results.

"If you have a light-feeding plant, it's not so much the nutrient ratios are off (when using GH feed-charts); it's just that plants simply can't handle the overall concentration of food," he says.

The feed charts, designed around the Flora Series, are meant to be versatile to encompass the needs of 80% to 90% of growers, he says.

For growers who are trying to determine their nutrient needs, Bateman suggests starting with a medium feed rate and keeping track of whether the plant is utilizing the nutrients.

© Courtesy of Hawthorne

Hawthorne General Hydroponics Feedcharts detail when a light, medium or aggressive feeding is most appropriate. 

"A good general rule to follow if you have never grown before, it is better to underfeed than overfeed your plants to start, obviously depending on the severity of it," he says. "If you overfeed your plants, you can't take the nutrients out of the plant. Nutrients are just tools for the plants to do things, so if you overload the nutrients, the plant has to store the nutrients, but they can only store so much. So, once you exceed that threshold, you'll start to see leaves die, spots on the leaves, or damage.

"But let's look at the opposite," he adds. "Let's say I underfed my plant by 20%, and the leaves are a little bit pale yellow, or the plants aren't growing as fast as they should. Well, that is a little bit easier to correct. You would just have to increase your feed by 20%. So, that's why I suggest starting with a medium [feed rate], and if you need to add more, just add more, but you can't take the nutrients out of the plants once they’re in there."

Although Bateman suggests starting with a medium feed rate, he says it's important to monitor the plant’s response to the feed rate throughout its lifecycle.

"You can go up and down in the feed strength during your crop's life depending on what the crop is telling you," he says. "Growers can go up to the aggressive feed rate, then go down to the light and then back up to the medium, etc."

Growers don't want to over-deliver or under-deliver nutrients. The goal is to give crops precisely what they need, which can be done by measuring nutrient levels and also checking key indicators like electrical conductivity (EC) and pH, he says.

Measure Rootzone Salinity Weekly

Growers can prevent overfeeding or underfeeding plants by measuring the salinity of the rootzone of their crops. They can use a conductivity meter, which reads in units of electrical conductivity (EC: mS/cm), he says.

Rootzone salinity influences plants' ability to absorb water. Highly saline rootzones can dry out roots and inhibit water and nutrient uptake, which is precisely why overapplying nutrients can be highly counterproductive, Bateman says.

If a grower waters plants with a gallon of water per day, about 10% to 20% of that water or leachate should come out of the bottom of the pot. This is to mitigate excessive salt buildup which can then be used to measure the rootzone salinity. However, the volume and frequency of irrigation will influence the leachate percent, Bateman says. The water holding capacity of the substrate will also influence this, as well. For example, heavy bark or peat dominant mixes will generally hold more water than a light, aerated mix like coco coir. Taller pots will tend to drain more readily than short, squat pots.  

"If the rootzone salinity level is significantly higher than the EC of the nutrient solution the grower has been applying, then we can infer that the plant is not using all the nutrients inside the pot. They're building up, and when you pour some nutrients through the pot, it pulls some of those salts," he says. "And the opposite is also true. If the rootzone salinity is low, then we can infer that the plant is using up all the salts in the pot, and there's almost nothing left behind, and then the plants could go hungry."

Bateman suggests growers check rootzone salinity weekly to give them precisely the right amount of food. They can accomplish this by simply collecting runoff leachate and measuring the conductivity of that solution, then comparing it to their nutrient solution EC, he says.

“For example, if a grower is using a medium growth feed chart and is in early bloom, EC is going to be about 1.8," he says. "So, you make up your nutrient solution, pour it over your plants, a little bit drains out of the bottom, and then you measure that, and it comes to 3.0, then you know there's almost double the nutrients in the pot."

If that were to occur, the grower could flush the plant out with a reduced strength nutrient solution to reduce the rootzone salinity and try to get them back to baseline, he says.

For new growers, using a measuring tool such as a conductivity meter is a simple way to understand if your crops are being over or underfed.

Measure pH Balance Weekly

Bateman suggests growers check their plants' pH balance weekly, using the same method he described for measuring EC, as pH determines the availability of most plant nutrients.

"There's a temptation when things don't grow, to want to start to play with nutrient ratios, add in other products or go online and look at deficiency or toxicity symptoms and try to match it with what you're seeing and start trying to correct things on your own," he says. “If your crops have many nutrient disorders simultaneously, I always recommend first checking your growing media/substrate pH before starting to mess with the nutrient concentration or ratios.” 

It's very complicated to know exactly what's going on inside the plants, as it's rarely just one nutrient missing that's causing the deficiency, he says.

"When you add more nutrients to try to fix the problem, you are likely going to [create an] imbalance [with] other nutrients, and then your plant will get fixed in one area but show an imbalance in another area. So, you'll start to go through a vicious cycle where your plants look worse and worse,” he says.

If the pH is not within an optimal range and is either too acidic or basic, the crop will likely start to exhibit nutrient disorders, even when those nutrients are being applied at appropriate levels.

"If you check the pH or EC of your plants and start to make minimal corrections as your plants are growing, your chances of success are going to be exponentially higher than if you just wait for things to happen and try to catch up later,” he says.

Water Plants Optimally

For new growers especially, knowing when to water plants is critical, Bateman says.

Over-watering plants can essentially "drown" them because it causes oxygen to leave the root zone, and the roots need oxygen to survive, he says.

"The way I break it down is that water is the delivery vehicle, and nutrients are the cargo," he says. "So, if you can't get water into your plant because your roots are dead, then you aren't going to get many nutrients either and the whole thing will come crashing down.”

He says he can't tell growers exactly when to water their plants because it depends on many factors; however, there are many tools available for growers, including soil moisture sensors, tensiometers and devices that measure the weight of the pots. 

Remember that the growing media is juggling three key factors for crop growth: air, water, nutrients,” Bateman says. “Watering maximizes water and nutrients but overdoing it will lead to a lack of air and vice versa.”