When we are considering how to improve our crops, one of our main concerns is that we want them to taste absolutely fantastic—sweet or tart, but never bland. To better understand the science behind this, let’s examine what it is that the roots actually take up. For example, if we want our fruits or veggies to taste sweeter, we might think that we should add sugar to our nutrient formulations—but would this actually work?
Providing an optimal supply of all the nutrients plants require is the best way to ensure good flavor. What these nutrients are and what level is optimal is the question—let’s take sulfur, for example. Sulfur is especially important, as it forms organic compounds within the plant that ultimately contribute to the flavor of the crops produced. Some soils are deficient in sulfur and you can address this by adding organic material like compost. Potting soil is generally rich in organic material, but the level of available sulfur can vary greatly.
Adding straight elemental sulfur has negative tradeoffs and this form of sulfur is extremely slow in becoming useful for your plants. When too much sulfur is added to a soil the pH can become too low—most plants prefer a pH of between 5.5 and six. If you are growing tomatoes, the optimum pH for that plant is slightly higher, around 6.5 to 6.8.
Sulfur is a structural subcomponent of amino acids, proteins and many micronutrients and is essential to the production of chlorophyll. Magnesium sulfate, for instance, is a chemical compound containing magnesium, sulfur and oxygen. Epsom salts are a form of this compound and are often used in nutrient formulas to promote rose blooms. Many of the micronutrients your plants need are delivered in this sulfate form in order to make them more available to the plant for uptake and also to add sulfur as a flavor enhancer.
Sulfur is not very mobile within the plant and a lack of sulfur can be responsible for a number of plant health issues: poor photosynthesis, poor nitrogen fixation in legumes, poor conversion of nitrates into ammonium and proteins and retarded formation of storage proteins in developing seeds.
We apply lots of potassium to our plants in order to keep them strong, but this nutrient actually exacerbates sulfur loss. Stunted plants and uneven crop development are often the result of low sulfur levels, which can aggravate nitrate toxicity as well. The visual symptoms of a sulfur deficiency can look like general chlorosis—similar to a nitrogen deficiency—except the young leaves stay yellow over time and leaflet yellowing is general and uniform rather than varying throughout.
Plant roots take sulfur up as sulfate SO42-. This is another reason why the best way to apply sulfur to the soil is in the form of sulfates. Copper sulfate, ferrous sulfate, manganese sulfate (not to be confused with magnesium) and zinc sulfate are all excellent nutrient supplements to enhance flavor in your veggies.
Sulfur is as important to a plant as nitrogen. Most growers pay plenty of attention to nitrogen for their crops but don’t realize that sulfur is essential in order for plants to be able to use that nitrogen—without sulfate amino acids, proteins can’t be built and plants won’t grow. Sulfur content is lost by leaching from the soil and through anaerobic volatilization, which happens when soil is kept too wet and insufficient oxygen is available.
Because both nitrogen and sulfur are building blocks of proteins, an insufficiency of either one will cause a shortage of chlorophyll, which will in turn result in the plant being unable to convert sunlight into energy. A lack of sulfur or nitrogen will also cause an inadequate supply of the enzyme rubisco, which changes carbon dioxide into sugar. It sounds odd, but if you want really sweet veggies you’ll have to make sure they’re getting enough sulfur in their diet!
Good soils are actually full of life—the unseen microbial organisms in the soil are responsible for the continued good health of your plants. The release of organic sulfur from soil humus is very slow and its benefit to plants is limited—mineralization is where sulfur is converted by these beneficial microbes to plant-accessible sulfates.
Here is where a potential conflict can get started. Too much straight elemental sulfur is antibacterial and can actually sterilize the soil to a degree—elemental sulfur is often used to sterilize injuries on succulents in order to halt the development of disease.
So even though you need to have a good level of sulfur in your soil, you also need to be very careful about over-application or you will undo the very process that you want to occur—mineralization. Poor organic matter, soil humus and low microbial activity (common with pH levels that are either too low or too high) will decrease the amount of sulfur available to your plants.
Using a nutrient formula with plenty of pre-mineralized sulfates is one safe way to deal with this conundrum. Elemental sulfur can only be plant-available after a long breakdown period (often several years).
In order to produce sulfuric acid, elemental sulfur requires an initial phase of microbial oxidation. The microorganisms that produce this elemental sulfur oxidation need most of the same nutrients that the plants need, plus a few more.
The oxidizing bacteria are mostly aerobic—which means they need plenty of soil oxygen in order to stay healthy and reproduce. When you water your plants too often you create soil with insufficient oxygen, which can definitely impede this vital process. The breakdown of elemental sulfur is a slow process even under ideal conditions.
The primary minerals—calcium, sulfur, manganese, magnesium, copper, iron, boron and zinc—are in general all players in the quest for superior taste. Finding a plant nutrient that provides these in sulfate form can be a real game changer if you are interested in maximizing organoleptic (taste) quality and plant health while keeping the process of growing as simple as possible.
Indoor grows are often done in potting soils—while rich in organic matter, these soils also exhibit rapid draining characteristics and this can make them susceptible to sulfur deficiency. Keeping your soil microbes alive and healthy is one way to help this organic matter convert to sulfates.
Complete soil tests can give you an idea of the amount of sulfur that is available for your plants, but it is still generally difficult to determine how much additional sulfur is needed—regular additions of mineral sulfates can be another way to keep ahead of this issue.
Find and use a plant nutrient formula that provides a complete spectrum of nutrients, including micronutrients—if you can also find one that uses the sulfate form of the micronutrients you will be on your way to a better-tasting crop.