Fertilizers: Natural or Synthetic?

First off, what are fertilizers? A fertilizer is any material that supplies water soluble-nutrient ions (plant “food”) for plant growth and crop production. These ions are made up of the essential elements (nutrients) that plants need to grow, live, and reproduce. There are also many other elements that may not be required but can still have a positive effect on growth and development (silicon for example). Fertilizers supply these nutrients in various amounts, at different rates, and in different forms, depending on the type of fertilizer used.  Not only do these fertilizers affect the plants they are feeding, but they also affect the soil and organisms that live within, and not always in positive ways (e.g., worms, fungi, bacteria, other microbes). In Canada, for something to be labelled and sold as a fertilizer, the label must have the guaranteed analysis, directions for use, safety information and a lot number for traceability. Not everything you find in the store is considered a fertilizer, even though it may contain some amount of nutrients and/or contribute to plant health (compost for example).

The three major or “macro” nutrients are Nitrogen (N), Phosphorus (P), and Potassium (K). These elements are what the three numbers listed on fertilizer labels represent (e.g., 15-30-15). If there is a fourth number listed (10-15-10-5), this most often refers to the sulphur content. The big three macronutrients (N-P-K) are used in relatively large amounts and are usually the limiting factors (especially N) in plants’ health and development.

The remaining nutrients are known as minor or micronutrients (sometimes called trace elements). They are still essential but tend to be used in much lower amounts compared t the major elements. These include calcium (Ca), magnesium (Mg), sulphur (S), boron (B), copper (Cu), iron (Fe), molybdenum (Mo), manganese (Mn), zinc (Zn), nickel (Ni), and chlorine (Cl). Silicon (Si) is also included in some lists as it is important in the growth of certain plants such as horsetail and many grasses and can provide other benefits (such as stress resistance).

Ultimately, each fertilizer will have different amounts of each of these nutrients as well as different rates of availability, and so it is important to understand what your soils (and ultimately your plants) are going to need more of, and what there is enough of already.  Doing a basic soil test as well as keeping records of problems or deficiencies encountered can go a long way in helping make the right decisions for your fertilizer application.

The term “availability” refers to the amount of the nutrient that is in a form that plants can readily absorb. This is determined by the type of fertilizer used, as well as the conditions of the soil environment -microbe populations, moisture content, and especially pH (how acidic or alkaline the soil environment is).

Gardener’s Note: The three elements calcium, magnesium, and sulfur are sometimes referred to as “minor” nutrients as they are used in lower amounts than the majors but in higher amounts than the other minor nutrients. For this article, the only distinction made is between major and minor nutrients.

The name says it all – these products are synthesized using various industrial and chemical processes. The starting material may or may not be found in nature, but the processes used to extract, concentrate, and create the product are not. These materials are almost always water soluble (dissolve in water) making them instantly available for plant uptake. Think of them like an intravenous drip for the plant – they provide nutrients that are immediately accessible, no digestion needed. And, just like pharmaceuticals, they are very pure and highly specific in their target application.

They are designed in laboratories and so come in many different formulations with specific N-P-K concentrations depending on their intended purpose. For example, lawn fertilizers tend to be very high in N and often have significant amounts of iron and sulphur in order to “green up” the grass. “All purpose” fertilizers tend to be more balanced and “complete”, also often containing a limited suite of trace elements, depending on their target application. Others have more specialized formulations and are not “complete”. Calcium nitrate, for example, is often used for to help prevent and treat blossom end rot in tomatoes but only contains calcium and nitrogen. These fertilizers tend to have very high amounts of nutrient concentrations compared to their organic or naturally derived counterparts.

Gardener’s Note: If a fertilizer is labelled as “complete” this simply means it has all three major nutrients in some amount in its formulation, but not necessarily any others.

These types of fertilizers are referred to by many names – natural, organic, naturally-derived, biological, among others. These products are derived either from animals or plants and, unlike synthetic types, usually have some organic matter (i.e., organic carbon) component to them. Some common examples are manures, blood or bone meals, fish emulsions, alfalfa meal, insect frass (poop). Rock dusts and powders are an exception as they are not animal or plant derived but rather extracted through mining. Because they are minimally processed (usually just ground up), they are often included in the natural or organic section rather than classified as synthetic.

To be labelled and sold as a fertilizer, these products also need to have a guaranteed analysis indicating their nutrient content, safety information, and directions for use. They are usually not as “specific” as synthetic fertilizers in terms of their nutrient content as they are often comprised of a multitude of both macro and micro elements in varying forms, with some immediately available to plants, and some not. This means that if one source runs out, the plants will have other options to access the nutrients they need. Unlike synthetic fertilizers, many of these nutrients are contained in compounds that are not water soluble, and so they tend to be released slower and over a longer period.

Due to the complexity of some of the compounds, additional action from both soil microbes and the plants themselves are required to access these nutrients. Similar to humans eating food, there are a multitude of processes (digestion) that must occur for plants to access the nutrients within these fertilizers. Soil microbes eat and convert these compounds into plant-useable forms through their own digestion and death. Plants themselves also excrete compounds from their roots (exudates) that both help to make the nutrients available to them, as well as attract beneficial microbes to further increase availability.

Gardener’s Note: Synthetic and mined products can also contain carbon but it is in an inorganic form and so isn’t usually considered in the overall soil organic matter content.

Plants are very efficient - maybe even lazy! If they have access to readily available food, they will consume it over other sources that require more work to break down and absorb. This might seem good from a plant perspective, but because the food is already there, they no longer need to send out signals (exudates) to call the microbes required to break down the more complex organic materials into absorbable forms. Microbes eat and poop nutrients, and so using organic fertilizers and amendments feeds the microbes which, in turn, feeds the plants. When this relationship is severed, the microbes will either relocate or perish. This now means that the plants will have a much harder time if the synthetic fertilizer ever becomes unavailable (i.e., the gardener stops fertilizing). Their signals to the microbes go unanswered and soil and plant health suffers.

Besides providing access to nutrients, a robust and diverse microbe population also helps defend the plants against pathogens and disease – it’s like having a personal army of microbe soldiers to fight off bacterial, fungal, and even some insect invaders. Research here at USask has indicated that plants grow much more vigorously and with less problems with a healthy native soil microbe community than those without and so maintaining this soil microbe suite is crucial to continued long term soil and plant heath.

Synthetic fertilizers are mostly water soluble and very easy to apply, and so they are also very easy to overapply. Using too much fertilizer can cause problems such as fertilizer burn where plants take up too much of a nutrient and suffer stress and poor growth. Overapplying can also lead to excess runoff into our water sources such as during heavy rain events or intense irrigation, where fertilizers can be both physically washed away through erosion, and/or dissolve in the water and be flushed away from the root zone and into unwanted areas. This can also be an issue with organic fertilizers such as in the case of manures being washed into water bodies, causing large algal blooms and subsequent oxygen depletion (eutrophication) due to excess nitrogen and phosphorus levels.

Overapplication of organic fertilizers, especially in large scale agriculture can be very problematic, but because they are slower release and not as soluble in water, they are often able to be used without as many potential issues, especially on smaller scales like a home garden. This isn’t to say problems can’t happen though. For example, high levels of phosphorus can cause issues with iron and calcium availability. If the pH is too low (acid soils), phosphorus binds with iron and forms compounds that do not dissolve, rendering both nutrients inaccessible to the plant. If the pH is too high (alkaline soils), phosphorus can bind with calcium, again forming compounds that make both nutrients unavailable for plant uptake.  

Some products labelled as natural also contain synthetically derived ingredients. Urea, for example, is often included in some naturally-derived products as an additional source of N. Urea is created using the Haber-Bosch and Bosch-Meier processes, which converts the nitrogen in our atmosphere into a form that, under the right soil conditions turns into plant-useable ammonium (NH₄⁺) This process uses natural gas (methane) both to power the conversion and in the chemical reaction itself. This is incredibly energy intensive, and not natural. On this same note, a product can be "organic" without being produced using organic standards and growing methods (such as are defined by OMRI, for example). Always read the labels!

Gardening at USask always strives to encourage regenerative strategies in terms of soil health and environmental stewardship. Anything that improves the overall longevity and productivity of our soils is best, and most organic fertilizers blend well with these goals. Always practice the 4 Rs when applying any fertilizer. These are:

• make sure it is applied using the Right source/type,
• at the Right rate
• at the Right time
• and in the Right place.

It may seem like common sense, but these basic rules are often overlooked, which causes inefficiencies in both fertilizer uptake and nutrient retention, and ultimately results in poor soil health and your money being wasted.

Getting a basic soil analysis is also a good practice. This will give you a baseline as to what your soils might need more of and what there is already enough of, which can help to avoid over-applying and damaging both the plants and your wallet. In a study done recently in Saskatoon, for example, it was shown that urban garden soils contain up to 400% more phosphorus than rural agriculturally based soils in the same area (so go easy on the bonemeal!).

Saskatchewan soils are also generally sufficient in potassium and so adding K-based fertilizers is usually not needed. Basic soil test kits are usually available at most garden stores. Labs specializing in soil analysis can provide a more comprehensive analysis and sometimes will give recommendations but can be expensive and/or inaccessible for the average gardener.

While some fertilizers are arguably ‘better’ or more complete, it is important to consider the overall carbon cost of the material. Is it better to use bat guano harvested in Indonesia than it is to use alfalfa meal grown and produced here in Saskatchewan? Everything has a cost, and so the more things we can do to source inputs locally and sustainably, the better. Engaging in practices like no-till gardening and using carbon rich, organic inputs (when needed) to both feed the plants as well as the soil food web helps encourage a more robust and healthy plant and microbe community.

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Canada. (n.d.). Fertilizers Regulations (C.R.C., c. 666). Justice Laws Website. https://laws-lois.justice.gc.ca/eng/regulations/c.r.c.,_c._666/page-2.html#h-560005

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