Understanding Synthetic vs Organic Fertilizers

Are organic fertilizers better than synthetic fertilizers?

The "best" nutrients for greenhouse cultivation have been hotly debated since the beginning of the internet (and probably before that too). The key to finding the best greenhouse nutrient system for you and developing a nutrient schedule is understanding the pros and cons of synthetic vs organic fertilizers.

Overview of Synthetic vs Organic Fertilizers

  • Synthetic nutrients make sense when soil is not going to be reused so there is no point in conditioning it and maintaining it over time
  • Synthetic nutrients can destroy microbial communities in the soil
  • It can take years for soil microbe communities to recover from synthetic fertilizer use
  • Organic amendments will maintain the health of the microbial communities and keep them productive
  • Greenhouse growers may also opt to purchase bulk pre-mixed soil that contains soil microorganisms and organic amendments

Chemical Fertilizer vs Organic Fertilizer

Do synthetic fertilizers harm soil? In general, synthetic fertilizers can be harmful to soil and pose environmental threats. However, they offer a level of control and precision that may not be as reliable in organic fertilizers. Organic fertilizers like bat guano, worm castings, animal manure compost, and blood or bone meals are often marketed as soil conditioners because the nutrient levels can be difficult to guarantee. More precise organic fertilizers that are refined into bottled solutions are gaining popularity. These innovative hybrids come from natural ingredients but are refined adjusted to achieve more precise nutrient concentrations.

Do Synthetic Fertilizers Kill Soil Microbes?

In addition to sunlight and water, plants need mineral elements to produce sugars and starches through photosynthesis. These elemental minerals can come in forms that are ready to be used by plants, or they can come in forms that need to be modified in order for the plant to use them. Mineral nutrients that are in a usable form are said to be bioavailable to the plant. Synthetic nutrients are bioavailable and do not require any microbial modification. Organic nutrients, mainly nitrogen, are not naturally bioavailable. The plant relies on the soil microbes to modify them.

Greenhouse growers have many options when it comes to soil amendment. If the soil your greenhouse uses is not naturally very fertile and does not contain abundant soil microbes, synthetic nutrients will provide fast nutrient delivery. On the other hand, if you plan on reusing soil that is fertile and has a healthy microbial community, it makes sense to maintain the soil’s health by using organic amendments. Microbes in the soil are like the probiotic bacteria in the human digestive system. They help break down nutrients so plants can use them and improve the texture of the soil by breaking down organic matter.

Synthetic Fertilizers: Pros and Cons

Highly refined nutrients that are not in their naturally occurring state can be beneficial because they are ready to be used by plants. If plants are grown in a controlled and contained environment synthetic fertilizers may be advantageous for their precision and fast results. Nutrients from synthetic fertilizers are immediately available for the plant and can show benefits faster than organic nutrients. Nutrient concentrations are precise in synthetic fertilizers, and synthetic nutrients can be supplied at the precise time in plant growth that they are needed. Generally, synthetic fertilizers are less expensive than organic fertilizers and their labeling is standardized and easy to comprehend.

Disadvantages of Synthetic Fertilizer

  • Synthetic fertilizers are mostly made from nonrenewable sources, though some naturally occurring
  • Synthetic fertilizers can cause long term damage to the soil of outdoor crops by depleting carbon stored and decreasing microbial activity
  • Detrimental effects on plants and the greater environment if more synthetic nutrients are supplied than the plant can absorb and utilize
  • Can create toxic build-up with long term use and overuse which can leach below root zone into groundwater under outdoor crops
  • Synthetic fertilization inhibits natural microbial activity which is vital to combating the activity of pests and pathogens (creates more need for use of pesticides)
  • Synthetic fertilizers can alter soil pH and decrease microbial activity which can result in increased pests and pathogens.

Organic Fertilizers: Pros and Cons

Non-living biological material, more generally referred to as organic matter, imparts benefits to soil health and plant nutrition. Types of organic matter include dissolved organic matter which is small in particle size and water-soluble, particulate organic matter which has distinguishable fragments of plant matter and microbes, humus which is indistinguishable fragments of plant matter and microbes, and inert organic matter such as charcoal, graphite, coal. The organic matter content in the soil plays a large role in providing nutrients to plants. It also alters the texture of the soil, making it less bulky, which provides better moisture retention.

Advantages of Organic Fertilizer

  • Organic fertilizers improve soil structure and moisture retention
  • Since nutrients are released slowly overuse is rare and there is minimal risk of toxic build-up and environmental harm when using organic fertilizers
  • Organic fertilizers are made from natural and renewable sources, because of this they can be expensive to purchase pre-made but can be made inexpensively by collecting compost and manure
  • In general, the nutrient content is imprecise and labeling is not standardized for organic fertilizers, but the long term supply of organic matter from manure fertilized soil is very stable
  • Organic fertilizers increase gas exchange and bolster microbial activity which promotes nutrient cycling and increases stored organic matter (carbon) in the soil
  • When organic matter stores in soil increase, total nitrogen levels increase as well
  • Synthetic fertilizers increase nitrogen levels but deplete organic matter stores
  • Carbon stores from organic fertilization increase soil fertility long term

Disadvantages of Organic Fertilizer

  • Organic fertilizers breakdown naturally over time with the help of soil microbes, they may not provide the benefits of nutrient enrichment within the appropriate growth stages of the plant
  • Warmth and moisture are necessary to encourage microbial activity. If a plant variety does not share in the conditions which are ideal for microbes, the plant may suffer other stresses that mitigate the nutritional benefits. 
  • The benefits of organic fertilization are not apparent as quickly as with synthetic amendments

Can Organic Fertilizer Burn Plants?

Most organic fertilizers will not burn plants. However, the risk of nutrient lock still remains with any nutrient source!

Can I Use Organic and Synthetic Fertilizers Together?

Combinations of fertilization systems can be utilized to reduce the harmful effects of synthetic fertilizers from outdoor crops. Using half synthetic nitrogen fertilizers and half manure or worm castings can produce good crop yields. There may still be some reduction to carbon stores in soil, but moderate supplies are maintained when a combination of synthetic and organic fertilizers are used. Organic fertilization and reduced use of synthetic fertilizers provide long term and lasting benefits to crops, humans, and the greater environment.
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