The Difference Between Fertilizer And Compost

Edible Landscapes, Food Forest Design, Food Forests, Permaculture

By Graham Towerton

At Food Forest Abundance we’ve been hosting strategy sessions all over the world with hundreds of people. If you are reading this and you haven’t booked a strategy session yet, get yourself to our home page to book one as soon as you’ve read this article!

A common question that we get in these sessions is “what do I do to fix my soil?” There are endless types of soil depending on sand, clay, and loam content; how much organic matter is present, the soil moisture content, and its mineral nutrient content. People who are practicing permaculture or other sustainable agriculture methods are always wanting to avoid the use of synthetic chemicals, whether for fertilizer or for pest and weed control. Is there a way to manage soil fertility without the use of chemical fertilizers? The answer is most definitely “yes”.

Chemical Fertilizers

So what are chemical fertilizers? As all nutrients used by plants can be classified as “chemicals” of some form, I prefer to use the term “synthetic fertilizers” to describe those fertilizers which have been manufactured to form a compound that might not be found in nature. 

The main elements needed by plants include nitrogen, phosphorus, and potassium. Often you will see these listed on fertilizers as N, P, and K (their elemental chemical symbols). Conventional agriculture considers calcium, magnesium, and sulfur also to be essential macro-nutrients and then considers boron, copper, iron, manganese, zinc, chlorine, cobalt, silica, and molybdenum to be “trace” or micro-nutrients. Rarely does conventional agriculture consider the necessity for plants to use any of the 70 or more additional elements found in nature.

Instead of considering soil as a habitat in which plants, fungi, bacteria, insects, worms, and many other creatures can exist together, conventional agriculture considers soil simply to be a sterile growing medium into which soluble plant nutrients are added to stimulate and control the growth of plants. Whether the “crop” is corn, soybeans, or the typical residential lawn, the approach is generally the same i.e. ignore the soil flora and fauna and feed the plant with chemical fertilizers.

Let’s discuss some common synthetic fertilizers.

Nitrogen fertilizers typically come in the form of ammonia, nitrate, and urea-based compounds. Ammonium nitrate and urea are the two main forms of nitrogen fertilizers used in commercial agriculture and both of these are synthetic chemicals made from other chemicals. Ammonium nitrate is made by reacting nitric acid and ammonium hydroxide and both of these chemicals are also manufactured or synthetic. Urea is made by reacting ammonia with carbon dioxide and ultimately comes from natural gas for the production of these ingredients.

Common phosphorus-based fertilizers include phosphoric acid and monocalcium phosphate (or “super” phosphate). The latter is produced by reacting sulfuric acid with rock phosphate. Essentially all phosphorus-based fertilizers come from this chemical digestion of phosphate rock, followed by other reactions to purify and modify the chemical. Most phosphate fertilizers formed in this way can contain impurities from other elements including zinc and other heavy metals, as well as fluoride and aluminum.

The most common form of potassium fertilizer is potassium chloride which is generally obtained from potassium salt deposits. Other forms of potassium fertilizer include potassium sulfate and potassium nitrate.

One thing that all of these chemicals have in common is a high level of water solubility. Quite commonly in commercial agriculture, these chemicals are applied in irrigation water, or by “drilling” the liquid fertilizer into the soil at the time of planting. Fertilizers for home and garden use are usually pelletized solids but are also generally water-soluble. By having all of these chemicals water soluble, the theory is that they are readily available for uptake by the roots of the plants, resulting in faster growth. While the faster uptake and growth are true, there are numerous problems created in soils by the use of these fertilizers.

Harmful Effects of Chemical Fertilizers

The water solubility of these chemicals which is described as a benefit for plants is also a detriment for many other reasons. With heavy rainfall or excessive irrigation, the water-soluble chemicals can leach right through the root zone of the plant making them unavailable for use and entering the groundwater table where the chemicals become a pollutant. Water soluble chemicals can also wash easily from the soil into rivers, streams, and lakes causing algal blooms and depletion of oxygen leading to fish kills and death to other aquatic species.

While phosphorus is soluble in the form of liquid fertilizers, as soon as it hits the soil it starts to react with calcium and other soil minerals and more than 70% of the phosphorus becomes insoluble and immobile in the soil. 

In order to make commercial farming more cost-effective, chemical fertilizers are generally supplied in high concentrations and misapplication of these can burn plants. Even with the correct application methods, fertilizers can kill all of the soil micro-organisms.

It is this death of the soil micro-organisms that are the most harmful effect of all. An understanding of how plants interact with the soil and its micro-organisms is important in order to realize this fact.