Organic Fertilizer or Chemical Fertilizer - Which is better?

Opinions on organic fertilizer vary from: "Chemical fertilizer poisons the Land" to "All you need is keep control costs". So what is the truth and how can gardeners make rational decisions about fertilizer?

The first mistake people commonly make in thinking about organic fertilizer is to start by comparing the amount of nutrient in various fertilizer types. For example, Chicken Manure is usually about 1% Phosphorus, while Single Superphosphate is 9% and Triple Super is closer to 20%P. On this basis, when you look at the cost of applying a certain amount of Phosphorus per hectare, you would be crazy to use Chicken Manure. And yet, people do – and get good results. So what is going on?

The answer lies in what happens to plant nutrients in the soil and the ways in which plants can get them out. In the late 1800s Justus von Liebig was one scientist interested in this and he developed what has become known as the "Law of Minimums" according to which plant growth is limited by the least available nutrient in the soil. This seemed to work well enough, but it overlooked two important factors. The first being that soil is a dynamic system, so even with luxury amounts of every nutrient applied, the amounts available can still change.

However, the most important limitation to von Liebig’s approach was that he considered only soluble nutrients. His interest was to find chemical treatments that would improve the effectiveness of organic fertilizer by making the nutrients more soluble.

In fact, soil nutrients occur in a number of different forms, such as Mineral, Soluble and Exchangeable. Not all forms are equally available, but we now know that plants can actively seek out the nutrients they need, rather than wait for them to be washed in with the water they take up. Also, nutrients cycle between these various forms at differing rates, but factors such as moisture, proportions of clay and organic matter, and interactions between the nutrient elements themselves can strongly influence these changes.

In the face of such a complex and dynamic system, how then can we hope to develop any practical means of managing soil fertility for optimum plant growth?

The short answer is that it is all a matter of ‘balance’. The soil is not just a lifeless sponge to be filled with water and nutrients, which are then replenished once the plants have soaked them all up. It is a Living System with requirements of its own (apart from the direct needs of our plants). These requirements must be met if the soil is to function as we need it to, and soil function is what helps deliver nutrients to plants, as they require them.

If you have not yet read our information on the Mikhail Soil Balance System, you may like to do that now before reading further about the best uses of organic fertilizer.

For our purposes, balance occurs at two levels – in the soil and in the plant. Of course, there are no prizes for guessing that the soil is the one to start with. If the soil doesn’t function properly, neither can any fertilizer, and it is the physical character of the soil – its structure, strength, friability and so on – that is the most important criterion.

Until recently, the widely held belief was that the physical character of any soil was determined only by its particle size distribution. That is, the proportions of Sand, Silt and Clay. However, research by Ted Mikhail in the 1960s and 1970s established that soils with essentially the same particle distribution could vary widely in physical characters.

It turned out that the predominant factor affecting these soil properties was actually the proportions of five soil cations (Calcium, Magnesium, Sodium, Potassium and Hydrogen) relative to the soil’s capacity to hold these elements in their exchangeable form (i.e. Its Cation Exchange Capacity).

This balance of cations is what determines if the soil is well structured or not, is hard when dry, perhaps sloppy when wet, whether it absorbs moisture or repels it and so on. Of course, the five cations involved are plant nutrients, but it is important to realize that they are also needed by the soil and these two activities – soil function and plant nutrition need to be dealt with separately.

Once the soil’s requirement for these cations (in their correct proportions) is met, the soil will function to allow proper access by plant roots, shift nutrients into more plant available forms, provide the optimum balance between drainage and moisture retention, maximize biological activity and minimize nutrient interactions. Together, these things promote good plant nutrition and fertilizer efficacy; without them you are stuck waiting for the next rain to wash any available nutrients either into or away from the roots.

In a properly balanced soil, organic fertilizer performs at its best. On the other hand, chemical fertilizers that are applied repeatedly and in large amounts will have the effect of progressively reducing soil function.

There is also the real risk that a proportion of nutrients from chemical fertilizer will be lost altogether. For example, Nitrogen from fertilizers like Urea can literally evaporate into the air and Phosphorus in a soluble form can be either locked up in the soil or washed off the surface to pollute waterways. These things are far less likely to be a problem with organic fertilizer.

In a well-balanced soil, organic fertilizer gives the best result. In these circumstances, chemical fertilizers should be used only sparingly, if at all, and then only with continuous attention to maintenance of the cation balance, but of course it is important to emphasize the need to balance the soil first!

The aim in applying fertilizer of any kind is, of course, to supply plants with the nutrients they require for optimal growth and development. This then leads us to the next level of balance: Balanced Plant Nutrition.

Once soil function improves, nutrients begin to cycle towards more available forms and interactions between them are reduced, soil test levels can often be seen to stabilize without the need for large and regular inputs of fertilizer. Which nutrients respond and to what extent will be different for every soil, but this is usually the first sign of a positive shift in the balance.

This is why organic fertilizer works best in balanced soils - it supplies a continuous trickle of nutrients to help maintain fertility without doing anything drastic that may upset the system.

At this point the gardener has two choices:

• Find plants with requirements that fairly closely match the natural balance of nutrients in the soil – a difficult task at best; or
• Carefully adjust that balance at critical times through the season so that your plants can perform at their best without harm to either soil function or sustainability.

In either case, it is important to avoid the heavy-handed use of any fertilizer and to only apply the nutrients actually needed to make up the plants requirements – where the natural soil levels are insufficient. Again, Organic fertilizer will perform best and the reason for this is twofold:

• Firstly, they tend to contain a wide range of nutrients, including all the trace elements. While these are not in large quantities, they are in a reasonably good balance so they are less likely to cause any disturbance to soil function.
• Secondly, the nutrients in organic fertilizers are not all in soluble form. Instead, they are combined into organic compounds that are consumed by microbes in the soil. In this way, they reach the plant indirectly via the nutrient cycling function that cation balancing has optimized.

At times, there may be a need to deal with a specific nutrient insufficiency and careful use of either a chemical fertilizer or foliar spray may be warranted, but once soil is properly balanced, organic fertilizer will do a better job of maintaining that balance.