Which is better - Organic or Chemical Fertiliser?

Opinions on this topic vary widely, from: "Chemical fertilisers poison the Land" to "All you need to do is keep the unit cost of nutrients under control". So what is the truth and how can farmers make rational decisions about fertiliser?

The first mistake people commonly make in dealing with this topic is to start by comparing the amount of nutrient in various fertiliser types. For example, Chicken Manure is usually about 1% Phosphorus, while Single Super 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 work was that he considered only soluble nutrients. He apparently believed that plants merely soaked up moisture in the soil and that their nutrients simply came along for the ride. In fact, soil nutrients occur in a number of different forms, such as Mineral, Soluble, Exchangeable and so on. 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 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 and then replenish once the crop or pasture has soaked them all up. It is a Living System with requirements of its own (that is apart from the direct needs of the crop). These requirements must be met if it is to function as we need it to, and this soil function is what helps deliver nutrients to plants, as they require them.

Of course, these days there is balance in everything from engines to shampoo! So where does the marketing hype end and reality begin?

For our purposes, balance occurs at two levels – the soil itself and 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 fertiliser, 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 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 realise 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, maximise biological activity and minimise nutrient interactions. Together, these things promote good plant nutrition and fertiliser 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 fertilisers perform at their best. On the other hand, chemical fertilisers 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 fertiliser will be lost altogether. For example, Nitrogen from fertilisers 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. It is a little known fact that the amount of phosphorus found in a match-head, if dissolved in an Olympic sized swimming pool, will be enough to produce an algal bloom!

In a well-balanced soil, chemical fertilisers should be used only sparingly, if at all, and then with continuous attention to maintenance of the cation balance, but of course it is important to emphasise the need to first balance the soil!

The aim in applying fertiliser of any kind is, of course, to supply the crop or pasture with the nutrients it requires for optimal growth and development throughout its productive cycle. This then leads us to the next level of balance: Balanced Plant Nutrition.

Once the 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 stabilise without the need for large and regular inputs of fertiliser. 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.

At this point the farmer has two choices:

• Find a crop that fairly closely matches the natural balance of nutrients in the soil – a difficult task, but the one that will most significantly reduce the ongoing need for fertiliser applications; or
• Carefully adjust that balance at critical times through the season so that the chosen crop can perform to its best without harm to either soil function or sustainability.

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

At times, there may be a need to deal with a specific nutrient insufficiency and careful use of either a chemical fertiliser or foliar spray may be warranted, but once soil is properly balanced, maintaining that balance will usually require a gentle hand rather than a sledge hammer.