Choosing a Fertilizer for Garden Use
By Dr Peter May

Sooner or later, anyone attempting to grow plants will consider using a fertilizer in an attempt to modify or enhance plant growth. This is an endeavour fraught with mystery and confusion and I thought I would attempt to provide a few clues to allow easier movement through the mire of information. For the sake of imposing some limits to the discussion, I will restrict my thoughts to the supply of the three major elements - nitrogen (N), phosphorus (P) and potassium (K). These nutrient elements are taken up by plants in the largest amounts and most fertilizer use involves them (to be technically strict about this, plants also absorb large amounts of calcium and sulphur but fertilizer application of these elements is often not necessary as they are either present in soil in large enough amounts or they are present in fertilizer salts used for N, P or K.

Some general observations before we start:

• In general, woody plants are less responsive to nutrients than herbaceous plants. Exceptions to this rule might apply where growing conditions are very favourable (eg high rainfall or irrigation, friable, well-drained soil) and where you are dealing with highly-bred horticultural species (eg fruit trees, roses).
• In many cases, the element you are most likely to see responses to will be nitrogen.
• Highly cultivated herbaceous plants (eg vegetables, bedding plants, some perennials, quality turf) have very high nutrient requirements.

Any random assessment of commercial fertilizers will show you that there are many products in the market place. The way that you determine their possible benefits will be by examining their nutrient content. This will be displayed as %N, %P and %K, in that order. A typical example might be a 20:5:18 which would contain 20% N, 5% P and 18% K. (Note that in some parts of the world, P is expressed as % P₂O₅ and K as % K₂O : my comments will not apply in those places.) By looking at this information you can compare products to see if they are similar or radically different.

Once you have sorted out the range of fertilizers into categories, the next question will be, what should you be using? This will depend in part on soil availability of nutrients and in part on what plants you are growing. For example, many soils in the drier parts of Australia have abundant supplies of potassium. Fertilizers in these areas need very little, if any, K. Many Australian soils are very poorly supplied with phosphorus, but years of application of P fertilisers can build up reserves of available P which reduce the need to add more. There is no way of deducing this situation. Soil analysis is the only solution. While this is routine for commercial growers, the expense is rarely justified for domestic users.

The other factor to consider is the relative proportions that plants absorb nutrients in. It is hard to generalise, but nitrogen is used in the largest amounts, potassium in slightly smaller quantities and phosphorus quite a deal less. While it is a ballpark estimate, typical absorption of N, P and K will be in the proportion of 5:1:4. If you are fertilizing to replace nutrient removal then the N:P:K content of a fertilizer should reflect this ( and you will find that many do just that.)

At the bottom line you may well find that many fertilisers will be more or less appropriate. With a bit of understanding of the background to your problem, you can make better decisions about products to use. Remember that for many of us, the use of fertilizers is fairly inexact, and if your nutrient ratio is a little imperfect, it probably won’ t matter in the long run. Avoid high P fertilizers on phosphorus-sensitive species (e.g. many Proteaceae), remember that high K fertiliser will not make a plant flower (flowering plants may use more K, but that is not the same thing) and trust me on the nitrogen (but be careful not to use too much.

Some of the issues raised in this article were triggered by an article by David Nichols - Why do fertilisers have different element ratios? Australian Horticulture, March 1998.
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About the Writer: Dr Peter May is Deputy Principal of Burnley College, University of Melbourne. He is a soil scientist with a range of horticultural interests including plant performance in the landscape, turf and viticulture.
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