Fertilizing gardens is a popular topic and every gardening book and website will give you advice. There are thousands of products on the market claiming they are the best ones for your garden. The problem is that almost none of this information is correct. Most gardeners are wasting time and money on fertilizer they don’t need. Even worse is the fact that they are wasting a valuable natural resource and polluting the environment.
If you garden, please take the time to understand fertilizers. I know this is not a glamorous topic, but it is very important to the garden and the environment.
What is a Fertilizer?
There are several definitions, some used in science, others by governments. For the purpose of this article I will consider any product that is labeled with NPK ratio numbers as a fertilizer. The NPK ratio is the 3 or 4 digit number on the container; for example in the above picture it is 16-16-8-13.
The numbers indicate the amount of nitrogen, phosphorus, potassium and if their is a fourth number, sulfur.
Some people do not consider things like compost and manure as a fertilizer, but they really are fertilizers and when sold in bags they usually have NPK numbers on them. Even without the NPK number it is best to think of them as fertilizers.
What is a Nutrient?
In the plant world the term nutrient is used to describe the food that plants need. These are simple chemicals such as nitrate, calcium, sulfur and magnesium. Fertilizers contain one or more of these. If the amounts are high enough they may be listed on the container but many minor nutrients are never analyzed or reported. Chemical fertilizers tend to be purer and contain mostly the nutrients on the label. Organic fertilizers like compost contain a wide range of nutrients, and except for the three or four in the NPK ratio, you never know how much of each you get.
What Nutrients Do Plants Need?
Plants use about 20 nutrients. This can be broken down into two groups, micronutrients and macronutrients.
The micronutrients are used in very small amounts and unless you know your soil has a deficiency, you can assume that it contains enough of these nutrients.
The macronutrients include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), and magnesium (Mg). Plants use larger amounts of these and if they are not available in sufficient amounts the plant will not grow properly. Calcium and magnesium are usually not deficient. Phosphorus, potassium and sulfur may be deficient, but usually are not deficient in gardens. Farms are another story.
Nitrogen is an unusual nutrient in that the amount in soil changes very rapidly. It is converted to nitrogen gas which escapes back into the air and it dissolved easily in water and then washes away with rain and irrigation. If any nutrient is deficient it is mostly likely nitrogen.
For a more detailed discussion about these nutrients, have a look at Trace Mineral Fertilizers – How Many Nutrients Do Plants Need?
There are many fertilizer myths – these are some of the more important ones you need to understand.
A balanced fertilizer is one with equal amounts of nitrogen, phosphorus and potassium, eg 10-10-10.
In the early days of fertilizer manufacturing it was discovered that nitrogen, phosphorus and potassium are the top three nutrients used by plants. Someone came up with the idea that a balanced fertilizer was best for plants. This was mostly a marketing gimmick and was not based on science. Plants don’t use a balanced fertilizer and your soil does not need a balanced fertilizer.
Plant Specific NPK Ratios
If you go to buy fertilizer you will see plant specific fertilizer. For example, many companies supply a tomato fertilizer. If you want to grow a specific plant you can look it up on the internet and someone will tell you that it needs a specific ratio, like a 5-1-2 for your daisies. Your beans need one ratio, and your tulips need another.
This is all based on the idea that different plants use different amounts of nutrients. It is true that different plants use different amounts of nutrients but this advice misses the main reason for fertilizing as explained in the next section.
Try buying the so-called tomato fertilizer (Google for images matching “tomato fertilizer”). You will find that every manufacturer has a different NPK ratio? How can that be? If there was such a thing as a tomato fertilizer would all companies not sell the same perfect NPK ratio? They don’t do this because the whole idea of plant specific fertilizers makes no sense – it is a myth that few gardeners understand.
Why Do You Fertilize?
Many people think that you fertilize to feed the plant, but that is completely wrong.
- When you fertilize you are replacing the nutrients that are deficient in the soil, you do not feed the plants.
This is a critical concept to understand.
Lets say you are growing tomatoes and your soil does not have enough potassium but it has lots of nitrogen and phosphorus. Which fertilizer should you use? You should use a 0-0-10, which contains only potassium. Why add nitrogen and phosphorus when your soil already has lots? Adding more of these will not make your tomatoes grow better, but it will waste a valuable natural resource and cause pollution.
Someone across town might have lots of potassium and phosphorus in their soil, but not enough nitrogen. To grow the same tomatoes, they need to add a 10-0-0 fertilizer – their soil needs more nitrogen.
When you fertilize you replace the missing nutrients in the soil, regardless of which plant you grow. Provided all of the nutrients are available, plants will use whatever they need to grow well.
Since the point of fertilizing is to replace the missing nutrients, nobody can tell you which fertilizer to use since they do not know which nutrients your soil is missing. Neither manufacturers nor gardening gurus know the specifics about your soil so they can’t recommend the right fertilizer for you.
Which Nutrients Are Missing?
If you follow and agree with the logic so far, you will be asking a very important question, “how do I know which nutrients are missing from my soil?” You can’t look at your soil to figure that out.
You can get the soil tested. This can be done by using kits sold for home use, which are virtually useless so don’t waste your money on these. Soil can also be tested by professional laboratories which will give accurate results. The problem with these tests is that they don’t usually measure nitrogen, which, as stated above is the nutrient that is most likely to be deficient. They are good for determining other nutrients and the test will also give you soil pH.
Many people promote that idea that you can tell a soil deficiency by looking at the plant. Certain symptoms indicate specific nutrient deficiencies and usually show up in the leaves. You can find charts on the net for this kind of identification, but don’t bother. You can’t identify a nutrient deficiency by looking at leaves, except in very special cases. This type of information is fun to use, but is far too simplistic to be of any real value in the garden. To better understand why leaves can’t be used to determine deficiencies have a look at; Fertilizer – What Do Plants Need.
The third option is a simple approach. How are your plants growing? If they are growing OK, and you are getting a reasonable amount of flowers or fruits, then assume you do not have a deficiency. In that case you don’t need to fertilize, except as noted below under specific garden situations.
This last approach is the one I use. My garden does quite well, so I don’t fertilize, even when the nursery, book or social media says I should – they don’t know my soil. Most established gardens do not need fertilizer.
Synthetic vs Organic
This debate rages on and for the most part the parties on both sides miss some key points.
As far as adding nutrients to soil there is no difference between the two sources. Both synthetic and organic fertilizers contain the exact same nutrient molecules – they are indistinguishable from one another. Neither labs nor plants can tell if a nutrient comes from a synthetic source or an organic source.
Synthetic fertilizers make nutrients available more quickly than organic sources. That may or may not be a good thing.
Synthetic fertilizers, when used correctly, do not harm soil organisms – that is a big myth promoted by the organic movement and people who do not understand basic chemistry.
Having said that, there are good reasons to use organic fertilizers, but it has nothing to do with providing nutrients. It is all about building better soil and you can find more on this topic here; Organic Fertilizer – What Is Its Real Value?
Fertilizing Ornamental Gardens
The term ornamental gardens is a general term that includes any garden except vegetable gardens, lawns and containers.
Unless you know you have a specific deficiency, as shown by a soil test, assume you have all the nutrients your plants need.
Mulch with an organic source like wood chips. These add small amounts of nutrients over time and will help correct any minor deficiencies. Any plant material you remove from the garden removes nutrients so it is best to return it all to the soil. This can be done by my cut and drop method, or by composting the yard waste and then returning the finished compost.
If you follow these suggestions, you will not need to fertilize. Adding a small amount of compost or manure is Ok, but you can add too much.
If you have a deficiency, follow the labs recommendation to fix the deficiency, and then follow the above advice.
Fertilizing Vegetable Gardens
A vegetable garden is different because you remove produce on a regular basis thereby reducing nutrients in the soil. In addition, you want things to grow quickly before the end of the season.
It is a good idea to add some fertilizer each year. The best option is compost or manure which will provide all the nutrients your plants need and improve the water holding capacity of the soil. Synthetic fertilizers are also a good option in the early years of the garden since they give a quick feed to plants. The main nutrient needed is nitrogen, so a ratio something like 10-1-2 works well for most soils. Most gardens have lots of phosphorus so keep the middle number as low as possible.
Containers are special gardens for many reasons; they don’t use soil, daily watering leaches out nutrients and we want plants to flower a lot. The article Container Gardening – Fertilizers provides specific fertilizer information for containers.
If you want to understand fertilizers and your garden even better, have a look at my book Garden Myths, which contains a full chapter on fertilizer myths.
- Photo source; Vinacam Stock Company