In my post Fertilizer: Selecting the Right NPK Ratio, I explained that you don’t feed plants – you feed the soil. Your job as a gardener is to add missing nutrients to the soil. If the soil contains all the nutrients plants need to live – the plants will do well.
The answer to the question, what fertilizer do plants need, is very simple. They need any nutrient that is deficient in the soil. If your soil is not deficient of nutrients – you do NOT need to fertilize.
But how do we know which nutrient(s) is deficient in the soil? I’ll try to answer this question in this post. In order to do that, we need to better understand what nutrients do in the soil.
Nutrient Availability In Soil
Most discussions about nutrients talk about them as if they are all floating around in the soil water ready to be absorbed by roots. This is true of artificial hydroponic growing conditions but it is a good description for your garden soil. It is important to understand that nutrients interact with each other as well as the soil. The whole thing is a very complex system. For example phosphorus will react with iron and aluminum and hold onto them so tightly that they become unavailable to plants, resulting in an iron or aluminum deficiency. The problem is not a lack of iron and aluminum, but an excess of phosphorus.
Nutrients also react with soil. Some nutrients, like nitrogen, don’t stick to soil very strongly and are therefore easily washed out of the soil. Other nutrients, like phosphorus, stick to soil so tight that plant roots have trouble using them.
To complicate matters, the way in which nutrients react with each other and with soil is very dependent on the pH of soil. At pH values higher than 7.5, phosphorus, reacts with calcium and magnesium making them unavailable to plants. Many other micronutrients are also less available in these alkaline conditions, but some micronutrients become less available in acidic conditions. What this means is that your soil may have lots of a certain nutrient, but because the pH is not ideal, plants can’t access the nutrient. This is one of the main reasons why the pH of soil is important for plant growth.
It might seem as if this system is so complex that the gardener will never be able to provide the right amount of nutrients to plants, and that conclusion is partially correct. Trying to figure out which nutrients are deficient and trying to apply the right fertilizer might work for farms, but it’s not practical for the average gardener.
Some gardeners solve this complexity by just adding all kinds of fertilizer. The reasoning here is that if you add enough different nutrients you will eliminate a deficiency. That may be true in some cases, but this approach can have some serious consequences. It is expensive and environmentally irresponsible. Fertilizer is a natural resource and using it when it is not needed is a waste and using excess fertilizer will contribute to pollution problems. More importantly, high levels of nutrients can be toxic and be more harmful to plants than low levels. Adding fertilizer when you don’t need it is a terrible idea.
What about the rose expert that recommends regular feeding 3 times a year? They don’t know about the deficiencies in your soil – so their recommendation is poor advice.
Nutrient Deficiencies in Plants
Here are some facts about plant nutrients.
- There are a lot of them – over 80 different ones.
- They are all critical for plant growth. A deficiency in even one will affect plant growth.
- Each nutrient plays one or more specific roles inside the plant.
- They react with each other and with the soil.
Given the fact that each nutrient has a specific job in the plant it seems reasonable that a nutrient deficiency should produce specific symptoms in the growing plant. You should be able to identify the missing nutrient by looking at the plant. That is in fact what many references try to do. The image below shows a list of some plant deficiencies and their corresponding symptoms.
The idea that you can identify plant deficiencies by looking at symptoms is, at least in part, a myth. It is true that a deficiency in one nutrient will manifest itself with certain symptoms. For example a deficiency of iron will cause chlorosis, a yellowing of the leaves. Iron is required to produce chlorophyll which is the molecule that makes leaves look green. Without iron, plants can’t make chlorophyll and therefore leaves look yellow.
However, chlorosis can also be caused by a deficiency of manganese which is also required to make chlorophyll.
A high soil pH can also cause chlorosis. Soil may have lots of iron and manganese, but in alkaline conditions where the pH is above 7.0, the iron and manganese are held so tightly by the soil that certain plants can’t use them. Since the plant can’t absorb the iron and manganese from the soil, they can’t make chlorophyll and the leaves look yellow. Adding more iron or manganese to the soil will not solve the problem because it is not a deficiency problem. It is a pH problem.
As you can see from this example chlorosis is not a reliable symptom for identifying a deficiency.
Myth: Red leaves show a phosphorus deficiency
Sometimes, plants that are normally green will produce red leaves in summer and that is not normal. Some people attribute the red color to be a symptom of phosphorus deficiency and it is a possible cause of the problem. Other nutrients can also cause the symptoms either because they are deficient, or because there is an excess of them. A lack of water and other environmental factors can also cause this problem. Red leaves in summer can be the result of many things.
So far we have discussed the effect pH and nutrients have on symptoms but many other factors can produce similar symptoms. Diseases, environmental conditions, insects, and pesticides can all produce symptoms similar to nutrient deficiencies.
Plant symptoms are a tool for predicting possible nutrient deficiencies, but in most cases they is not something you can rely on. Soil and plants are just too complicated and problems can’t be identified by a simple list of symptoms. Simple chats like the one displayed above provide interesting facts about plants, but they should not be used to figure out which fertilizer to add.
How do you figure out if your soil has a nutrient deficiency? You can’t tell by looking at the soil, and plant symptoms aren’t reliable. The best way to determine a nutrient deficiency is to do a soil test, but even these have limitations.
Soil tests done by a proper laboratory do not normally check the level of nitrogen in the soil. As discussed in Fertilizer – Understanding Plant Nutrients, nitrogen changes too fast, and by the time you get the sample to the lab, the nitrogen levels in the sample and in your garden have already changed. Unfortunately, nitrogen is the one nutrient that is most likely to be deficient in your garden.
Plants use some 80 different nutrients. The average soil test will look at three to five of these. Laboratories can measure the other nutrients but the cost is too high for most home owners.
Soil tests have value, but it is important to understand that they don’t measure for nitrogen nor do they normally measure for the micronutrients.
Myth: Home soil test kits are a good way to test soil
Several companies produce home soil test kits for N, P and K. The accuracy of the less expensive non-lab quality kits is not reliable and they don’t provide actual numeric values. Instead they indicate that levels are sufficient, adequate, deficient, or depleted. If the results are deficient or depleted you have no idea how much fertilizer to add. These kits are even less accurate for measuring pH. If you want your soil tested, use a professional lab.
Fertilizer – What Do Plants Need?
Published information about fertilizer is too focused on just N, P, and K, giving a false impression that adding a balanced fertilizer of these three nutrients solves all of your plant’s nutrient requirements. Now that you have a better understanding about nutrients and soil you will realize that the whole topic of providing the right nutrients is much more complicated. You might even feel that it is too complicated and want to give up gardening. The good news is that there are simple solutions, but they rely on doing things differently than suggested by many experts.
Gardeners should follow this advice.
- Assume that your soil has enough nutrients unless a soil test indicates a deficiency.
- Don’t spread commercial fertilizers in your garden unless you are sure that you need to fix a problem. Don’t do it because your granny or your neighbor says you should and don’t do it because a book or web site tells you to do it. Your neighbor may be using fertilizer simply because they have always done it. Other reference material knows nothing about your soil so they can’t make a correct recommendation. Remember, you are feeding the soil, and not the plants.
- Don’t sweat the micronutrients—you probably have plenty of them in your soil.
- Lawns don’t need to be fertilized but they will grow thicker with fewer weeds and look greener if they get some additional nitrogen. Use fertilizer that contains as little P and K as possible. A 24-0-0 works great.
- Vegetable beds don’t need fertilizer if you are following the other rules listed here, but may benefit from some added nitrogen. A 24-0-0 works great but too much will result in lots of leaves and low crop yields.
- Flower and shrub gardens should not be fertilized unless you have a known deficiency.
- Nitrogen is the only nutrient you need to worry about. All other nutrients are usually available in sufficient quantities, unless you have very sandy soil.
- Add organic matter like compost and manure. Leave grass clippings on the lawn to feed the lawn. Mulch with woodchips. These are all slow release fertilizers that usually take care of adding all the nutrients needed, except for nitrogen, which may still be deficient.
The above rules are general rules and are based mostly on my knowledge of soils in North America. Other regions maybe different. For example, readers have indicated that Australia has low levels of P. If you know this to be true for your soil, then follow the above rules and add additional phosphorus. Feel free to add fertilizers if you know you have a deficiency.