Nitrogen for Leaves, Phosphorus for Roots and Flowers, Potassium for Health

I am sure you have heard the saying, plants need nitrogen for growing leaves, phosphorus for growing roots and flowers, and potassium for overall plant health. Well, it is not only wrong, but very misleading because it causes gardeners to use the wrong fertilizer.

In this post I will have a close look at this and show you why it is wrong. Along the way you will learn a lot about plant growth and how to get more flowers.

Plants Use Nitrogen, Phosphorus and Potassium

How do plants use these three nutrients? Nitrogen is used mainly for making proteins. Proteins are large molecules that carry out most of the reactions in plants, microbes and even in animals. Quite literally, nothing happens in a plant without a protein. What this means is that proteins, and therefore nitrogen, are needed in every part of the plant including roots, stems, leaves, flowers and fruits.

Phosphorus is part of the DNA molecule so it is in every cell. Another key function for P is in the molecule ATP, adenosine triphosphate. ATP is the source of energy in an organism – think of it as a rechargeable battery for life. It provides the energy needed by proteins and each molecule of ATP contains three molecules of phosphate. Clearly phosphorus is found in every cell of a plant.

Potassium has a lot of uses in a plant that contribute to plant health, including the movement of water and the control of some proteins. It too is found throughout the plant.

You can find out much more about these nutrients in my book Soil Science for Gardeners, but the main message here is that all three nutrients are required in all parts of the plant, at the same time. It is true that a lot of nitrogen is need to grow leaves, but that growth also needs a lot of phosphorus and potassium.

Nutrient Uptake Curves

Another way to understand nutrient requirements is to look at how plants absorb nutrients from the soil. To some extent plants control when they absorb nutrients and how much they absorb and nutrient uptake curves are a good way to understand this. The diagram below shows the relative amount of nitrogen taken up over time.

The same shape of curve is found for most plants. The plant starts out as a seedling and at that stage it needs very little nitrogen because it is small and has only a few cells. As the plant grows it enters a vegetative stage during which most of the roots, stems and leaves are formed. This requires a large amount of nitrogen and that is illustrated by the almost vertical line. The plant then reaches a botanical maturity and it now focuses on flowers and fruits (seeds). At this stage it removes much less nitrogen from soil, in part because these processes need less nitrogen, but also because the plant moves excess nitrogen from leaves into flowers and fruits.

Now lets have a look at the uptake curve for other nutrients.

The above nutrient uptake curve is for corn. Note that the left axis is a percent, not the actual mass of the nutrient. This curve shows that all three nutrients are absorbed at about the same rate.  Low levels in the seedling stage, high rates during rapid growth and less once the plant reaches maturity (flowering).

If the mantra “nitrogen for leaves, phosphorus for roots and flowers and potassium for health” were true, you would see a lot of the phosphorus being absorbed during or just before flowering, and early on in growth when the plant is make a big root system. Instead what you see is that all the nutrients are needed at the same time, more when the plant is actively growing and making new cells, and less when growth slows, and that makes sense given the way plants use these nutrients.

Nutrients Move Around The Plant

Nitrogen, phosphorus and potassium are mobile nutrients which are moved around the plant as needed. The diagrams below for soybean, show how these nutrients are distributed in different above ground parts of the plant over time. The movement of each nutrient varies a bit, but they follow similar trends. During the main growth period they accumulate in leaves and stems. As the plant reaches maturity, they are moved away from the green parts and into flowers, and at the end of the season a good portion is located in the grain (seeds). This trend is seen in most annual plants. Perennials are similar but they also move nutrients into root storage systems towards the end of the season.

Notice that the rate of uptake for all three nutrients is about the same throughout the growth phases of the plant.  Similar charts for corn can be found here and charts for beans, peas, potatoes, sugar beets and sunflowers are here.

Production of Flowering Plants

Commercial producers of flowering plants know something about making plants grow and bloom. Gardeners should follow their lead. Orchids are fed the same fertilizer all year long. The same is true for growing African Violets. Higher phosphorus levels may be useful in specific cases such as high salinity, where increased P does modulate the negative effects of salt on the growth of African Violets.

Nitrogen levels are changed for different growth phases of houseplants and bedding plants, but commercial growers do not increase the relative phosphorus level, for plants such as geraniums, impatiens, mums, bedding plants and perennials.

Why Bloom Boosters Don’t Work

Nitrogen also impacts the uptake of phosphorus, potassium and sulfur. When nitrogen levels are high enough, the plant is able to take up more of the other nutrients. Excess P or K will only be absorbed by the plant if there is also a matching amount of nitrogen. I will discuss bloom boosters in more detail in another post but this partially explains why they don’t work.

Should You Use High Phosphate Fertilizers?

Probably not. Most non-agricultural soil has adequate amounts of phosphorus unless the soil is very acidic. Most gardeners add too much phosphorus and some have very high levels in the garden. Unless a soil test indicates a deficiency, don’t add extra.

Nitrogen for Leaves, Phosphorus for Roots and Flowers, Potassium for Health

This message needs to change in gardening circles. It’s wrong. Instead we need to say” nitrogen, phosphorus and potassium for plant growth, flowers and health”.