Fertilizer – Understanding Plant Nutrients

Plant nutrient is a term used to describe plant food. We all know compost is good for the garden, but most of the good stuff in compost is not ready for plants to use – it is not plant nutrients yet! Over time, the complex molecules in compost, like proteins and carbohydrates, will be broken down by microbes into smaller molecules and eventually they become nutrients.

In this post I will take a close look at the main nutrients used by plants. What are they? What happens to them in soil? The answers to these and other questions will help you understand the process of fertilizing plants better.

Plant Nutrients: Carbon, Hydrogen and Oxygen

Most authorities use the term nutrients to refer to N, P and K as well as other molecules like sulfur, magnesium and calcium. To gain a better understanding of your plants and the role of soil it is important to understand that carbon, hydrogen, and oxygen are also important nutrients. These last three elements are just as important to the health of plants as the other nutrients, and hydrogen and oxygen can be in short supply.

If you take a plant and dry it, 96% of what you have left is made up of carbon, hydrogen and oxygen. All of the other nutrients together, including N, P, and K, make up less than 4% of the plant.

A plant gets most of its hydrogen from the water that is absorbed by the roots. Water is made up of hydrogen and oxygen, and during photosynthesis the hydrogen is released and made available to the rest of the plant so that it can be used to build large molecules.

You might guess that the oxygen produced during photosynthesis is also used to build large molecules, but that is not what happens. Most of this oxygen is expelled by the leaves into the air. The oxygen used by plants to build large molecules is absorbed by the roots from the air in the soil. This is one of the main reasons why it is critical for soil to contain lots of air.

The main source for carbon is the CO₂ in the air.

Plants rarely have a problem getting enough CO₂ from the air but getting enough air and water from the soil can be a problem. Poor soil and improper cultivation of soil makes it harder for a plant to get enough water and air and if that happens, the plant is starved of hydrogen and oxygen – key food nutrients for the plant.

Plant Nutrients: Nitrogen

Nitrogen (N) is the most important nutrient for a gardener to be concerned about. Next to carbon, hydrogen, and oxygen, plants need a significant amount of nitrogen and it is the nutrient that is most likely to be lacking in your soil.

Nitrogen is used as a building block for all kinds of large molecules as explained in Fertilizer – Selecting The Right NPK Ratio. Enzymes, which are a special type of protein, contain quite a bit of nitrogen. Enzymes control chemical reactions and therefore control just about everything that happens in a plant, from photosynthesis to growing. Nitrogen is also part of DNA, RNA and chlorophyll—all vital for plant growth.

Atmospheric nitrogen, N₂, is a gas which makes up 70% of the air we breathe. There is a lot of it, but most plants can’t use it. Nitrogen fixing bacteria are able to convert N₂ into ammonium, which plants can use. This process is critical to plant life and is one of the main ways in which plants are fertilized in nature.

Commercial fertilizer can contain a variety of different chemical forms of nitrogen including ammonium (NH₄), nitrite (NO₂), nitrate (NO₃), and urea. Plants are only able to use nitrate and ammonium directly. If you add a fertilizer containing nitrite (NO₃) or urea the plants cannot use these forms of nitrogen. So why would fertilizers contain them? The answer involves an important concept in gardening. Microorganisms in the soil are able to convert one form of nitrogen into another and this goes on all of the time in the soil. See the image at the beginning of this post. Microorganisms take nitrogen forms which plants can’t use and convert it to a form they can use.

The various forms of nitrogen mentioned above are converted from one form to another so quickly that by the time you get a soil sample to a lab it has changed so much that the analysis would be meaningless. If you want nitrogen analyzed in your soil you need to freeze the sample as soon as you take it and then take it to the lab frozen.

Ammonium (NH₄), nitrite (NO₂), nitrate (NO₃) and urea are all very soluble in water. What this means is that they easily mix and dissolve in water, and then move along with the water. When it rains, the rain moves through the soil taking soil nitrogen with it. If you add too much nitrogen fertilizer, rain will wash it away from your garden into the rivers and lakes, causing pollution.

What does all this mean for the gardener? Nitrogen is a critical nutrient for your plants but it is very unstable in soil. It is easily and quickly converted from one form to another and it is easily washed away by water. Your plants can have lots of nitrogen available in the morning and after a good rain have a shortage. As a gardener you never know how much nitrogen is available to your plants because the situation changes so quickly. Nitrogen is the nutrient that is most likely to be in short supply for your plants.

Plant Nutrients: Phosphorus

Phosphorus (P) is a critical part of plant growth. It is part of DNA and RNA the genetic material in plants and it is incorporated into cell walls. It is also a component of a molecule called ATP which is like a rechargeable battery for the plant, storing and releasing energy as it’s needed. ATP is involved in many reactions taking place in the plant, including photosynthesis.

Phosphorus naturally comes from dissolved rock and, with the exception of sandy soil, it is present in most soils. Unlike nitrogen, it is not very soluble in water and so natural forms of it do not leach out of soil quickly. In fact most phosphorous sticks to soil so tightly that it is difficult for plant roots to extract it.

There are two forms of phosphorus in soil. The majority is insoluble, tightly held by soil, and unavailable directly by plants. Another form is slightly soluble in water and is available to plant roots. The two forms of phosphorus are continually converted back and forth so that there is always a small amount available to plants.

Some fertilizer uses a soluble form of phosphorus, called phosphate, which has also been used in making soap. Phosphate dissolves in water and can be leached out by rain. Phosphate pollution from both fertilizer and soap is a main cause of pollution in lakes where it causes algae blooms, which in turn reduces the amount of oxygen in the water, killing fish. This is one of the main reasons that phosphate was removed from soap products.

Non-soluble forms of phosphorus are found in bone meal and rock phosphate, both of which are sold as fertilizer. They just sit on the surface of the soil, moving deeper very slowly at about an inch per year. These forms of phosphorus break down very slowly. In alkaline or neutral soil, rock phosphate can take 100 years before it breaks down and makes the phosphorus available to your plants. Clearly, rock phosphate is not a good short term fertilizer.

When P levels get too high in the soil, it becomes toxic to microorganisms, especially mycorrhizal fungi.

Some agricultural soils are deficient of phosphorus because they have been heavily harvested for many years, but this is not usually true of home gardens. Most home gardens have enough phosphorus to support plant growth. There are two common exceptions. Very sandy soil has few nutrients of any kind and may benefit from the addition of more phosphorus on a regular basis. The amount added should be based on actual soil tests. Secondly, new homes built on land that was intensely farmed may be deficient of P. In this case adding phosphorus for a couple of years will replenish the phosphorus level in the soil. Once this is done, no more P needs to be added in future years.

Even governments and fertilizer companies have come to realize that phosphorus is not required by most home gardeners. Some states in the United States have banned phosphorus in lawn fertilizer and some companies have reduced or illuminated the level of phosphorus in garden fertilizers.

The bottom line for home gardeners is simple. Too much phosphorus is bad for the environment, and may harm soil organisms. Unless your soil test indicates a shortage of phosphorus you should assume that your soil has plenty of it and don’t add more in the form of commercial fertilizers.

Some common phosphorus myths.

Myth: Adding phosphorus stimulates root growth.

Soil that is deficient in phosphorus will benefit from the addition of more P and will result in better root growth. However, if the P levels are satisfactory, as is the case in most soils, adding more phosphorus will not stimulate root growth.

Myth: Plants showing a phosphorus deficiency need more phosphorus.

This may or may not be true. A lack of nitrogen reduces the plants ability to absorb phosphorus. Consequently, the plant shows symptoms of phosphorus deficiency. In this case adding more phosphorus will not not help the situation since there is no phosphorus deficiency. Adding nitrogen to the soil solves the problem, and will make the phosphorus deficiency symptoms go away.

Plant Nutrients: Potassium

Potassium is not used very much as a building block for large molecules, but it is critical for controlling the movement of other molecules. It regulates the movement of CO₂ and water by controlling the opening and closing of stomata in leaves. It regulates the formation of ATP, proteins, starch, and activates enzymes responsible for plant growth. It plays a key role in photosynthesis and in preparing plants for a cold winter.

Potassium is clearly an important nutrient for the plant and it is logical to think that a plant needs a lot of it, but that is not true. Think of potassium as being recycled in the plant. The same molecules is used over and over again to control things but very little of it is used up in process.

Potassium dissolves easily in water as a potassium salt and plant roots can absorb it in this form. Potassium also sticks tightly to soil particles, especially clay. As plants use up the portion in the water, more potassium will move from the soil particles into the water making it available to the plants.

When fertilizer is applied to the soil, potassium sticks to soil and moves slowly through the soil layers. Each time it rains, a bit of potassium is dissolved and moves deeper in the soil. It moves faster than phosphorus but not nearly as fast as nitrogen. This means that the potassium in fertilizer sprinkled on top of the soil takes a while to reach plant roots but once there it is available to plants for a extended time.

Except for very sandy soil, most soil contains enough potassium and rarely needs to have more added.

Plant Nutrients: Sulfur

Sulfur is not normally considered to be one of the main nutrients but it probably should be. That is why some countries are starting to show the value of fertilizer as NPKS, where the ‘S’ stands for the % of sulfur.

A plant uses as much sulfur as phosphorus. Sulphur is an important component of proteins and various oils. The flavor and odor of onions and garlic is due to sulphur containing oils. Sulfur also regulates important processes such as the formation of chlorophyll which is critical for photosynthesis. It plays a critical role in the formation of the root nodules in legumes that are responsible for converting nitrogen gas in the air to a form of nitrogen plants can use.

Like nitrogen, sulfur moves quickly through the top layer of soil but then it gets trapped in the sub soil. Plants with longer roots can use this trapped sulfur, but short rooted plants may not be able to reach it. Soil tests are normally performed on the top layer of soil and are therefore not very useful in determining the total amount of sulfur available to plants.

Except for sandy soil, your soil probably has enough sulfur.

Micro and Macro Nutrients

In addition to the main nutrients discussed above there are two other macronutrients: calcium (Ca) and magnesium (Mg). Both of these are critical for plant growth and plants require significant amounts of both. Calcium and magnesium are common in many types of rock and therefore most soils contain lots of both of these nutrients.

There are also about 72 micronutrients including chlorine (Cl), boron (B), iron, (Fe), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mo). These chemicals are called micronutrients because they are used by plants in very small amounts.

The micronutrients may not be required in large quantities, but each one is critical for proper plant growth. For example iron is critical for photosynthesis and boron moves carbohydrates around inside a plant.

With the exception of boron most non-sandy soil contains enough micronutrients. Boron is deficient in many soils around the globe but surprisingly it is rarely discussed in gardening circles.

References:

1. Nitrogen cycle photo source: Wikipedia
2.  Phosphorus cycle photo source: Wikipedia