Fertilizers – What you need to know

Plants require nutrients to grow and develop, much the same like people. We eat so our bodies can get the vital nutrients and minerals to grow healthy. Most of the food we eat is in the form of large molecules such as protein, carbohydrates, DNA, and fats. Our digestive system takes these large molecules and breaks them down into smaller molecules like simple sugars and nitrates. Our bodies then use these small molecules as building blocks to build new large molecules which are then used to create complex body parts. Plants differ from humans and other animals in that they do not have a digestive system. Therefore, they are not able to break down large molecules.

Plants take small molecules such as carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium and dozens of other minor nutrients, and use them to build large molecules such as sugars, carbohydrates, oils, protein, and DNA. These large molecules are used for everything that happens in the plant. Carbohydrates are used to build cell walls, which in trees, eventually turn into wood. Enzymes are proteins that make all of the chemical reactions in a plant work. Sugars and carbohydrates are the energy source that allows the plant to grow. The production of flowers and fruit require many different types of large molecules and all of these are made by the plant using the small molecules we call nutrients.

The figure below shows a list of some of the large molecules found in plants along with the nutrients used to make the molecules. All of the molecules contain carbon, hydrogen and oxygen. Nitrogen and phosphorus are also common. Other nutrients are used less often, and in smaller amounts, but they are critical for building the large molecules.

• If a single nutrient is missing the plant will not be able to produce all of the large molecules it needs, and plant growth will slow down or even stop. A gardener’s job is to make sure that plants always have access to the nutrients they need. That seems like a daunting task, but it is easier than you think. There are two important concepts to understand:
• Plants need much more than just the nitrogen, phosphorous and potassium found on fertilizer labels.
• They need to eat all of the time—not just once or twice a year when you fertilize.

We need to supplement the nutrient value of the garden soil by making soil amendments in the form of adding fertilizers, preferably organic fertilizer. These nutrients include Carbon, Nitrogen, Calcium, Hydrogen, Phosphorous, Magnesium, Oxygen, Potassium, Sulphur, Iron, Zinc, Chlorine, Manganese, Boron, Copper, and Molybdenum. Soil is the most important source of plant food.

Macro elements

Macro elements refer to the main elements that are required by the plant for its basic functioning. These main elements are:
a. Oxygen and Hydrogen which can be found readily available in water as well as the air.
b. Nitrogen and Carbon which can be found in the air and in organic soil. (Especially the carbon dioxide present in the air.)
c. Potassium and Phosphorous which can also be found in organic soil.

Of these macro elements Carbon, which is obtained from carbon dioxide of the air; Oxygen, which is obtained from air and water; and Hydrogen, which is obtained from water, are required by plants to build its basic cell structure. Thus it is most fortunate that these elements are the most commonly found elements and are required by all living creatures, therefore it is seldom that you need to provide additional carbon, oxygen or hydrogen. Potassium, Phosphorous and Nitrogen, on the other hand, are usually lacking and supplements of these need to be provided for the plants. These are thus the main ingredients in the most basic chemical and organic fertilizer:

Nitrogen (N)
Nitrogen encourages development of plants, it is responsible for healthy green leaf growth which is the result of the formation of chlorophyll, which is the main unit for the production of carbohydrates, proteins and oxygen. Therefore, plants that exhibit a Nitrogen deficiency will show symptoms like stunted growth and pale green and yellow leaves.

Phosphorous (P)
Phosphorous is responsible for cell development and the promotion of good root growth, particularly in fibrous roots, the vigor of the plant. Plants that suffer from a phosphorous deficiency will have poor root development and show symptoms like stunted growth.

Potassium (K)
Potassium is responsible for chlorophyll formation which plays an important part in the strength of cells and encourages flower and fruit formation. Plants that exhibit symptoms of Potassium deficiency will have like weak stems. Other symptoms of Potassium deficiency include older leaves that are floppy with yellow tips and brown margins.

Secondary macro elements

There are also secondary macro nutrients such as Calcium, Magnesium and Sulfur

Calcium (Ca)
Calcium is responsible for the construction of cell walls and promoting proper functioning of growing tissue. Fortunately Calcium occurs naturally in organic soil and there is usually no need for any calcium supplements.  A Calcium deficiency in soil only occurs in extremely acid soil. This is why most plants struggle to grow in acid soil.

Magnesium (Mg)
Magnesium (Mg) also occurs naturally in organic soil which usually makes adding any magnesium supplements to garden soil rather superfluous. Magnesium deficiency symptoms are manifested as the yellowing of older leaves.

Sulfur (S)
Sulfur (S) also occurs naturally in organic soil. Most chemical and organic fertilizer also contains Sulfur which makes Sulfur deficiency very rare. If Sulfur deficiency does occur, it shows up in the form of stunted growth and yellow foliage, much the same as nitrogen deficiency.

Micro elements

Micro elements are also known as trace elements and sometimes people even call them the “multi vitamins” for plants. The micro elements of plants are made up of Iron (Fe), Zinc (Zn), Manganese (MN), Boron (B), Molybdenum (Mo) and Copper (Cu). A Molybdenum deficiency produces whip-tail in some vegetable crops such as cauliflower and broccoli. The Molybdenum deficiency will reduce the activity of the symbiotic and non-symbiotic nitrogen-fixing micro-organisms. A Boron deficiency can manifest itself in varying forms that are dependent on the type of plants and the age of the particular plants. It is seldom that a micro element deficiency occurs. However, if there is a deficiency it is normally manifested as discolored foliage, poor leaf maturation and poor fruiting.

Types of fertilizers

Keeping soil healthy is an essential part of gardening. The materials, i.e. the types of fertilizer, most often used in most areas where cultivation takes place, to maintain and improve soil fertility may be classified as follows:

The natural, organic fertilizer types in the shape of manures. These are usually the relatively bulky fertilizer types, like animal or green manures. Animal and the green manures are usually added to garden soil to; not only improve the physical condition of the soil, but also to replenish and keep up its humus status, as well as to maintain the optimum conditions for the activities of soil micro-organisms and make up some amelioration to a small part of the plant nutrients removed by crops or otherwise lost through leaching and soil erosion. The plant-nutrition that can be found in manure is released in an available form after it is applied to the soil and is decomposed by soil micro-organisms. Similarly, the green manures add not only substantial amounts of organic matter but also nitrogen.

Concentrated organic manure fertilizer types. This fertilizer type is comprised of concentrated materials, such as oil-cakes, bone-meal, urine and blood. It is important for the home gardener to remember that the use of manures and fertilizers is complimentary and should not be viewed as a substitute for each other.

Bulky organic manure type of fertilizers. The properties and role of organic matter and humus in the soil have been explained already in our organic matter page.

Chemical, inorganic fertilizers. Fertilizers are inorganic materials of a concentrated nature; they are applied mainly to increase the supply of one or more of the essential nutrients, e.g. nitrogen, phosphorous and potash. Fertilizers contain these basic elements either in soluble or readily available chemical compound form and can usually be purchased from commercial sources such as garden centers, nurseries, and the like.

Organic ferilizers

Organic Matter. Organic gardening is the practice of using only organic matter in your garden. Organic matter is quite diverse, it is the living and dead particles found in organic soil.

Living particles: – these are the organisms like earthworms, insects and micro organisms such as bacteria and fungi found in organic soil. All living particles are fundamental factors in creating fertile soil.

Dead particles: – these are the decaying remains of old, once-living plants and animals and include bird droppings, grass cuttings, leaves, and even dead, decaying animal remains.

Organic compost is part of the natural cycle of plant growth, where dead and decayed organic matter is returned to the soil, broken down by micro-organisms, and is thus recycled. By adding organic compost to your garden soil you are actually imitating nature. Compost soil is fertile soil. In rare cases organic matter may not supply sufficient plant nutrients and chemical fertilizers are thus required to supplement the nutrient value of the garden soil. The key factor is to make use of organic fertilizer as much as possible and only use chemical fertilizer where organic fertilizer is insufficient.

Compost/ Humus

Humus means mature compost, or natural compost extracted from a forest or other spontaneous source for use to amend garden soil. Compost is regarded as the gardener’s best friend and can be described as well-rotted organic matter that is absorbent, humus-rich and will result in improved soil fertility, structure and water-holding capacity when applied to soil – in other words think of compost as an organic fertilizer. Incidentally compost is formed in nature all the time as plants and animals die and decompose. This natural compost is generally called humus.

Benefits of compost:
a. Improved water retention –humus soil and organic compost will hold water well, and will also absorb water well due to its fibrous texture. This is especially beneficial for garden soil that is sandy in nature and in areas where water is a scare commodity. As mentioned in the following benefit, humus soil will also combat soil erosion. For a greener world we need to all strive towards organic gardening and being soil conservationists.

b. Improved soil texture – soil compaction is prevented due to the soft, fibrous nature of organic compost. The fibrous nature of humus soil keeps the smaller soil particles apart and prevents compaction. Organic compost can also help to bind larger soil particles and prevent soil erosion and water loss, while facilitating rapid root development.

c. Increased nutrient value – organic compost contains high percentages of macro- and micro-nutrients, akin to a “natural” organic fertilizer.

d. Improved nutrient holding capacity – due to the nature of organic compost, the structure of the compost will allow it to hold on to nutrients which could easily be leached away in soil that is not composted or sandy soil and will thus also help in addressing drainage problems.

e. Improved plant health – there are many beneficial micro-organisms, fungi and bacteria in organic compost that will help to break down organic matter. This then results in adding new life to soil and thus improving plant health – really in keeping with the soil conservation sentiment.

Green manure

If your budget does not allow you to indulge in spending copious amounts of money on purchasing organic fertilizer, then green manure is the answer. It is essentially a supplementary means of adding organic matter to the soil. The types of soil that stand to gain most by green manure are the light sandy soils. Green manure is achieved by growing a crop on the site where organic matter is needed and worked directly into the garden soil without having to go via the whole process of decomposing first. The green-manure crop supplies organic matter as well as additional nitrogen. Crops like legumes are especially suited for green manure purposes. Legumes can be cut down and left as an organic mulch on the ground as well. In this way the green manure crops also perform a protective action against erosion and leaching.

Mulch

Organic mulch is quite a loose term that describes coarse organic compost or organic matter that is spread over the surface of the soil. Organic mulch can consist of materials such as compost, peat moss, leaves, grass clippings, bark chips, nutshells, decomposed animal manure, pine needles, wood shavings, sawdust and even straw. Organic mulching holds a multitude of benefits.

Organic mulch adds organic matter to depleted garden soil.
Organic mulching increases microbial activity.
Organic mulching serves as protection against harsh sunlight that dries out the soil, and against soil erosion.
Organic mulching reduces the need for irrigation as it preserves soil moisture.
Organic mulch acts as a weed deterrent because it inhibits the growth of unwanted plants.
Organic mulch protects plants from cold damage as it acts like a blanket.
Organic mulch is visually appealing since it makes the garden look neat and cared for.
Organic mulch is also beneficial in that it increases crop production and helps to cultivate chemical free food with higher nutritional value.

Chemical or inorganic fertilizers 

What is NPK?

It is time to feed your plants and you go to the store to pick up some plant food which is called fertilizer. Most packages of fertilizer show the letters NPK followed by some numbers, for example: NPK 10-5-5. NPK stands for nitrogen, phosphorous and potassium which are three of the most important nutrients required by plants. The numbers following NPK are the percent amounts of each nutrient. An NPK value of 10-5-5 means that the fertilizer contains 10% nitrogen, 5% phosphorus and 5% potassium. NPK is the common way to describe fertilizer in some countries while but some use an NPKS value where the S stands for the amount of sulphur. Sulphur can be as important as the other three nutrients.

The letters N, P and K are the symbols used by chemists as a short hand to describe the elements. N is used for nitrogen and P for phosphorus. The letter K is used for potassium and stands for kalium, the original Latin name for potassium. If you have trouble remembering whether P is for phosphorus or potassium, remember that the three nutrients are listed in alphabetical order. Phosphorus comes before potassium alphabetically and so the last letter in the list, K, is short for potassium.

A bag of plant food that contains 10% nitrogen, 5% phosphorus and 5% potassium, also has 80% of some other material, but you can ignore it since it has no effect on your plants. If the fertilizer is in a dry form like lawn fertilizer the 80% may be small stones or other dry inert material. If the fertilizer is in liquid form the extra material is water.

How Does a 10-10-10 Fertilizer Compare to a 5-5-5 Fertilizer?

This is a very common question. Should I use a 10-10-10 fertilizer or a 5-5-5? What is the difference? The answer is simple. It doesn’t matter which you use because they provide the same relative amount of nutrients. It is however important that you use the correct amount of fertilizer. When dealing with fertilizer there are two things that are important: the actual amount (the weight) and the ratio of nutrients. The ratio is the relative amount of each nutrient. In the two examples above, there is an equal amount of each nutrient and so the ratio is 1:1:1 for both of them. In comparison, an NPK value of 20-10-10 has twice as much of nitrogen as phosphorus or potassium and so the ratio is 2:1:1. A 10-10-10 fertilizer has 10% of each nutrient and a 5-5-5 has 5% of each nutrient so a bag of 10-10-10 contains twice as much fertilizer as the same sized bag of 5-5-5. The 10-10-10 is more concentrated, but both have the same ratio.

Which one is best for your garden? It doesn’t matter since they have the same ratio. If you need to add equal amounts of N, P and K, either one works just as well. However, you will have to use twice as much of the 5-5-5 to provide the same level of nutrients as the 10-10-10. The most important thing when buying fertilizer is to buy the correct ratio so that you get the correct relative amounts of nutrients. In general, a fertilizer with higher numbers is cheaper. Many of the liquid fertilizers on the market are very dilute, in the range of 1-1-1, and they are also some of the most expensive fertilizers you can buy. From a price point of view, always buy the one with the higher numbers, provided it has the correct ratio. 

What is the correct NPK ratio for your plant? 

Fertilizer is now available in many different NPK ratios and knowing which one to use can be very confusing. As you begin to understand fertilizers and plants you will start to realize that the answer is actually very simple. It is however very instructive to look at the question in more detail. Lawns need one formula, tomatoes need a different one. Tress and perennials are different again. It all gets very confusing and it is all bad advice.

Statements such as the following are wrong:
Use 5-10-5 fertilizer for flowers.
Use 34-10-10 on grass
Use 5-7-3 for vegetables.

The advice is wrong for several reasons.

Flowering plants might in fact need more phosphorus—the middle number—but it is just as likely that this advice is based on old myths and not on actual science. At best these recommendations are based on plant tissue analysis and not on nutrient levels in your soil. The main reason the above recommendations are wrong is because you don’t add fertilizer to plants – you add it to soil. Read on and this will make sense shortly.

You Don’t Add Fertilizer to Plants 

The idea that we feed the plant what they need seems to make perfect sense but it ignores one very important point. Plants get their food from the soil. You don’t add fertilizer to plants—you add it to soil. This is an extremely important concept that is not well understood and is best explained by a simple example. Assume that your soil is naturally very high in phosphorous. The fertilizer you use at transplant time, or for flowering plants, or to get more blooms does not need a high amount of phosphorous because your soil already has more than your plants can use. The fertilizer you add to the garden should be a supplement to what is lacking in your soil.

If your soil has high phosphorus levels, any fertilizer you add – for any type of plant – should contain no phosphorus because you already have too much. Plants absorb the nutrients they need from the soil. If grass needs more nitrogen, it takes more nitrogen from the soil than a plant that needs less nitrogen. If a plant is ready to make flowers and it needs more phosphorus, it takes more phosphorus from the soil.

Matching fertilizer to what a plant needs does not make sense. Instead you should match the fertilizer to what the soil needs. If your soil is low in nitrogen and high in phosphorus you should use something like a 20-0-5 fertilizer for all of your plants no matter what type they are. Despite all the different types of fertilizers that are available from local nurseries and garden centers it is easy to work out which fertilizer is best for which types of plants by simply reading the container. If you are in doubt regarding the suitability of the fertilizer for your garden then make use of the NPK ratio rhyme.

N – Leaves and shoots
P – Stems and roots
K – Flowers and fruit

Applying Liquid Fertilizer

Liquid fertilizer is usually concentrated. They need to be mixed or dissolved in water before application to the intended plants. When properly diluted you also negate the chances of fertilizer burn. Properly diluted liquid fertilizer is ideal for use of soft, sensitive and young plants. Fertilizer for indoor plants, specialized food for seedlings, bonsai fertilizer, and orchid fertilizer are all water soluble.

Applying Granular Fertilizer

Granular fertilizer can be used in its dry form sprinkled on the soil around the intended plants. After application these types of fertilizer should be watered in immediately to avoid the plants burning. Using granular fertilizer is a trifle more complicated than using liquid fertilizer. In many cases gardeners would just scatter a handful or so around each plant and hope that it will suffice. This brings on problems like burnt plants and even overfed plants, or alternatively not applying enough fertilizer for the size of the plant.

Credits for information: Robert Pavlis, Landscape and Garden