The tree letters NPK stand for the three most essential nutrients in soil that are needed by plants. N stands for Nitrogen, P stand for Phosphorus and K stands for Potassium. In addition, there are other macronutrients in soil such as calcium (Ca), magnesium (Mg) and sulfur (S). The proper fertilization is essential to keep your houseplants happy and growing well.
Nutrient concentration in fertilizers is indicated by numbers. The higher the number of one nutrient in relation to the others, the higher is the concentration. Labels and description on fertilizer use the NPK format to indicate the strength and concentration of each of these nutrients in the fertilizer by a series of numbers.
An example would be a ratio of 10-10-20. Since the order is NPK, this would mean that the concentration of Potassium is twice as high as the concentration of Nitrogen 10 and Phosphorus 10. In addition, 10-10-10 compared to 5-5-5 contains twice as much of all the nutrients: Nitrogen, Phosphorus and Potassium.
Let’s now have a closer look at each of these three macronutrients that are also essential nutrients in soil:
Nitrogen is a major component of chlorophyll in plants. Chlorophyll is used to produce sugars from water and carbon dioxide using sunlight energy. Nitrogen is essential for plant growth. Although it is one of the most common elements on mother earth, it is the main nutrient deficiency plants suffer from. Plants contain about 3-4% of nitrogen. This is measured in the plant tissue above the soil.
Pale green or yellow leaves, as well as poor growth, are common symptoms of nitrogen-deficient plants. The poor growth is a function of the yellow and pale leaves. These lead to a reduced ability to produce chlorophyll. The yellowing first happens in older leaves. Plants have the ability to transport available Nitrogen (N) to new leaves and growth points. This is why older leaves get pale or yellow first. They eventually fall off completely.
In some plants, the leaves may turn purple instead of yellow. Nitrogen deficiency also leads to plants growing smaller and being more susceptible to pests and other deficiencies.
How to spot Nitrogen Deficiency
- Yellow leaves / Purple leaves
- Pale leaf colour
- Upright leaves
- In extreme deficiencies, the leaves may appear burnt
- Older leaves turn yellow first
- The yellowing starts from the centre of the leaves
- Stunted growth
- Leaves start falling off
- Stems and bases of plants start to turn red or purple
How to prevent Nitrogen Deficiency
Intense growth or stress can encourage Nitrogen deficiency. To prevent a Nitrogen deficiency it is best to make use of a fertilizer with a high Nitrogen (N) value. There are different types of fertilizers. Foliar sprays that are applied directly on the leaves, liquid fertilizers that are indeed liquid as well as slow-release fertilizers that are solid and are applied to the soil.
Phosphorus plays an important role in cell division and plant growth as it fosters the growth of new tissue. It converts other nutrients into building blocks that are essential for plants to grow. It is also used in different key structures in plants. A future effect of Phosphorus is capturing and converting the suns energy into different plant compounds. It is essential for the health of plants and is responsible for:
- Accelerated root growth
- Stem strength
- Resistance to diseases
- Better flower and seed production
- Crop quality
Phosphorus is the second most essential nutrient for plants after Nitrogen. A deficiency leads to limited plant productivity during early growth. It is therefore spotted early in the vegetative development as it leads to an unnatural dark green and sometimes red and purple foliage. It further delays the maturity of plants and reduced yields if plants or their fruits are planted for consumption. Phosphorus deficiency can be more difficult to spot compared to Nitrogen (N) and Potassium (K).
How to spot Phosphorus Deficiency
- Unnatural dark green or red/purple foliage
- Purpling is often concentrated to veins
- Yellowing of leaves may also occur
- Loss of leaves
- Bronze colour under the leaves
- Older leaves are affected before younger ones
- Growth of deficient plants often is only 1/2 of healthy plants
How to prevent Phosphorus Deficiency
Appropriate temperatures adjusted to the natural habitat of the plants / the plant’s needs is essential as cold temperatures favour a Phosphorus deficiency. In addition acidic soil as well as strong rainfalls (in outdoor conditions) or heavy watering foster a phosphorus deficiency. Applying bone meal to the soil is a great way of increasing the pH level.
We are using Espoma Bone Meal because it is not only natural but also available at an affordable price and we see great results.
Potassium regulates C02 uptake and plays an important role in various processes such as growth regulation. It plays an essential role in opening and closing of stomata. In botany, a stoma or stomata (plural) are pores in leaves stems and other organs that regulate the gas exchange. Potassium is responsible for:
- Plant growth regulation
- Essential for CO2 uptake
- Used to fight plant diseases
- Essential for fruiting
Symptoms of Potassium deficiency are yellowing leaves between the veins (chlorosis), curling leaf tips and brown scorching and curling on leaf tips. An additional sign is yellow leaf spots on the underside of leaves. These symptoms often appear on older leaves first as plants have the ability to transport Potassium (K) to support younger leaves when it gets scarce. Potassium deficient plants often have reduced growth, stunted root development as well as decreased seed and fruit development.
How to spot Potassium Deficiency
- Yellowing between the veins (chlorosis)
- Curling leaf tips
- Brown scorching
- Yellowing or browning leaf edges
- Symptoms appear on older leaves
- Reduced growth
- Stunted root development
- Decreased seed/fruit development
How to prevent Potassium Deficiency
Humidity is important for successful Potassium uptake. The amount of water in the soil plays a crucial role. Increased organic matter in soils ensures that potassium retention is increased. A further method is to bury banana peels in the soil as bananas are rich in potassium.
When buying or applying an NPK Fertilizer it is important to know that the nutrients in soil different plants need varies. It is therefore essential to know the optimal concentration of Nitrogen, Phosphorus and Potassium that your plant needs. As an example, a Monstera Deliciosa has different NPK needs compared to a Fiddle Leaf Fig.
Applying too much of a certain nutrient or using the wrong balance between NPK can lead to negative effects. If you are unsure of the needed concentration you can either use compost or a slow-release fertilizer as these methods lead to limited fertilization and the chance of overfertilization is limited.
A different methods is to use a balanced fertilizer:
Balanced fertilizers have equal ratios of Nitrogen, Phosphorus and Potassium. An example of a balanced fertilizer that would add nutrients in soil would be 5-5-5 where each macronutrient has the same concentration. So in simple terms, if all the three number are the same, it is considered to be balanced.
The problem with a balanced fertilizer is that the need of plants varies and most plants do not need the same amount of all three nutrients. In fact, the phosphorus concentration in a balanced fertilizer is often higher than what most plants need. In addition, Nitrogen moves easily in the soil while Phosphorus and Potassium don’t. By applying a balanced fertilizer over time, you are increasing the amount of Phosphorus and Potassium to a level that is too high eventually.
We, therefore recommend using an uneven distribution of Nitrogen, Phosphorus and Potassium such as 5-1-2 or 5-1-3. This reflects the smaller need for Phosphorus and increased need for Nitrogen in most plants in relation to Potassium.
As a hydroponic grower, you are growing using a soil-less system. This means that you are providing everything plants need in liquid form. As soil contains decaying matter, nutrients and minerals you have to provide these nutrients usually present in soil in liquid form to your plants. Growing plants in a hydroponic system require using liquid fertilizer applied to the water.
The nutrients plants need can be segmented into two groups. Firstly, we have the macronutrients. Macronutrients are NPK or Nitrogen (N), Phosphorus (P) and Potassium (K). These are needed by plants in big quantities. Secondly, we have micronutrients that are needed in lower quantities.
There are different ratings on fertilizers. If they are marked with “grow” or “vegetative” this means that they help in the vegetative state of plants. They increase growth and ensure lush and green foliage.
When they are marked with “flowering/fruiting” the fertilizer will support to induce the flowering and/or fruiting process. These fertilizers usually are rich in phosphorus (P) and potassium (K).
There is a wide array of NPK test kits to measure nutrients in soil on the market that test the occurrence of nitrogen, phosphorus and potassium in the soil. One we use ourselves all the time is the Luster Leaf Digital 1605 Soil Test Kit as it not only tests the availability of NPK individually but also measures the pH level in the soil. It has a handy LED display (indicators) reading nutrients in soil and comes with test tubes. The results it delivers are very accurate even compared to actual laboratory results.
NPK Monstera – Or what fertilizer to use for nutrients in soil for a Monstera
We generally recommend using a slow-release fertilizer that you can put directly into the soil. The big advantage is, that it releases nutrients over time and not all at once. This way the risk of overfertilization is minimal.
If you want to use a liquid fertilizer to add nutrients in soil, it is best to use it at half-strength. Indoor plants such as a Monstera Deliciosa are confined in a pot and it is very easy to overfertilize in such a scenario. In addition as most fertilizers will be balanced, there is a chance that your plants will get more Phosphorus and Potassium than they need. These are the reasons why most houseplant care guides will suggest to use fertilizers at half the recommended strength.
Nutrients in Water
Lets not just look at nutrients in soil but also at nutrients in water. Unlike soil that contains nutrients, water itself is an inorganic, odourless and transparent substance. It is the main ingredient our planet consists of. It is essential for all life although it doesn’t contain any nutrients or calories.
However, water takes up nutrients by coming in contact with soil and rocks and due to the geological setting such as the climate according to the WHO. Also, the chemical composition of water is impacted by the contamination through multiple effects such as industry, agriculture, human civilization and others. In addition, different chemicals are added such as chlorine, control corrosion treatments as well as chemicals to adjust the pH level of water.
Besides chemicals, tap water can contain multiples nutrients:
- Potassium – usually only found in very low concentrations
Water is generally differentiated into two categories. Hard water and soft water:
Hard water is rich in minerals such as magnesium and calcium. The main use of hard water is internal consumption.
Soft water is low in mineral content. It is perfect to be used for cleaning and in conjunction with soap.
Besides hard and soft water, let’s have a look at tap water:
Water coming out of the tap has been purified and treated. This process removed most of the nutrients present in water. In addition chemicals such as chlorine and sodium may have been added to make the water drinkable. That is why gardeners and indoor plant enthusiasts recommend using rainwater or using a water purification system such as a reverse osmosis filter or a water distiller.
It is important to note that even rainwater is not free of any chemicals and pollution as these can also be present in the atmosphere.
Frequently asked questions regarding Nutrients in Soil
How do nutrients get into the soil?
Nutrients get into the soil in multiple ways. By decaying and decomposing animal and plant material in the soil. By bacteria and weathering rocks, by rainwater and from the atmosphere.
What nutrients are in the soil?
Some of the essential nutrients in the soil are NPK. N stands for Nitrogen (N), P stands for Phosphorus (P) and K stands for Potassium (K). In addition, other nutrients in the soil are calcium (Ca), magnesium (Mg) and sulfur (S).
Why do plants need nutrients in the soil?
Soil is essential for plants as it holds the roots of plants. Roots are essential for nutrient uptake. In the soil are many essential nutrients that plants need for growth, flowering and for fruit production.
Fertilizing plants is essential no matter if you are an outdoor gardener or are taking care of plants indoors. Studying nutrients in soil and knowing the needs of your specific plant are the key to good looking and happy growing plants.
We are sorry that this post was not useful for you!
Let us improve this post!
Tell us how we can improve this post?
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.