How to Choose the Best Soil and Fertilizer for Indoor Plants

How to Choose the Best Soil and Fertilizer for Indoor Plants
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Choosing the right soil for your indoor garden sounds as simple as heading down to your local garden centre and grabbing any ol’ bag of dirt, but soil selection should be a much more involved process and a variety of factors need to be considered. What kinds of plants are you growing? How much sun are they going to be receiving? How much water do they like? Will they need extra fertilization? These are just a few of the questions you should think about before deciding on a potting mix for your indoor garden.

Choosing the Best Soil for Indoor Plants

It might seem like a good idea to grab some soil from your outdoor garden, but garden soil is quite different than potting soil and could kill your indoor plants. Not only will outdoor soil have drainage issues when used indoors, it may contain bacteria that could be harmful to your plants when trapped within a moist, enclosed space. An outdoor garden is exposed to more airflow within the soil and has better natural drainage than you’ll have with your potted plants, so stick to potting soil for your indoor garden.

Not all potting soils are created equally. Some may have very little organic material, some are chalked full of nutrients, some are designed for specific types of plants, and some contain no soil at all, which technically makes them a “potting mix” rather than “potting soil”. The price of potting soils and potting mixes can range from dirt cheap to very expensive. You don’t need to go with the priciest option, but you do need to be aware of what to look for within your soils.

What’s in Potting Soil (and Why Should You Care?)

Knowing what should be in a bag of potting soil and understanding all the different potential ingredients will help you choose which soil is best for your indoor garden. Potting soils will often contain peat moss which will hold moisture and can help retain nutrients. You can use peat moss on it’s own as a potting mix for some types of plants, but it may be too acidic on it’s own for others.

Pine bark is often added to potting mixes to provide some moisture and nutrient retention and to allow for better drainage. Perlite or vermiculite can be added to help retain moisture and to increase aeration as well.  While these materials play a similar role, vermiculite will tend to retain more moisture and perlite will provide superior drainage.

Organic materials are necessary for your plants to receive nutrients so you will often find compost or humus in the mix. A nutrient rich, moisture retaining, and well draining soil is usually the ideal mix for most plants. However, some plants require less drainage and more moisture than others and vice versa.

What Type of Plants Are You Growing?

Different plants like different soils. Species that don’t like a lot of water, like cacti and succulents, will prefer coarse soil that contains some sand allows for maximum drainage. If their roots are allowed to soak in too much moisture, it can lead to root rot and death for these types of plants. You can purchase potting soil intended specifically for cacti and succulents.

Orchids like moisture but they need plenty of air around their roots or they can rot as well. Soils containing plenty of peat moss, perlite, or fir bark will allow for the necessary drainage. Some plants, like roses, prefer a slightly more acidic soil, so pH is also something you should be watching for, depending on how sensitive your specific plants are.

Sun-loving plants who receive plenty of direct, natural light will need a heavier soil that retains plenty of moisture as they’ll tend to dry out faster. The opposite can be true for low-light plants that spend most of their days hiding in the shade.

When to Fertilize Your Indoor Plants

Your potting mix could be sufficient to grow healthy, luscious plants if they already contain the necessary nutrients, but there are times that you may require the use of fertilizers. The nutrients in outdoor soil is naturally replenished through decomposing plant and animal matter but the soil in potted plants lives a much more sheltered life. Once your plants have sucked the soil dry of nutrients, or if they’ve slowly been washed away when watering your plants, fertilizer can be added to the soil to replenish the nutrients.

During the high growth seasons, such as spring and summer, your plants may require additional nutrients to help them flourish. Keeping them well fed during these periods will increase their growth rate and keep them strong and healthy. Seedlings will require fertilizer as well, so when you’re growing new plants from seeds, give them a little nutrient boost after their first true leaves appear.

Over fertilizing can cause yellowing, browning, or wilting leaves, brown or blackened roots, and can slow or stop additional growth. Potted plants will often require a much smaller amount than what you’d use in an outdoor garden so be aware of how much fertilizer you’re giving to your indoor plants. Some potting mixes will come with fertilizer added so if this is the case you probably won’t need to add more as you’ll risk over fertilizing your plants.

Understanding the Ingredients in Fertilizer

NPK Nutrient System Diagram

The three macronutrients in fertilizer are nitrogen (N), phosphorus (P), and potassium (K). You’ll see the ratio of these elements listed on bags of fertilizer as an NPK value and they will always read in order of nitrogen, phosphorus, then potassium. The higher the number of each element, the higher the concentration. Some values may even read as zero, meaning only one or two of these elements are present is that specific fertilizer.

Other secondary and micronutrients may be present as well and these would include any combination of calcium, magnesium, sulfur, boron, chlorine, copper, iron, manganese, molybdenum, and zinc. These elements are needed in smaller quantities than three main macronutrients.

Choosing the Best Fertilizer for Indoor Plants

The NPK ratio you’ll need for your plants will vary between the growth stages of your plants and their species. Nitrogen promotes leaf growth, making it essential for any plants with lots of green foliage and it will be needed in smaller amounts for flowering plants. Phosphorus helps roots, fruits, and flowers grow and can be added to the soil before planting. Potassium will help with the overall strengthening of the plant and can stimulate early growth.

Once you know which elements to look for in a fertilizer, you’ll need to decide between a liquid fertilizer or a granular, slow release fertilizer. Liquid fertilizers will provide nutrients immediately, which can be convenient if you want to get nutrients into your soil quickly. However, too much liquid fertilizer can damage and burn your plants so it’s important not to add too much at once.

For an ongoing nutrient supply, liquid nutrients will need to be regularly added every few weeks. Granular, slow-release fertilizers are released into the soil more slowly.  They aren’t as fast acting but they’ll release nutrients for a much longer period of time and can be applied just once or twice in a year.

Your Knowledge Will Grow With Experience

Understanding soils, potting mixes, and fertilizers can be a little tricky at first. Once you know what your specific plants need and match that up with the right mix of soils and fertilizers, you’ll be able to provide your indoor garden with everything it needs to be as luscious and vibrant as possible. As you become more familiar with the various materials and nutrients your plants need, you may even be inspired to concoct your own potting mixes.

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