Can I use compost for my indoor plants?

Photo by Karolina Grabowska: https://www.pexels.com/photo/putting-soil-in-pots-4750383/

When we think of compost, we think of top-dressing for vegetables. We might also think of mucky odors, including the smell of animal manure and other rotted delights. So what’s the consensus on using compost for indoor plants?

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What is compost?

Compost is organic matter that has decomposed into its simplest chemical parts. Anything that came from a plant or animal can end up as compost: paper, leaves, grass, fruits, vegetables, wood, or coffee grounds can all be added.

Microbes found in the natural environment are what speed the process of decomposition along. Once broken down into their simple chemical parts, the organic matter releases all the valuable minerals that it once contained into its surroundings, ready to be taken up by plant roots.

Compost is full of the three main nutrients that plants need to grow and maintain themselves. These are nitrogen, phosphorus, and potassium.

An important ingredient in compost is humic acid, and good quality compost will contain about 5 percent of this substance. The chemical increases the negative charge of soil particles, allowing them to cling onto positively charged ions of elements like sodium, potassium, magnesium, calcium, and zinc that plants need to grow.

Can I add compost to indoor plants? 

Some indoor plants will love compost in their potting mixture. These will generally be rainforest species like ficus elastica, philodendron, and begonia or inhabitants of damp forests like Pilea.

Ferns from both tropical rainforests and temperate woodlands will really benefit from the moisture retaining properties of a high quality compost.

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These plants evolved in the dark, dense undergrowth of tightly packed forests where lots of rotting organic matter from trees and animals form a natural compost rich in mineral nutrients.

Since tropical rainforests also tend to be very moist and humid places, plants like ficus or monstera do well in a compost-rich potting mix. Compost is a naturally heavy material compared to other potting mediums like perlite, coconut coir, or sand and it is much better at sucking up any water you put in it.

Orchids are one tropical species that likes high-quality compost but due to their specific needs, it is best to buy a purpose-made orchid compost.

Drought-loving cacti and some succulents will prefer less compost in their mixes.  For these species, mineral materials like perlite and sand should be the dominant component of their potting mixtures. 

Cacti that like the driest conditions will do well in almost pure mineral materials like pertlie with just a few tablespoons of compost mixed in for added nutrients. Those that need slightly more moisture will appreciate a mixture of one part compost to two parts mineral potting material.

Problems with adding compost to indoor plants

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There are many benefits to using compost for your indoor plants, but there are caveats.

Houseplants can't thrive in compost alone. You should always mix the compost with materials of a different texture in at least a 2:1 ratio (2 parts compost to one part lighter material). This even applies to tropical houseplants that generally need heavier organic potting mixes.

Adding lighter materials like horticultural sand, perlite, or coconut coir to compost is necessary because, unlike in a natural tropical environment, your plants are growing inside a container with a limited volume of soil. Too much heavy potting material like compost can easily lead to waterlogging.

In these conditions, excess water cannot drain away easily and can easily become waterlogged. Too much water in the soil pushes out those all-essential air pockets that provide a flow of oxygen to the roots. Depriving roots of oxygen leads to root rot and, eventually, your plant will die.

Adding other materials into the compost mix is also advisable because, over time, the compost will continue to break down as it sits in the pot. As this happens, it can start sinking to the bottom of the pot. Horticultural sand or perlite will prevent the soil from becoming too hard and compacted.

Another potential problem with using compost indoors is that it may be harboring unwanted pests that can infect your plants.

To prevent pests from sneaking into your home, you should bake the compost in the oven at temperatures between 275 to 300 degrees Fahrenheit. Make sure you turn the compost over after ten minutes of baking so that it is sterilized right the way through.