News release: Potting Soil Basics

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640

Release Date: February 1, 2018
Headline: Potting Soil Basics

Selecting a good potting soil for your indoor plants is an important decision.  The proper soil for your plant will provide the best conditions for good root growth and nutrient uptake.  On the other hand, poorly mixed soils will give you less than satisfactory results, with short-lived plants.  To select the proper soil for your plants, you need to think about the plant's requirements and your own plant care habits.

The plant's requirements are dictated by where the plant is found in nature.  Is it native to an arid desert?  Or a tropical rainforest?  In what types of soils does it grow in its native environment?  If you are not sure of your plant's preferences, pick up a copy of Guide Sheet G6510, "Caring for House Plants", from your local MU Extension center.  This reference lists many common houseplants and their soil requirements.

Your own plant care habits can also influence potting soil selection.  Do you tend to overwater?  You may want to use materials in your soil which improve drainage.  If your plants dry out too fast, you should add ingredients such as sphagnum peat moss or other organic materials to increase water retention.

If you have only a few plants, you may choose to buy a ready-made mix.  This saves time and work.  If you have a plant with special soil requirements, you may start with a commercial mix, and modify it to suit you needs.  If you have a lot of plants, you may choose to make your potting soil from scratch.  This will save you money, and will give you exactly what your plants require.

Often the basic starting ingredient is garden loam or topsoil.  Loam soils are classified between sandy soils and clay soils.  Since most soils are not pure loams, they are usually classified as sandy loams, silty loams, silty clay loams, or clay loams.  Knowing how much sand or clay is in your basic mix will help fine tune your final potting soil.

So where do you start?  A good basic mix for most houseplants will have equal portions of garden loam, sphagnum peat moss, and clean sharp builders sand.  Be sure that the sand is not too fine.  Coarse sands provide better drainage.  If you can't find a good coarse sand, perlite may be substituted.  A third choice would be vermiculite, if neither builder's sand or perlite can be found.

Some people prefer soil-less mixes, such as vermiculite, perlite and sphagnum peat moss.  These may work well, but be sure to fertilize frequently and carefully.  These mixes do not hold fertilizer well.  You may also need to add micronutrients from time to time.

What about using soil from your own garden?  This can be done, with caution.  The problem is that you may be introducing unwanted pests or plant diseases.  The solution is to pasteurize the soil before using it in your mix.  You may do this in your oven, or even in an outdoor covered barbecue pit.  But, be forewarned… this is a very smelly process.

The soil should be placed in a large covered kettle.  It should be moist, but not too wet.  Bake it at 180 degrees until the center has reached a minimum of 140 degrees.  Let the soil cool before using.  Then use the soil as a base for your mix, modifying it by adding the appropriate amendments.  While pasteurization may seem like a lot of trouble, it may eliminate problems later on.  This is especially true when you are starting plants from seed, where damping off problems may occur.

What about compost?  This can be an excellent material to add to your potting mix.  Be sure that it is well-composted, and has heated up well during the composting process to provide pasteurizing protection.  Leaf compost is especially good, providing a good source of organic matter.  Use it alone or in combination with sphagnum peat moss.