University of Maryland Extension

Urban soils and growing media

Author: 
Neith Little, Extension Agent, Urban Agriculture


Urban Ag home | Table of contents 

 

One of the common denominators I’ve observed among farmers who grow plants is that they will talk for hours about what they grow their plants in. Rural farmers? They complain about how many rocks they have in their soil and brag about their organic matter content. Ground-based urban farmers? They swap tips and tricks for building up healthy soil from construction rubble, reducing compaction, and avoiding contamination. Hydroponic urban farmers? They argue passionately about whether coconut coir plugs are better than peat moss plugs, or whether it’s best to put the roots directly in the water (nutrient film technique) or in air with misted water and nutrients (aeroponics).

Whatever you grow your crops in, how well you understand it and manage it will have a huge impact on how well your plants grow.

What is soil? What is a growing medium?
Soil is the result of a mind-bogglingly long process wherein the rocks of the earth’s crust are gradually broken down into very small particles by the environment and living organisms. Soil is made of tiny particles of rock (sand, silt, and clay are size classes of tiny rock particles), dead biological material (organic matter), and living organisms (from “macroinvertebrates” like worms down to microbes, fungi, and viruses).

In some places, growers are blessed with soil that is well suited to growing plants. In other cases, the native soil requires amendment with other materials to improve its ability to support life. Many outdoor urban growers have added so much organic matter in the form of compost or wood chips, that the "soil" they grow in might more accurately be called a growing medium. In some cases, growers start from scratch by importing soil from elsewhere or using a "soilless growing medium:" a mix of materials like peat, coconut coir, vermiculite, perlite, wood chips, course sand, or compost. Hydroponic or rooftop growers might use specialized growing media such as rockwool or clay pebbles (hydroton).

Historically, rural farmers grew in soil and plant nurseries grew in pots of soilless growing media. But, as in many other areas, urban farmers are trying new things and blurring the lines between what used to be distinct categories. Applied researchers are working hard to expand our scientific understanding of the growing media urban farmers use, and what best practices we can recommend.

If you are growing outdoors in soil, you’ll probably need to learn about soil science: what size your soil particles are and how they affect how well your soil holds water and air, how your soil’s chemical properties (pH, fertility, cation exchange capacity, organic matter) affect your crops, which organisms that live in soil are beneficial and which cause problems, and how you can work to improve some of these things. Various soil tests can tell you a lot about these soil properties.

If you are growing primarily in something other than soil, whether outdoors, in a high tunnel, or using hydroponics, you may be able to learn from the experience and research of the plant nursery industry. You’ll still need to understand your growing medium’s chemical properties, but you’ll need to use tests designed for growing media, not soil. You’ll also need to pay particular attention to your growing medium’s salinity (electro conductivity) and the quality of the water you use. And when deciding what growing medium to use remember that, as in many parts of life, a mix is usually better than too much of one thing. For more information on growth media, go to https://go.umd.edu/growingmedia

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