What is soil?

A soil profile to a depth of about 50 centimeters

Soil is a mixture of broken rocks and minerals, living organisms, and decaying organic matter called humus. Humus is dark, soft and rich in nutrients. Soil also includes air and water.

Organisms in the soil need air and water to survive. Having these essential materials - air, water, and organic matter - makes it possible for plants, bacteria, fungi and small animals like earthworms and insects to live in the soil.

All the living things in the soil, plus essential materials that these organisms use to survive, form the soil ecosystem. Scientists study the soil ecosystem because they want to understand how organisms relate to one another and to the environment that surrounds them.

How do we study soil?

Scientists who study the soil ecosystem - soil and the living things in it - are called soil ecologists. Soil ecologists study the soil ecosystem in a variety of ways. What all of these ways have in common is that they rely on careful observation and measurement.

With surveying, scientists watch a small part of a natural area in detail, making observations and measuring things like the soil temperature and moisture. Then, they might come back to the area again later, or they might move on to a nearby area. Surveying lets scientists get detailed observations, but these measurements are a few snapshots in time, and so do not say anything about how the area changes.

With experiments, scientists control some parts of the environment and watch how other parts of the environment change as a result. Soil ecologists conduct experiments either in the field or in a lab. In field experiments, scientists compare nearby areas under different conditions. In lab experiments, scientists collect soil and bring it back to their labs, where they can control the conditions more carefully. Experiments let scientists control the environment and take careful measurements, but the experiments might work differently from what would have happened naturally in the study area.

With sampling, scientists collect soil or organisms in the field and bring them back to the lab. They might identify and count different types of organisms, or they might examine soil in the lab. One type of sampling is to study the DNA of the collected samples. Like surveying, sampling can offer detailed information about organisms, but it gives only a few snapshots in time.

With long-term monitoring, scientists look at a natural area for a long period of time and record what they see. They measure the soil conditions and record any organisms they find there. Long-term monitoring lets scientists observe organisms in their natural environment over a long period of time, but monitoring can be expensive, and the presence of the scientist disrupts the environment they are trying to study.

Team members Razvan and Lijun set up a sensor network in Leakin Park in Baltimore, Maryland, USA

We are researching a new method, for long-term monitoring, called sensor networks. In a sensor network, small sensors are placed in the ground to automatically measure conditions such as temperature and soil moisture, directly in the soil. These measurements happen every few hours for several months. The measurements are then uploaded onto computers, from which they can be maintained and searched through. Sensor networks allow for regular measurements over a long period of time, without disturbing the area to be studied. See the About Sensors section for more information on sensor networks.

Soil photo from soil-science info's photostream.