Ask a Soil Scientist

Dr. Terry Firma

I am a Professor of Soil Science at Earth University. I grew up on a small farm and ever since I was a kid, I had my hands in the soil. I helped my family plant and harvest crops such as corn and other vegetables like carrots and zucchini. I studied chemistry in university and now I am a soil scientist. I research how different types of fertilizers help plants to grow, and how this depends upon the soil type. I have had the opportunity to travel all over the world to see how different farmers grow food for us to eat! (See organic agriculture, urban agriculture) If you have any questions about soils, feel free to ask me by filling the form below! I’d be happy to help you!

How do fallen leaves become part of the soil?

When leaves fall from a tree to the forest floor, they are slowly decomposed by organisms that live in the soil. Leaves are broken down by macroorganisms (organisms you can see) such as ants, beetles, caterpillars and earthworms. But they are also decomposed by microscopic organisms such as bacteria and fungi, which can better colonize smaller leaf pieces because of the greater surface area (more surfaces or edges to react with – think of a sugar cube versus a teaspoon of sugar). As the cells in the leaves are broken down, energy and nutrients such as carbon and nitrogen are released into the soil and can be made available to plants and other organisms in the soil. The leaves get broken down so small that they end up as a bunch of little pieces stuck together in the soil called humus.

Decomposition by microorganisms happens much faster in warm, wet conditions and slows down as it gets cooler and/or drier. Forest floors here in Canada are often covered by humus, whereas in the tropics (such as the Amazon Rainforest), the humus is decomposed so fast into the soil, that it doesn’t stay on the forest floor for very long.

What makes a soil red in colour?

Red soils have built up an accumulation of iron oxide, which is essentially rust. The more red the soil, the older it is. Sometimes it is derived from red-coloured rocks, or parent material, that slowly weather in place over thousands of years to form soil (e.g., soils along the coastline of Prince Edward Island). But sometimes, a red soil’s parent material is not red. In this case, dust that contains iron is blown by the wind from another location and is deposited on the soil. The iron in the dust then oxidizes (rusts), and turns the soil red. Over time, more dust containing iron falls on the soil and the soil becomes increasingly red. Red colour also tells us that the soil is dry for most of the year, so we often see red soils in deserts (think of the Australian Outback).

What different particles does soil contain?

Soil particles are defined as less than 2 millimeters in diameter. Particles larger than that are called gravel or rocks.

Soil particles come in three sizes:

  • Sands (0.063 – 2 mm diameter) are the largest and are coarse and gritty. They allow water and air to move into and through the soil because there are large pores (holes) between them.
  • Silts (0.002 – 0.063 mm diameter) are the mid-sized soil particles and are smooth. They feel much like flour or baby powder. They hold water that plants can take up through their roots.
  • Clays (<0.002 mm diameter) are the smallest soil particles, and are fine and sticky. They hold a lot of water very tightly, so they don’t give as much to the plants as silts. They also hold nutrients. They help hold soil together in clods which scientists call aggregates. Clays can be molded and shaped, much like modeling clay or Play-Dough.

An ideal soil would have equal amounts of sand, silt and clay particles. But most soils are not ideal. Some are mostly sand, others are mostly clay or silt, depending on where they are found. Soils that have close to equal mixtures of soil particles are called loams.