Urban Agriculture

Organic Gardening Fundamentals:

Soil Ecology and Plant Life

Part 1

By Brian Barth

 Real farmers don’t grow crops, they grow healthy soil….

This is an old adage that beginning gardeners eventually learn, and seasoned gardeners continue to develop their understanding of. Over the next few weeks, I will outline the ABC’s of soil science—including the structure, chemistry and biology of soil fertility—and give an overview of the best soil-building practices for home gardeners. Each week’s posting will build on the information from the previous week and together should illuminate some of the fundamental relationships between soil life and the growth and good health of your vegetables, herbs, fruits, berries, and other crops.

Structure

Next time you have the opportunity to be in the forest, find your way to a tree that is at least two feet in diameter. Kneel down and pull back the layer of duff with your hands to see the soil that nature has created in the time since the landscape was last disturbed. Scoop up a handful and you will know first-hand the holy grail of organic gardening—the fertile loam.

The cookie crumb texture, dark color, and earthy smell of the soil in a mature forest are the end results of a natural process that can be mimicked and accelerated in a garden environment. Farmers use the word tilth to describe the water- and nutrient-holding capacity of fertile soils with a loose, crumbly texture. If you used a microscope to examine the structure of a soil with good tilth, you would see something like the texture of a sponge—an immense labyrinth of cracks, crevices, and crannies where water molecules (and the nutrients they carry) can seep in and be stored. The immense surface area of each particle of a loam soil is easily penetrated by tiny rootlets, the active tips of a root system that seek out and absorb moisture and nutrients.

To unravel the mysteries of fertile soil and the processes that create it, we must begin with understanding the three physical components of soils and their properties. Sand, silt and clay particles each add different qualities to soil based on their relative concentration:

  • Sands are the largest soil particles and are essentially tiny pieces of weathered rock. Sandy soils have a lot of air space, which allows moisture to dissipate quickly, leaching away nutrients in the process. Thus, extremely sandy soils have a low concentration of nutrients, but offer excellent drainage for the plants that need it.
  • Silt particles are medium-sized and have moderate of nutrient- and moisture-holding ability. Silt results from the weathering of sand particles and is often deposited in large quantities in flat, low-lying areas by the natural process of erosion. The world’s great river valleys are agricultural powerhouses because of the high silt content of their soils.
  • Clays are the tiniest soil particles and are formed by a chemical process rather than weathering. They are flat, plate-like particles held together by chemical bonds. Clay particles attract and bond with water and nutrient molecules,  providing a slow and steady release to plants. However, the dense nature of soils with a high clay content can impede drainage, limiting the growth of some plants.

A perfectly balanced loam has a 40-40-20 ratio between sand, silt, and clay, respectively. Knowing the relative concentration of the three particles in your soil is important for choosing the best strategies to boost fertility. Fortunately, this information is easy to come by:

  1. Fill a glass jar with approximately 50% soil and 25% water.
  2. Put the lid on and shake for at least thirty seconds.
  3. Place the jar on a table and observe what happens.

Sand particles fall immediately to the bottom. The silt particles will settle on top of the sand within about 30-60 seconds. The clay is the last to settle and can actually stay suspended in the water for 24-48 hours. Once the water is perfectly clear, there will be three distinct layers that show the proportions of the different size particles.

Next week, we will dive into the chemical reactions in garden soil that transform mineral compounds into naturally-occurring fertilizers and learn the properties of the most important plant nutrients.