Soil Structure and Its Effect on Plant Growth

One of the major tenants of garden lore that is mentioned but rarely talked about in detail is the need to pay attention to garden soil structure. Too often a beginning gardener becomes disappointed with their early efforts, not because of anything they did, but rather due to some shortcoming in the soil they planted in.

Plants require certain type of soil to grow well in. Plain dirt dug up in the back yard may or may not cover all the bases, so let’s take a look at some of the key characteristics to look for in a good as opposed a poor soil. Let’s start by taking a look at what makes a soil suitable for growing crops. Soil that is of the right overall composition or mix is termed soil that has good tilth. Let’s explore some characteristics of tilth.


Basic Soil Composition

In the most general of terms, a soil can be broken down into two components: solids and spaces. The solids would be composed of things like minerals, sand and organics while the spaces would contain air and water. Now, about ninety percent of the solids could be further broken down into three classes or types; sand silt and clay which are really just rocky minerals divided up according to their size. The remaining ten percent is composed of organic material and it is this material that is vitally important to the future health of your plants. It’s also the one thing about soil that you can easily do something about! Included in the organic part are latterly billions of soil microbes that directly participate in the breakdown or decomposition of organics and then often aid in the transport of nutrients to the plant roots. We call decomposed organic material humus and it is the health of the tiny microbes that are what really matter. Good microbe populations equal healthy plants. The presence of humus in a soil is what gives it that great smell.

Air spaces constitute or make up about 50% of the volume of a healthy soil so they are very important also. In the spaces you will find both air and water. Yes, roots require air to breath and if the spaces become compacted such as is seen from over tillage, you will get terrible results. The plants will latterly die for lack of air and water. The moral of this story is that once you get your soil in good condition, please don’t ever walk on it or do anything to compact it!

A Closer Look

Now that we see that soil is both spaces and solids, lets take a little closer look at how the solid part can be put together. Remember when I used the word tilth? Well, good tilth depends on the overall mix of soil texture, structure (or aggregation), density, drainage and water holding ability. This is influenced greatly by the exact proportion of clay, sand and silt that make it up. To the right is a chart of how different soils are made from varying the composition of these three basic ingredients. When you first look at it, you might be a little confused. So, what’s the best soil? Well, the answer is not to think so much as what is best as it is to think in terms of avoiding any extremes. So, if your soil is somewhere in the middle of the chart that’s good. If it’s tending towards one of the corners, then you might want to consider some soil management techniques that could apply depending on your circumstance. If you have small raised beds like I do, then you can always buy bags of different kinds of soil to alter the structure. If, on the other hand, you have a large backyard garden, this might not be a viable alternative. The good news is that almost any soil can be improved by the addition of compost. That will be subject of a future blog.