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.

It's All About the Soil

“So why is the condition of your garden soil so darn important when it comes to growing stuff? Hey, aren’t all soils basically nothing more than weathered, decomposed rock and mineral fragments mixed with air and water?”

Yes, that’s a good definition of sterile soil alright. However, fertile soil (the kind you must have to grow stuff) contains a whole slew of nutrients that are in a readily available form which plants require to flourish. The roots of a plant act as little miners moving through the soil and bringing needed minerals into the plant roots. However, by themselves the roots are not able to transport the nutrients through their cell walls. They require a number of environmental factors be present in order for everything to work. Chief among these factors is the soil pH which is a measure of acidity and alkalinity. The closer the soil is to a neutral pH (about 7.0), the better the uptake of minerals for plant growth will be all things considered. Other factors that need to be presence in a proper balance are; air, humus (organic matter), water and microbes. The clay and sand particles supply what is know as ‘crumb’ structure which supplies tiny spaces that allow for the containment of air and water in close proximity to the humus organic matter. If all these factors are ‘in balance’ you have a soil that will be able to ‘feed’ your plants and a healthy garden will result.

However, please remember that if things get out of balance, the plant will suffer. For example, one of the things that can go wrong in the garden is soil that becomes too compacted either by foot traffic or improper tillage. This action removes those tiny spaces which decrease the amount of air and water the soil can hold. So tread lightly.

Another common problem is caused when improper soil amendments are added which may change the pH to either too acid or too alkaline. If the pH is not close to neutral, the nutrients which are in the form of anions (negatively charged particles) or cations (positively charged particles) become tied up and therefore not available to the plant roots. Still other things that can go wrong involve the use of so-called fertilizers that contain high amount of nitrogen, phosphorous and potassium (NPK). In high concentrations, they can also upset the overall balance you are trying to achieve. In addition to this, they tend to leach quickly from the soil, after a rain, to go on and pollute waters downstream.

It is far better to use natural compost in your garden as this material acts as both a nutrient buffer and reservoir for the plants to draw substance from as they need them. If you do not have a compost pile already, you should go ahead and start one today. They are easy to setup and more importantly you can supply them with all of your vegetable peelings and leftovers (anything but meats which tend to attract animals) rather than throwing them into the trash where they will end up in some landfill.