Container Gardening – Improving “Pot” Performance with Soil Amendments

Written by David S.

Container gardening for vegetables, flowers, and fruit-producing plants are popular. A common question is whether you need to amend the soil in a container or replace it. The answer is not static and mostly depends on the type of plant you are growing. Inside, we discuss the soil structure, the philosophy that a container is an artificial environment, the role of nutrients in the soil, Plant health, and aged soil. 

Soil Structure
Soil consists of five or so layers. They are the top layer, the humus layer, the topsoil layer, subsoil, parent rock, and bedrock. 

When you plant in a container, you are recreating the humus layer and topsoil layer. The humus layer is where organic matter begins to breakdown into topsoil- Last year’s leaf fall is decomposing. The humus layer is very airy and light, while the topsoil layer is both airy and more compact. The subsoil layer is generally very compact with less airflow. The deeper you go, the harder the soil becomes. In a container, you want to create those airy layers. You want to soil to be full of organic material that feeds the plants and holds water or allows water to drain.

How you construct these layers of soil depends on the soil requirements of the plants you put there. It also means that the depth of the pot is essential. If you plant a short-rooted plant in a tall pot, it wastes soil. Match your plant to the pot so that the pot provides an exact growing environment. The soil you make for a citrus tree will be different from that of an apple tree and much different from a cacti garden. The pot shape and depth will also differ. 

Artificial Environment 
A pot or container is an artificial environment, much like a fish tank is. For the most part, artificial environments are not self-sustaining. You have to add water with a fish tank, remove water, clean the gravel, and clean the filter on a fish tank. In a container garden, you have to amend or replace the plant’s soil, water, and care. So, when do you replace the soil, and when do you amend it? You amend the soil to address the daily or monthly needs of the plant. You replace the soil when it no longer is offering full support to the plant. That will change based on plant needs, the environment around the container, and the quality of the substrate you’ve added to the pot. Here’s a closer look at how that occurs.

Soil Nutrients 
The three big names in soil nutrients are Nitrogen, Phosphate, and Phosphorus. These are what support the leaf, bud, fruit, root, and trunk/stem development. All three of these nutrients you can use to amend the soil. These are all plant foods and come in various ratios as fertilizers. A 10-10-10 formula is a balanced blend, while A 12-10-10 formula is nitrogen-rich. Fertilizer is a product you use to amend the soil. Lye is another product that you use as a soil amendment. 

What Are Soil Structure Products?
Soil structure products include potting soil, gravel, sand, clay, compost, manure, perlite, bark, and a few other items. These products do help to feed the plant, but they also have a use as part of soil structure. When it comes to planting in containers, you want to pay attention to soil structure – The layers of the soil in the pot. You could dump in a bag of potting soil. Eventually, it would act as the humus layer and the topsoil layer. Potting soil tends to dry out quickly in hot climates; it can hold too much water for some plants and may contain too much food, spurs on weed growth. 

Instead, consider adding a bottom layer of small-size gravel or coarse sand. Either of these in the bottom of the pot will act as the subsoil layer. Gravel promotes drainage even when you add a high concentration of water-absorbing material above them. The process is due to gravity, which continually pulls water down thanks to the gravel’s airspace. The gravel layer helps to support well-draining soil and prevent plant health issues such as root rot. 

The next layer in the pot is the topsoil layer. You want to build topsoil based on the needs of your plant. You can find ready-mixed potting soil in a comprehensive array of features. Some offer ways to hold more water near the roots; others offer specialized pH for specific types of plants. You can also mix your soil, making a unique mix that supports specialty plants, such as roses, succulents, cacti, or citrus. The container’s top layer acts as the humus layer, though we don’t expect it to break down and create topsoil. What the goal is for the humus layer in a container is to help water loss from occurring thanks to evaporation. 

Shop for the Perfect Soil Products
Green Thumb Nurseries carry a wide assortment of soil and soil conditioners so that you can grab a bag or collect ingredients. FoxFarms offers soil for plants that like to grow in moist soil, and EB Stone offers a quality bag of mix designed just for cacti.

You can also buy just a plain bag of topsoil or manure. The best option is to make a list of what the plant in the container needs and loves and then stop by one of the Green Thumb Nursery Southern California locations. Not only will you find a large assortment of soil and soil amendments, but you also have access to plant professionals who can help you find the best soil for your project or answer questions you might have about soil products. That sounds like a small job, but it’s much more than just that. A moist
environment helps the soil biota stay near the surface. Soil biota includes earthworms, nitrogen-fixing bacteria, fungi, and other beneficial soil organisms. Their presence helps prevent plant disease, convert soil into nutrients for the plant, and aerate the soil layers. Products that help create a humus layer include bark chips, stones, mosses, and peat moss. Most will break down over time. 

Aging Soil 
You might notice that the soil level in the pot is shrinking. That’s a good sign the soil needs help. When you see that the soil level drops, it is time to rebuild the soil rather than amend it. If the plant was doing well and then begins to struggle, bring in a soil sample and let the crew here test it for pH. The plant might be struggling because the pH has risen or dropped. It can also mean that the soil now has poor nutrition. If the plant is a heavy feeder, you can amend the soil to replace the plant’s food. 

If you have a container project, stop by and shop the beautiful selection of pots, soils, soil amendments, and tools offered by Green Thumb Nurseries. Each of our locations throughout Southern California is well-stocked to help you create the perfect soil for your project. If you need something specific, give us a call, and we can make sure it is in stock.

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