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Has it crossed your mind to start planting in compost without soil? It’d be a lot easier for the practical gardener to fill a container with homemade compost from the backyard, plant some seeds, water the soil and wait for the plant to grow. It’s a nice idea, but does it actually work?
Unless you’re growing small vegetables that don’t need a lot of structural support like onions or smaller herb plants, plants are going to need some soil. How much depends on the type of plant you’re growing.
Growing Vegetables in Compost
Some fruits and vegetables could be grown in compost only but they would need to be lightweight such as baby tomatoes, baby carrots, spring onions, or small herb plants.
One of the things soil has over compost is an abundance of micronutrients. Research into hydroponic gardening– a soil-free method of growing plants using water – there’s several micronutrients that need to be added to the water solution. Mainly, nitrogen, magnesium, phosphorous, calcium and sulfur.
The same is true for any type of soilless gardening method. These are micronutrients that can be lacking in compost, or be in such high doses that they become toxic to plants instead of helping them. Boron and hydrogen are two examples of nutrients that can be out of balance in compost mixtures.
If you’re going to try planting vegetables in compost only, the one thing to invest in is a handheld pH meter. Vegetables do best grown in soil or compost with pH levels in the range of 6.0 and 7.0.
Discovering the nutrient load of any compost would need lab testing and that’s something most people won’t be able to justify the cost of.
Problems with Planting in Compost Without Soil
Compost drains fast. In fact, it’s one of the reasons it’s added to soil… to improve the drainage. When you take the soil out of the equation and plant in compost without any soil, you’re going to find your plants are constantly thirsty.
As fast as you’re adding water, it’ll drain right through compost. No matter the quality of compost you use, it is always going to drain fast.
Compost decomposes at different rates. The rate of decomposition is affected by elements such as temperature, humidity and the components added to a compost pile, such as food scraps.
As part of the composting process, materials shrink until they’re reduced to crumb size, then those crumbs eventually clump together. A pile of compost left alone for up to a year can be just a third of the size by the time it’s ready for use.
Compost without soil is going to lack the foundational strength many types of plants require for their roots to anchor to. What is more likely to happen when planting without soil is the root ball of the plant won’t have the structural support to hold the roots in place.
So, even if you do manage to get your plant growing, you’ll likely find, as it does, it’ll be leaning to one side, or the stem will totally flop as it won’t have the foundations to hold the plant upright.
Meeting the Nutritional Needs of Plants
The purpose of compost is to help the plants absorb the nutrients found in soil. The quality of soil is based on nutrient load. A good quality soil is mostly minerals.
The largest nutritional source plants need is from the soil as nearly half of it is minerals that just aren’t balanced in the majority of composts.
These are mainly from decomposing plants, including foods and animal produce. It’s this 5% organic matter that leads to soil fertility helping plants to grow.
Without the organic matter, most compost won’t have as much nutrition needed to sustain healthy plant growth.
The minerals and the organic will make up to half the contents a plant needs. The rest are from air and water, about 25% of each.
As compost is crumbly, the water is going to drain faster so the more compost you’re using, the more frequently the plant will need watering. Additionally, the thicker and heavier a plant is, the more strength it’s going to need in the soil to support the weight of the plant so a good thick and strong soil base may be required for tall flowering plants.
You may be able to grow lighter-weight plants efficiently in compost only, but do keep an eye on soil acidity using a pH meter and don’t get your hopes up. The vast majority of plants require the organic matter only found in quality soil.
Use compost to help plants feed on the nutrients and fertilize as a top-up when the mixture is lacking.
Compost is only an addition to soil. It’s an amendment to improve soil quality and fertility and doesn’t usually work well on its own. Some people have had success planting in soil only, but these are only on lightweight small plants.
The bigger and more nutritious the vegetables or potting plants you are trying to grow, the more soil you’ll need. You can go compost heavy for growing plants but rarely compost only.