How to Improve Clay Soil and Poor Garden Drainage
How to Improve the Structure of Clay Soil
If you can roll a handful of your garden soil into a sticky ball, it is clay. Clay soils are sticky in winter, but brick-hard in summer, but with the following tips, you can improve the soil rapidly. However, it may take from several months to a few years to completely change the structure of heavily impacted soil.
Add Organic Material
- I've found that garden compost is best, but soil conditioners such as seaweed, farmyard manure, or bagged manure products like Scott's Organic Dehydrated Manure can also improve the soil quality.
- Spread the soil conditioner across the surface, and use a garden fork to mix it in.
- This is easiest to do during spring and autumn, after the rains.
- If using manure, spread to an 8 cm (3") thickness.
- If using dried seaweed meal, apply 100-200 grams per square meter. (3-6 oz per square yard).
- If using proprietary compost mixtures, follow the instructions on the package carefully. Improper use can do more harm than good.
After a year or two of repeated applications, the soil will become more friable and easier to work with.
What Is the pH of Clay Soils?
Clay soils tend to be acidic, but not always. My current garden soil is clay but alkaline. A simple way to find the pH of your soil is with a pH kit. They are very easy to use, and you can find out whether your garden soil is acid or alkaline in just a couple of minutes.
Can I Change My Soil pH?
While it is possible to adjust the pH of your soil to grow certain plants, the process can be time-consuming and very expensive. Adding compost and other organic materials tend to neutralize the soil, so you'll have trouble growing plants that love either acidic soil or alkaline soil. This is where raised bed gardens are useful.
While adding in compost and organic material is always beneficial, and does tend to help neutralise the soil, you are still going to have trouble growing acid loving plants in alkaline soil, or alkaline loving plants in acid soil, and this is where raised bed gardens come in useful.
Whether you choose to buy a ready built raised bed garden, or have one constructed from wood or brick, you can choose to fill it with a compost of your choice, and grow your plants of choice.
Improve the texture of your heavy clay garden soil, but make a point of growing flowers and vegetables best suited to the natural pH of your soil.
Are There Plants That Can Grow in Clay?
There are certain plants that can grow in clay soils. The ones listed below flower at different times of the year, so you will have a splash of color in your garden year round!
Plants That Can Grow in Clay
- Flowing currants (Ribes sanguineum)
- Hardy geraniums (Geranium sanguineum, G. pratense, G. macrorrhizum, G. enderssii)
- Hellebores (Helleborus atrorubens)
- Ivies (Hedera spp)
- Japanese anemomes (cultivars of Anenome hupehensis or Anenome x hybrida)
- Lungworts (Pulmonaria saccharata)
- Snake's head fritillaries (Fritillaria meleagris)
- Snowdrops (Galanthus nivalis)
- Winter aconites (Eranthis hyemalis)
Plants That Cannot Grow in Clay
Bulbs: Avoid the smaller, delicate ones that need light, warm soils.
- Alpine species that are usually grown in rockeries.
- Lilies, except the Madonna lily (Lilium candidum and L.pyrenaicum). Lilium regale can also be grown if it is cultivated and well-drained.
- Rock roses (Helianthemum)
- Sea hollies (Eryngium maritimum) and other Eryngiums that naturally grow in sandy soils.
- Irises, especially the smaller ones. However, Iris foetidissima can grow well in clay.
How to Drain Clay Soils
Clay soils tend to become waterlogged in wet weather; the grains tend to stick together, preventing water from escaping. Most plants, except bog plants, do not like having their feet wet because they can quickly rot. You may also find that it's hard to walk on the waterlogged soil.
A quick fix is to make drainage holes with a garden fork or similar tool. However, if the problem persists, consider installing land drainage pipes to remove excess water permanently.
Creating a Herringbone Drainage System
This involves digging series of deep trenches across your garden, in areas where you might walk.
- Start by digging the main drainage trench—2-3 feet deep in a straight line across your garden. Be mindful of the slopes of your garden when positioning the trench. You want to have the trench at a downward slope.
- Dig trenches that will feed into the main trench in a herringbone fashion.
- Cover the surface of the trenches with gravel or small stones.
- Lay piping in each trench and interconnect them. Use special drainage piping with drainage holes across the top to collect the water.
- Angle the pipes so that water will flow into the main pipe.
- Build a soakaway—a deep circular hole with sloping sides—at the end of the main pipe. The soakaway should be at least 6 feet deep and filled with rubble to collect the water.
- Position it well away from the house.
- Cover your soakaway with thick plastic sheeting.
- In areas of poor rainfall, you may want to build a concrete soakaway to store water for gardening during drier times.
Alternative: French Drainage System
An alternative method is to create french drains. French drains are like herringbone drains without the pipes. Instead, the trenches are filled with rubble and gravel and replace the topsoil.
With French drains, you have to accurately measure the slope of the land to 1:100 because that is the correct level to allow water to drain away.
Clay soil can be a nuisance, but the addition of organic matter, and where flooding is a serious problem, the addition of land drainage can make your clay soil garden easy to maintain and enjoyable to use.