Gardening with Clay Soil

Clay soils have soil particles of less than 0.002mm and therefore have the capacity to retain water really well due to the capillary attraction of the tiny spaces between the numerous clay particles. Whilst this is good as nutrients are not leached away there are many downsides as the soil may become:

• compacted and cloddy when you work with it while it’s waterlogged 
• poorly aerated, because of the tight bonding of clay particles 
• as hard as brick in extremely dry weather. 

Clay soils often test the gardener to the limits, but when managed properly with cultivation and plant choice, can be very rewarding to work with. However a highly compacted clay soil may need several years of care and back breaking work before it can be any good for growing. Do not despair though it is unlikely your soil is this bad.

Water logging and drainage issues

Since the water moves through the soil very slowly if drainage is impeded the movement of water is not just slowed down it is blocked and the soil is water logged for long periods during the winter months. This is an additional problem that will be discussed elsewhere but is usually caused by:

• Non-porous rock below the soil line (Horizon C) 
• Formation of a soils pan below the soil line 
• High water table 

If the issue of soil drainage is a problem you may need to consider adding artificial drains, soil drainage will be covered in a later post. 

Improving Clay Soil

Whilst clay soil isn’t ideal gardening material the retention of nutrients means it is quite likely to be fertile. This creates a good foundation to build on when conditioning clay soil. The objective is just to improve the soil structure making it easier to work.

This can be done by making it looser and a more crumbly growing medium by mixing in large amounts of organic matter. The organic matter stabilises the aggregation of the soil particles in the top soil, reducing crusting, improving water handling and nutrient availability. It also reduces the problem of erosion and runoff.

The plan is to roughly dig the soil thoroughly in the autumn (fall); this exposes the clumps of soil to frost in winter (if you get frosts). The cold causes water to expand and the effect of this on clumps of soil is to break them up therefore you let nature do some of the work.

To help retain the new crumb structure the addition of a generous dressing of lime/ Gypsum will help prevent the soil compacting back down.

Gypsum is calcium sulphate and lime is the hydrated form of calcium sulphate, calcium carbonate. It works by loosening the clay particles which serves to improve soil structure, aeration and the flow of water deep into the soil. The addition of lime/ Gypsum will need to be repeated every five years to help maintain the soil’s structure. Gypsum will not be of much benefit with acidic soils working best with alkaline soils so before treating your garden with gypsum, make sure your soil’s at the right pH. This topic will be covered on another page.

It is not advised to mix sand into the clay soil to improve drainage. It is a possibility that if the proportions are correct you could turn your garden soil into something resembling concrete.

The next step to improving your soil is the addition of organic matter which will help to preserve the crumb structure you (and nature) have created. You will need to add 15 to 20cm (6 to 8 inches) of organic matter to the entire bed. Stable manure with a high straw content is perhaps best but grass clippings, shredded leaves, other rotted manure, and compost are all perfect choices. 

Spread the organic matter on top of the soil and then start digging. The organic matter needs to be mixed into the top 15 to 30 cm (6 to 12 inches) of soil. Digging it in and mixing the soil and organic matter with a spade (shovel)is best as it moves a lot of the earth without pulverising the soil particles the way rotovating (tilling) can. However, if digging is just too hard on your back, using a rotovator (tiller) will be good enough the trick is to get the organic matter into the soil. 

Once the digging is complete the garden bed will be several inches higher than it was originally but it will settle over the course of a season and from this point the soil structure will keep improving as microorganisms in the soil work to break down all of the organic matter you've added. 

The bed can be planted immediately, and to help maintain the soil structure you will need to continue adding more organic matter on the top of the soil once or twice a year. This will continue the process of improving the soil's structure and offset any settling that happens.

Watering Clay Soil

There are some seasonal considerations to keep in mind when watering clay soil:

• Because clay soil is water retentive, water within the soil will freeze up in winter. 
• During summer’s hot, dry conditions, soil may develop a crust from which seedlings can’t emerge and from which their roots cannot burrow deeper into the soil. 
• In spring, clay soil takes a long time to warm up so planting schedules may be delayed. 
• The best time to dig or till is in autumn (fall), when clay is at the right moisture level for handling. 

Drip irrigation works best with clay soils. It allows you to deliver water at a very slow rate, giving it enough time to soak through to the root regions and drain away.

Tilling Clay Soil

Tilling is the initial critical step in soil preparation. It supports good structure for seedling growth and rooting, aeration, water regulation and oxygen distribution. When tilling, make sure that clay soil moisture is just right. That is the soil fractures, breaks or crumbles when pinched between your fingers.

A combination of air-drying, wetting, and raking will break up clods in moist clay to create a good soil structure.

Loosen clay soil with a garden fork before working with it. Puncture the soil and then gently wiggle the fork tines out. Turn over the soil, break the clods using the side of the fork, then leave the soil exposed to sun and air to dry. Once water has evaporated and the soil clods have crumbled, soften the clay again with a fine spray of water. Work the soil with a rake once it’s dried out.

Continued tilling can use up the organic matter in the soil as it encourages microbial life in the soil to consume the organic matter in large quantities. Once clay soil is friable and the plants are rooted deeply, you could consider switching to a minimum-till system. It will augment organic matter, carbon, total nitrogen and microbial life 5 to 10cm (2 to 4 inches) from the soil surface.