Solabiol's gardening advices

Understanding your soil for better growth(NL)

A lot more complex than it seems, soil is a vital component that you need to understand if you're looking to improve plant growth in your garden.

A lot more complex than it seems, soil is a vital component that you need to understand if you're looking to improve plant growth in your garden.

A true link between the animal, vegetable and mineral worlds, it plays a key role in maintaining the quality of water and air. You should therefore learn how to understand it better, and also respect it, as it represents a non-renewable resource. In fact, it takes around 1,000 years for soil to be produced, but only around 10 years of poor farming practices to destroy it.

The composition of soil

Life at soil level can be measured on a microscopic scale. The dozen or so centimetres of thickness representing soil on Planet Earth house a hive of intense biological activity. Each element comprising it has a role to play in the production and recycling of all organic matter making up living beings.

Every soil type is comprised of four families of elements:

  • 10% to 40% gas: nitrogen, oxygen, carbon dioxide and water vapour
  • 10% to 40% water comprising dissolved substances
  • 35% mineral compounds
  • 15% organic compounds

The proportion in each family varies depending on the weather conditions and nature of the soil in question.

The role of mineral and organic compounds in your soil is to filter and store nutrients and water, avoiding leaching and evaporation losses.

Organic compounds also include several categories of organisms, such as bacteria, fungi, insects and humus. As is the case for mineral compounds, their role is to filter and store nutrients. But they also have the ability to recycle and create new nutrients, pigments, hormones and antibiotics involved in plant growth and pest resistance.

The various soil types

There are four main types of soil. Each has its own characteristics which calls for suitable agricultural and planting methods.

Clay soils

This is a heavy soil type containing high quantities of clay. It is widely considered that a 15% to 25% clay component is suitable for fertile soil. The nature of clay soils varies depending on the season; it becomes claggy in winter, while it has a cracked look during periods of drought.

Its water and nutrition retention ability makes it the best type of garden soil.

Tip: Scatter organic fertiliser, lay down mulch and regularly add compost to make it lighter. On the other hand, avoid adding too much sand to it initially – this can cause your soil to turn into concrete!

Silty soils

Silty soil is rich and fertile, yet it remains fragile owing to its lack of clay (less than 10%) and sand. It is also sensitive to erosion. You will need to add compost once a year if you want to maintain a good supply of vegetables and vegetation.

Owing to its permeability to water and air, silty soils must regularly be aerated and covered by organic mulching to prevent a crust from forming on the surface after rain. Scattering organic fertiliser between each crop also offers a solution to prevent this crust from forming.

Tip: Avoid working with silty soil when it is still very damp, you run the risk of compacting it.

Sandy soils

This is a light soil that’s easy to work. One of the specific characteristics of sandy soil is that it is water and air permeable. This allows it to warm up quickly in spring, yet the downside of this is that it is not able to retain water and nutrition. Sandy soil also tends to dry out in summer. You therefore need to water it regularly with small quantities of water during the warm seasons.

Tip: Remember to regularly condition your sandy soil areas to prevent crops from depleting the nutrients in it.

Chalky soils

This type of base soil is easily recognisable by its whitish colour. Many gardeners consider it to be disastrous as it has a tendency to dry out quickly in summer, forming characteristic cracks. The presence of clay makes this soil claggy and compacted in rainy periods.

Chalky soil also tends to hold on to and not freely circulate certain nutrients, such as iron, which causes yellowing of leaves (chlorosis).

Tip: To improve the nature of chalky-clay soil, organic matter should be added to it, by scattering organic fertiliser, or adding compost to it. However, don’t use fertiliser with a high lime content, as this type of soil already contains 10% to 30% of calcium carbonate.

How do you know what type of soil you have?

Tell me what type of soil you have, and I'll tell you what to grow... Often we try to change our soil to enable us to grow the particular plants we like. We should actually adapt what we grow to suit the soil type we have. Here are the various ways in which you can identify the type and composition of your soil.

The texture of the soil

Using touch is an effective way to understand the nature of soil. To do this, you just need to do the “Sausage test”:

“Sausage test”:

Take a handful of soil from your garden and rub it between your fingers.

  • If you’re not able to form a sausage, you probably have sandy soil,
  • If you form a sausage without being able to join its ends, you have silty soil,
  • If the edges of your sausage join up, then it’s likely to be clay soil.

The colour of the soil

Sometimes, simple observation of the soil in one's garden can provide valuable information on the nature or condition of soil.

  • Whitish soil is probably rich in lime or sandy.
  • Black earth is the sign of bog soil,
  • Dark brown soil is rich in organic matter,
  • Blue, green or grey-tinged soils indicate that it is waterlogged.

The plants that grow in it

Plants are also a good indicator of soil type. In fact, some of them will only grow in certain types of soil. You therefore need to take note of plants growing naturally in your garden area, in addition to studying the behaviour of the plants you are growing.

Laboratory analyses

If you would like to be fully informed on the soil in your garden, the best way is still to contact a specialist laboratory. After in-depth analysis, you will be given information on several different aspects of it.

  • Its texture (proportion of clay, limestone and sand)
  • The concentration of calcium
  • Exchangeable elements
  • The C/N (carbon over nitrogen) ratio
  • The concentration of organic matter
  • The pH (degree of soil acidity)

Before contacting the laboratory, consider buying a commercially-available sampling kit. This will help you to collect a soil sample, enabling you to send it off for laboratory analysis.


Whatever soil type you have, simple routines will allow you to optimise it and improve its fertility.

  • Scatter organic fertiliser
  • Regularly add compost
  • Mulch the soil so as to protect the earth and prevent it from lying exposed.