Soil is the foundation of a herb garden, and for herbs it matters more than fertiliser, more than feeding, and almost as much as sun. Herbs are not fussy about richness — most of them prefer poor soil to rich soil — but they are very fussy about drainage. A rosemary plant in wet, heavy soil will die in its first winter, however well you feed it, while the same plant in lean, gritty, free-draining soil will thrive for years with almost no attention.
The work of soil preparation is therefore mostly about drainage and structure, not about fertility. This guide explains what herbs want from soil, how to test your soil, how to improve clay and sandy soils, and how to prepare a bed or a pot for each group of herbs. If you are unsure about your soil type, check your local extension service — they can test your soil and tell you what it needs.
What herbs want from soil
Most herbs want soil that is free-draining, near neutral in pH, and modest in fertility. Mediterranean herbs such as rosemary, thyme, sage, oregano and lavender want lean, gritty soil that dries out between waterings — the thin, stony soil of a sunny hillside is their natural home. Soft herbs such as basil, parsley, cilantro and mint want richer, moister soil, but they still need drainage — none of them wants to stand in water.
Soil pH matters too. Most herbs grow best in soil that is near neutral, roughly pH 6.0 to 7.0, and they struggle in very acidic or very alkaline soil, because the pH affects whether the plant can take up nutrients. Lavender and rosemary prefer slightly alkaline soil and dislike acidity, while parsley and mint are more tolerant. Our guide to soil pH testing explains how to test your soil, and a test through your local extension service will give you a full analysis, including the nutrients your soil already has.
Testing your soil first
Before you dig anything in, test your soil. A simple pH test kit will tell you whether your soil is acidic, neutral or alkaline, and a squeeze test will tell you its texture: take a handful of moist soil and squeeze it — if it forms a sticky ball that holds its shape, it is clay; if it falls apart, it is sand; if it holds together loosely and crumbles when you poke it, it is loam. Clay soil holds water and nutrients well but drains badly and is hard to work; sandy soil drains fast but dries out quickly and holds few nutrients; loam is the ideal in between, and most gardeners would like more of it than they have.
A laboratory soil test through your local extension service is worth the small cost, because it tells you the pH, the organic matter, and the levels of the main nutrients, and it comes with recommendations for what to add. Test every few years, and test before you plant a new herb bed, so you prepare the soil on the basis of facts rather than guesses. Do not add lime or fertiliser to a herb bed without a test — lime on soil that is already alkaline, or fertiliser on soil that is already rich, does more harm than good.
Improving clay soil for herbs
Clay soil is the hardest soil for herbs, because it holds water and suffocates roots. Rosemary, lavender and thyme will rot in clay over winter, and even mint and parsley will struggle. The fix is organic matter and grit, worked in over time — not sand, which can make clay worse by turning it into something like concrete. Fork in well-rotted compost every year, add coarse grit to the planting holes for Mediterranean herbs, and raise the bed so water can drain away. Our guide to clay soil improvement covers the method in detail.
The fastest solution for clay is a raised bed or pots. A raised bed filled with a gritty, free-draining mix gives Mediterranean herbs the drainage they need, whatever the soil underneath, and pots give you complete control. If you do plant herbs direct into clay, plant them on a slight mound, so water drains away from the crown, and mulch with gravel rather than with moisture-holding materials. And never dig clay when it is wet — it smears, compacts, and destroys its structure. Wait until it crumbles in your hand, then work it.
Improving sandy soil for herbs
Sandy soil is the opposite problem: it drains too fast, dries out quickly, and holds few nutrients. Mediterranean herbs such as rosemary, thyme and lavender actually like sandy soil, because it gives them the sharp drainage they want — though they may need a little compost to get established. Soft herbs such as basil, parsley and mint struggle in sand, because it dries out before their roots can take up enough water.
The fix for sand is organic matter. Fork in compost, well-rotted manure, or leaf mould every year, and mulch the soft herbs with compost or straw to hold moisture in the soil. Organic matter acts like a sponge, holding water and nutrients that sand alone cannot hold, and it feeds the soil life that makes nutrients available to plants. Do not add clay to sandy soil in the hope of balancing it — the two do not mix well, and the result is often worse than either alone. And water sandy soil little and often, rather than in one big soak that drains straight through.
Preparing a bed and preparing pots
To prepare a herb bed, clear the weeds, fork the soil over, and remove stones and roots. Work in compost — a 5 centimetre layer forked into the top 20 centimetres is a good starting point — and add grit to the areas where Mediterranean herbs will grow. Rake the bed level, water it, and let it settle for a week or two before planting, so the soil has time to settle and any weed seeds that germinate can be hoed off. Do not walk on the bed once it is prepared, because that compacts the soil you have just loosened.
For pots, use fresh multi-purpose compost, with grit or perlite mixed in for Mediterranean herbs and extra organic matter for soft herbs. Do not use garden soil in pots — it compacts, drains badly, and can bring pests and weed seeds indoors. Our potting mix calculator can help you work out quantities, and our soil volume calculator can help you work out how much soil a bed or a raised bed needs. Top-dress established herbs in spring by scraping off the top few centimetres of old compost and replacing it with fresh compost, which feeds the plant without disturbing its roots.
Common mistakes
The most common soil mistake is planting Mediterranean herbs in heavy, wet soil and wondering why they die over winter — drainage is the first requirement, not fertility. The second is adding sand to clay, which can make it worse. The third is digging clay when it is wet, which destroys its structure. The fourth is fertilising without a soil test, so nutrients build up to levels that lock up other nutrients. And the fifth is using garden soil in pots, which compacts and drains badly. Test your soil, improve it with compost and grit, give Mediterranean herbs sharp drainage, and your herbs will reward you for years.
| Soil type | Problem for herbs | Fix |
|---|---|---|
| Clay | Holds water; roots rot | Compost and grit; raised beds; plant on a mound |
| Sandy | Dries fast; few nutrients | Compost and leaf mould yearly; mulch soft herbs |
| Loam | Ideal | Light compost dressing each spring |
| Acidic (low pH) | Lavender and rosemary struggle | Test first; lime only if a test recommends it |
| Alkaline (high pH) | Some nutrients locked up | Test first; add compost; choose tolerant herbs |
Frequently asked questions
What soil do herbs need?
Most herbs want free-draining soil near neutral pH, roughly 6.0 to 7.0, and modest fertility. Mediterranean herbs want lean, gritty soil that dries out between waterings, while soft herbs such as basil and parsley want richer, moister soil — but all herbs need drainage.
How do I test my soil for herbs?
Use a pH test kit and a squeeze test for texture, and get a laboratory soil test through your local extension service for a full analysis of pH, organic matter and nutrients. Test before you plant a new herb bed, and every few years after that.
Can I grow herbs in clay soil?
You can, but Mediterranean herbs such as rosemary and lavender will rot in wet clay over winter. Improve clay with compost and grit, plant on a slight mound, or use a raised bed or pots with a free-draining mix. Never dig clay when it is wet.
Should I add sand to clay soil?
No — sand can make clay worse by turning it into a dense, concrete-like mix. Improve clay with well-rotted compost and coarse grit instead, and raise the bed so water can drain away.
What soil should I use for herbs in pots?
Fresh multi-purpose compost, with grit or perlite mixed in for Mediterranean herbs and extra organic matter for soft herbs. Never use garden soil in pots — it compacts, drains badly, and can bring pests and weed seeds with it.