Understanding Soil Types for Vegetable Gardens

5 November 2010, written by Kate Bradbury

Soil type can determine which vegetables grow best for you. Knowing your soil type enables you to overcome problems since you can prepare the ground accordingly. Both the texture and pH of the soil are worth knowing and these can vary throughout your garden or plot.

Soil Texture

Soil is made from three main components: clay, sand and silt. The ideal soil (or loam) has equal amounts of all three, making a fertile soil that is free draining and easy to dig. However, each type of soil has its own advantages as well as disadvantages and different varieties of plants are suited to different soils.

Sandy soils have large particles and gaps between them. This allows water and nutrients to drain away freely, making sandy soils less fertile than heavier soils. Sandy soils tend to dry out in the summer but they warm up quickly in spring (allowing seedlings a good start) and they are much easier to dig than clay-based soils. If your soil is sandy, you should have no trouble growing root vegetables such as carrots and parsnips, but you may struggle with nutrient-hungry brassicas such as cabbages and broccoli. Plants with shallow roots are prone to drying out as sandy soils lose moisture faster than heavier soils.

Clay and silt soils – ‘heavier’ soils – have small particles. This means water is less likely to drain away but the soil is more likely to become waterlogged. For such soils, raised beds can greatly improve drainage. Heavier soils are fertile, but take longer to warm up in the spring and are harder to dig. If you have clay soil, you should find that brassicas grow well, but root vegetables are likely to struggle as they have to push through the heavy, often compacted soil. Shallow-rooted trees such as pear trees are likely to thrive in this soil as it holds moisture better than sand.

All soil types benefit if paths are well defined in the garden since keeping to paths ensures that soil is not compacted. This is particularly important for heavy soils.

Testing Your Soil

You can test the texture of your soil by checking it in wet and dry conditions. If the soil is hard when dry and sticky when wet, it is likely to be clay. If it is light, easily drained and easy to dig, it is probably sand or loamy sand. For a more precise test, take a small amount of soil in your hand and wet it. Knead it into a smooth paste and then roll it about between your hands to form a ball. The following results will reveal the soil texture:

Sticky and gritty – loam, the perfect soil
Easily rolls into a ball, but feels rough – clay loam
Easily rolls into a ball, shiny when rubbed, but still gritty – sandy clay
Easily rolls into a ball and becomes shiny but not gritty – clay
Doesn’t roll into a ball well, and feels gritty – sand
Easily rolls into a ball but it falls apart easily – loamy sand
Feels slippery and silky – silty loam

Improving Your Soil

Whatever your soil type, organic matter such as homemade compost and leafmould should be added regularly to improve structure and nutrient content. Organic matter helps to break up heavy clay soils, improving drainage, and binds together sandy soil, improving retention of water and nutrients. If added once a year, organic matter will improve your soil and overcome any problems associated with texture. Adding organic matter can also slightly lower the pH of the soil (see below) to a level perfect for most vegetables.

Soil pH

Soil pH is a measure of the soil’s acidity or alkalinity. Knowing the pH of your soil helps to determine which vegetables to plant. Blueberries, for example, only thrive in an acid soil (with a pH of around 4-6). So, if your vegetable plot has alkaline soil, they should be grown in pots with ericaceous (acidic) compost. If, on the other hand, your soil is acidic, brassicas such as cabbages benefit if lime is added a few weeks before sowing since they enjoy an alkaline soil and lime make soil more alkaline. Adding lime also helps prevent clubroot disease, a major problem with the brassica family.

Alkaline soil has a pH of around 8.5 whilst the pH of neutral soil is 7. Most plants grow best in soil with a pH of between 6.5 and 6.8. You can identify the soil’s pH using a testing kit. These vary from a cheap soil pH meter, which is simply pushed into the ground and examined, to kits that include colour charts and tubes. The latter provide more reliable results. For the best results, take a small samples of soil from various areas of your garden or vegetable plot. Avoid taking soil from waterlogged areas or frozen ground. Place each sample in a polythene bag and label it with where in the garden it came from. Allow each sample to dry out and then follow the instructions on your testing kit carefully.

Once you have the results of these tests you can plan which areas to prioritise for soil improvement and where best to grow the more fussy vegetable types. Using the GrowVeg.com Garden Planner you can easily identify plants of a particular family (brassicas, for example, always have a green circle round them) and match them to the best area of your garden.