A beginner's guide to starting a vege garden. Part two: It's all in the soil
We began our "Start a Garden" series with advice on where to site your vege patch. In part two we are talking about soil and, if you want to grow your own fresh and healthy fruit and veges, you can never say too much on this topic.
To get to know your soil, dig a hole about twice the depth of a spade. The dark layer on top is topsoil; the paler, denser stuff underneath is subsoil, which is often rich in nutrients but structurally poor. If the topsoil is missing – it's often been removed in new sections and subdivisions – you'll need to replace it or restrict yourself to gardening in pots and raised beds.
If there is a line of reddish-brown soil below the surface of the earth, you have hardpan. This is a layer of compacted soil that will block fine roots and not allow water to drain. If the soil above the line is grey or smelly, it is probably waterlogged and nothing you plant in it will thrive. You will need to dig down to the level of the hardpan in order to break it up or use a digging fork to punch holes through it at regular intervals.
Is your soil clay or sand? Roll a handful between your fingers. If it forms a solid sausage, you have clay soil. If it feels gritty and falls apart, it is sand. If it holds together at first but falls apart when you poke it you have lovely loamy soil – and your crops should thrive! You can also put a little soil in a jam jar filled with water and shake it. If the water is clear on top after an hour or two, you have sandy soil. As the sediment settles you will be able to see your soil constituents –stones and sand will settle first, then organic matter, whereas sand and silt can cloud the water for hours.
The main soil improver is humus (well rotted organic matter). This not only contains nutrients and feeds beneficial soil microorganisms, it allows air into heavy soils and retains moisture in light soils. So no matter what kind of soil you have, add organic matter at every opportunity: layer on compost (half a wheelbarrow full per sqm), aged animal manure, green manures and chopped-up seaweed. You can even just dig a trench and pile in compostable material, such as vege scraps, grass clippings and newspaper.
How to improve clay soil: For a long-term fix, add grit or coarse sand (one barrow full per sqm) to improve the drainage. Don't add fine sand; this can make clay soil worse as it blocks soil pores. Add lime or gypsum too, but not every year. For a short-term fix pile a 10-20cm layer of good topsoil on the top and plant directly into it.
How to improve sandy soil: If you incorporate organic matter into it and mulch heavily, sandy soil's water- and nutrient-holding capacity will gradually increase. With sandy soil, use slow-release fertilisers such as organic blood and bone rather than highly soluble store-bought varieties, which will quickly be washed away.
Digging your soil is only necessary when you want to incorporate soil improvers such as organic matter or if the soil has become compacted. Never dig (or walk on) wet soil, as you will destroy its structure.
Healthy soil also contains air and water. All plants need air and water to grow, as do the good bugs, soil fungi, microbes and earthworms that are essential for a healthy soil. Too much water and plant roots will not be able to respire (or breathe); too much air and organic matter will decompose quickly. Want to improve drainage or water retention, and soil aeration? Add more organic matter!
Is your soil acid or alkaline? Get a testing kit from a garden centre to reveal your soil's pH, or how acid or alkaline it is. pH determines the solubility of soil minerals and their availability to plants. Acidic soils (pH 0-7) tend to be low in phosphorous; alkaline soils (pH 7-14) may lack iron and manganese. Soil pH also affects which soil organisms are living there. Worms shun strongly acidic soils while the microorganism that causes club root in brassicas will only live in acidic soil. Most Kiwi soils are slightly acidic but luckily the optimum pH for most plants is 5.5-6.5, which is just on the acidic side. Apply lime to raise soil pH, or sweeten the soil.
Look out for part three of our Starting a Vege garden series tomorrow. It's all about seeds and seedlings.
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- NZ Gardener