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Soil Testing for Vegetable Gardens

Around about February / March serious gardeners should be taking soil samples and sending away for analysis. And here is why!!.. Before the new growing season really gets going, it is good policy to know just what are the deficiencies and surpluses of nutrients.


Take 5 samples from each plot 10cm below surface level in a W shape, evenly across the plot and blend together. Avoid stones and lumps of organic matter. Bag into small plastic bags to avoid contamination. About 0.5 litres of soil is sufficient.


The first and most important thing to know is the soil pH. This may well seem like a daunting subject, but it only needs to be kept to the basics. The ideal pH is considered to be 6.5. This is very slightly acid (neutral being 7.0 which is the pH of pure water) This allows for the broadest range of plants to be grown and thrive. With vegetables a pH of 6.5 to 7.0 is ideal (with a few exceptions, ie potatoes pH 5.0 to 6.0) and with most ornamentals 6.0 to 6.5 (except ericaceous plants - below 5.5) is ideal.


Low pH (acidity) can easily be raised with the addition of ground limestone or ground chalk (sold as garden lime). To lower a pH, large amounts of organic matter – preferably stable or farmyard manure, needs to be added on a regular basis. Also fertilizers containing sulphur (sulphate of whatever – all inorganic) will have an acidifying effect on soils.


Testing for Nitrogen, Phosphates and Potash is also highly recommended. To use guesswork will result in an unbalanced nutrient content. Many gardens have massive surpluses of phosphate. This is due to gardeners using general purpose fertilizers all the time, as well as Organic Gardeners love of Bone Meal, which on many soils can be considered as good as useless, as it simply locks up and builds up, but never is available to the plants. Far better to use Superphosphate if phosphates are required. However plants do not require a great deal of Phosphate, and it is not washed out by rain, as in the case of Nitrogen. Such surpluses can cause other nutrients to be locked up, particularly trace elements.


Testing for trace elements is wize but not essential. By adding manure regularly you can ensure that the trace elements are maintained. Many brand name fertilizers have trace elements added to them as well. It shouldn’t be overlooked just how important they are, and by adjusting pH to the optimum, you will make available to the plants the trace elements present. A pH over 7.0 is as bad as a soil which is too acidic. Above pH 7.0 nutrients and particularly trace elements become unavailable to plants, even if they may be present in the soil.


In the case of Nitrogen, most soils are likely to show a deficiency, as it is usually all washed out over winter. In the case of Potash a deficiency is most likely, as plants use a lot of it and will therefore need more. Manure and Woodash do both contain very small amounts of Potash, but not enough. To add sufficient Potash will mean using inorganic fertilizer, although Rock Potash is totally natural and should be acceptable to Organic Gardeners.


Organic gardening and well balanced soil can go hand in hand, although it should be remembered that organic matter has a tendancy towards acidity, so unless the soil is naturally neutral (pH 7.0) or alkaline (over pH 7.0) the regular applications of ground limestone or ground chalk will be necessary.


Older and less scientifically minded gardeners from the past refered to overly acidic soils as ‘sour’ and soils with a higher pH (after liming if needed) as ‘sweet’. To test they poured sulphuric acid onto the soil. If it fizzed then lime wasn't required. That was as good as soil testing got then (early 20th, Victorian and before) and I'm not suggesting that is the way to do it nowadays.


Another advantage of maintaining the soil pH to the optimum 6.5 to 7.0 is that worm population and their activity is greatly improved. Worms love limed soil. The more worms the better, as they break down and distribute organic matter throughout the soil, as well as keeping it well airated. Airation is essential for good rooting and general plant health.


Of course ground limestone and ground chalk are not organic, (being crushed rock), but they are totally natural and should be considered by organic gardeners as acceptable. If not use Calcified Seaweed (seaweed which has been turned to lime). Chemically limestone and chalk are the same thing, (calcium carbonate) the only difference being, limestone is a hard rock and chalk a soft rock. Ground limestone gives a slower adjustment than chalk but maintains the level for longer.