Soil pH for Plants: All Things You Need to Know
Any plant suffering from a deficiency of lime (i.e. growing in a pH group that is too low) will show stunting symptoms and the growing tips of the plant are yellow and deformed whilst the lower parts of the plant remain unaffected. This is often accompanied by short and stubby root growth instead of long fibrous ones.
At very low pH levels, aluminum and manganese are dissolved by acids and escape into the soil. These are poisonous to some plants.
On the other hand, plants growing in lime soil (where the pH is too high) can also be adversely affected. This is generally more because of a secondary effect on other plant foods (see later) than because of an excess of Calcium.
Symptoms of high pH levels in soil usually show as deficiencies of Iron, Boron, or Manganese. Unfortunately, these also result in yellowing of the leaves which makes detection a bit more difficult.
Boron deficiency shows up as twisted, distorted growth and often the terminal buddies. In turnip and beetroot, hollow, brown areas develop.
With a Manganese deficiency, the terminal bud stays alive, but the older leaves show yellow patches between the veins, and often dead spots appear on the leaves.
If these deficiencies are induced because your soil has too much lime present, they cannot be corrected by applying the appropriate element, because no matter how much is applied, the presence of the excess lime in the soil locks up the elements and makes them unavailable to plant roots. The way to unlock them is to modify the pH level. This frees the elements in the soil eliminating the need to add “extra” amounts of those that were deficient.
The chart shows this quite clearly. You can see how the main plant foods of Nitrogen, Phosphate, and Potash are made more or less available at different pH levels. The availability of Iron, Manganese, and Boron is also depicted.
From the chart, you can see that it is pointless to apply Phosphate fertilizers if your pH is very low because it would not be available.
By increasing your pH to around 6.5, more Phosphate is available (FREE!). This is OK provided that the plants you want to grow are happy at pH 6.5 (see lists). You will, no doubt have noticed that the majority of plant foods are at their most available at pH 6.5 which may be why it is often quoted as being the best general pH level for most soils.
An application of lime also has other advantages provided that it does not raise your pH to a level that is higher than that needed for the sort of plants you want to grow.
Lime helps to improve soil drainage, aeration, and workability of clay soils by making them less sticky and more open. It does this by creating sand sized multi-particles. It encourages worm activity which itself significantly increases the organic content of the soil as food pulled into the soil by worms decomposes. It also helps to prevent some diseases, e.g. Club Root of Brassicas.
Finding out about your pH
There are three ways. Firstly, you can send a sample to a lab and have it tested for you. This will give a technically accurate result, assuming the sample you sent was representative of the whole area. It usually a cost a bit more to have a lab do a test for you than it does to buy a home test kit. It is less work, however, and a good lab will provide some advice as well as the result itself.
As an alternative, you can get an inexpensive kit from the Garden Centre that will give quite a reliable result. The usual arrangement with this sort of kit is that you put a bit of soil typical of the area that you want to test in a little tube, then you add a few drops of a test solution, shake it up and leave it for an hour or so to settle. Once it has cleared, the solution in the tube changes color according to the pH of your soil, and you compare the color with a chart that comes with the kit. Different kits work in different ways, but usually, acid soils give a kind of yellowy-orange result, and alkaline ones give a greeny-blue result. You usually get a color card, rather like a paint color chart to compare the color against to give you the result. The better kits also provide advisory booklets about how to interpret your result.
The third and least reliable method for the inexperienced gardener is to use the plant lists to identify the sort of weeds and plants that grow well naturally on your soil. From this, you can often develop a good guess as to what the pH of the soil is. For example, knowing that cabbages and wallflowers do well in your garden, but Rosemary keeps dying off would lead you to the view that the soil is alkaline (Cabbages like alkaline soil, but Potatoes prefer it more acid).
Raising the pH to Make the Soil More Alkaline
It is generally easier to make soils more alkaline (i.e. increasing the pH) than it is to make them more acid (i.e. reducing the pH).
A movement of 0.5 is quite easy but, because the pH scale is logarithmic, a movement of say, 2.0 points becomes difficult because there is a factor of 10x between each full point, so pH 5.0 is actually 100 times more acid than pH 7.0.
Also, different soil types react in different ways to the application of lime. To achieve the same result, clay soils and peaty soils require more lime than do sandy soils.To increase your pH by 1 point and make your soil more alkaline
NOTE: Applications in excess of 20 oz per square yard must be split
Applications in excess of 20 oz per square yard are best provided by splitting the job into two equal applications which are spaced at least six months apart, putting on half the total amount on each occasion.
It is quite safe to apply amounts of less than 20 oz per square yard as one dressing, preferably in the autumn, and this should be incorporated into the soil by digging. Hydrated lime should be available from Garden Store, garden centers or builders merchants.
Some safety precautions such as gloves and goggles to keep the powder off exposed skin and out of eyes would be sensible, and of course, you should always follow the instructions on the packaging. Ground limestone is less fine and flyaway.
Correction of an over acid soil should be considered as a longer-term job than just one year, and it is advisable to test your soil each year if a large alteration is planned over several years.
Lowering the pH to Make the Soil More Acid
Reduction of pH levels tends to be a more expensive and complicated process than increasing them. There are, however, a couple of emergency measure which are sometimes used to good effect. These are outlined later.
There are no specific chemicals which can safely be applied to the soil to make it more acid, so the acids must come from increased quantities of organic matter as they are decomposed by soil bacteria. The use of acid natured fertilizers will also help.
The speed at which organic matter breaks down is affected by soil type, temperature, and bacterial content, so the amounts required can only be approximated.
To reduce the pH by approximately 1.0 unit, use:
Peat – dug into your soil at the rate of approximately 2.5 lbs per square yard Compost – dug into your soil at the rate of approximately 14 lbs per square yard Manure – dug into your soil at the rate of approximately 5 lbs per square yard
When you want to increase the nitrogen level in your soil, use a fertilizer that exerts an acidic effect, such as Sulphate of Ammonia. If it is used at around 2 oz per square yard, this should reduce the pH by about 1.0 although the effect is short-lived, and you might not want to use so much fertilizer anyway.
Change the soil
One possible alternative if only a section of your garden is required for acid-loving plants is to change the soil by building a raised acid border on top of the existing soil. Such borders are generally made of peat block retaining walls and filled with peaty, acid soil brought in from elsewhere. The beds should be at least 12 inches (300 mm) deep above the existing ground level and should not have the imported soil dug or mixed with the existing soil.
Although not always easily available, it is possible to lower soil pH with Flowers of Sulphur or Ground Rock Sulphur. American research led the way in this technology, although some British research has been undertaken. Suggested rates are: To reduce pH by 1.0 units (e.g. to go from 6.5 to 5.5), apply Sulphur at 1.2 oz per square yard on sandy soils, or 3.6 oz per square yard on all other soils.
Ideally, the sulfur should be thoroughly mixed into the soil before planting, but it can be used as a top dressing hoed or forked into the soil around existing plants, in which case a booster dressing may be needed every few years. The greatest benefit comes when it is thoroughly mixed in amongst the soil particles. Clearly, there is a trade-off here with the risk of mechanical damage to the roots of existing plants.
It is advisable to check your pH at least once each year to see how you are going on.
Plants which have been planted in a soil which is too alkaline for them can have their problems eased by an annual application of Sequestrine of Iron (available from Garden Centres and shops), although this is not a permanent solution. (Click Next)