Acid or Alkaline?

Your soil may contain all the elements for optimal plant growth, but if its acidity/alkalinity level is wrong for what you grow, plants will not be able to access the elements, or they may get too much. This blog post covers how to measure acidity/alkalinity, and how to adjust it, if it needs adjusting.

Acidity/Alkalinity is measured in pH  

pH stands for potential Hydrogen, which explains nothing to most food gardeners, including me. 

The pH scale goes from 0.0 (extremely acid) to 14.0 (extremely alkaline). 

A soil with a pH of less than 7.0 is acidic, also called ‘sour’. 

A soil with a pH of more than 7.0 is alkaline, also called ‘sweet’.

pH 3.2             : the pH of pure vinegar 

pH 4.5             : the pH of some native Tasmanian bushlands 

pH 5.0             : preferred by blueberries 

pH 4.8 – 5.5   : optimum pH-range for potatoes 

pH 5.75           : the pH of my mushroom compost 

pH 5.5 – 6.5   : blackberries, strawberries, apples prefer this pH-range 

pH 6.0 – 7.0  : the pH-range most vegetables prefer 

pH 7.0             : the pH of neutral soil. It is neither acid nor alkaline

pH 7.0 – 7.5   : okay for apricot, artichoke, asparagus, beans, beetroot, brassicas, cucumber, 

                            garlic, parsnip, pear, peas, pumpkins, spinach 

Based on these numbers you can conclude that …

• Potatoes are ideally grown in relatively acid soils, unlike most vegetables.
• Blueberries also prefer a low pH, so are best not grown right next to vegetables.
• It makes sense to group together plants that like a similar pH, because then you can aim for that pH, and as a result have bigger and better crops.

My potato crops have not always been great. I did not realise this before, but this might be because the pH of most of my garden beds is around 6.7 

It is good to be aware what the ideal pH is for various plants, but there is no need to go overboard. You will have okay results with most food-plants if the pH of the soil they are in, is between 6.0 and 7.0.

Testing pH  

You won’t have the same pH throughout your garden because you would have added manure here, put new soil there, limed various beds, added compost here and there, so test the pH of each bed or area.

Even within one garden bed there will be small differences, but let’s not get too fussy. Ideally you do one test for each area or bed in your garden, and then re-test a year later, after trying to adjust the pH (see below).

The pH of soil can be tested with a soil pH test kit or digital pH meter. Both can be purchased at hardware stores and nurseries. For this article, I used a Manutec Soil pH Test Kit (around $19) and a Gardman pH Meter (around $13), and compared results. Both are October 2018 prices.

Using the pH soil test kit

• The kit came with one container containing a liquid, and another one containing a white powder, a colour chart, and a little platform on which to put your soil samples.
• Do the testing outside in good daylight.
• Where you want to test soil pH take a number of small soil samples from various depths, no deeper than 10 centimetres. Mix them up, and take from that mix a small quantity for your actual test.
• Remove from this sample any ‘big bits’, so you only have fine soil.
• Add some liquid from the container until the soil sample becomes a thick paste.
• Now sprinkle the provided white powder over the top of this mix.
• Some seconds later, compare the colour of the powder with the colour diagram provided.

I used kits like this in the past. Like on previous occasions, I did not find it easy to match the colour the sample took on with a colour on the chart. For example: 

The colour of the sample was green, but was it ‘7 green’, or ‘6½ green’, or ‘6 green’?

To me it was not obvious. Okay, it was green, so somewhere between 6.0 and 7.0, and that is good. 

As I tested more samples, they all turned green, which is good, but I began to wonder whether the kit would ever show me any different colour, when the next sample gave me …

This was clearly grey, so the pH of this soil was somewhere between pH 7.0 and pH 7.5 - a bit high. 

I then decided to see whether the kit would indicate extremes if I tested extremes. In the photo below I tested on the left, very fine sand with rice vinegar (extremely acid), in the middle, a bit of garden soil, and on the right, some wood ash from our wood heater (extremely alkaline).

The resulting colours, after applying the liquid, were quite convincing:

I had to put a little bit of the white powder on the sample in the middle and push it gently into the soil to see the result.  I did not have to any powder on the extremes on the left and right to see their result.

My conclusion is that the kit provides reliable rough indications of soil pH.

Using the pH meter
With a pH meter you need to make sure that ..

• You clean the probe with a wet cloth after each test.
• The soil you test is moist.
• The soil you are going stick the probe in is easy to penetrate.
• You push the probe into the soil with a screw action to create a tight fit.

It took around 30 seconds for the dial to settle.

To see how it responded to an extremely acid situation I poured vinegar down a hole. The result was this:

 

After testing six soil samples with both methods I concluded: 

• Both methods were reliable and did not contradict each other. 
• The pH soil test kit gave me only rough indications of soil pH.
• By its very nature, the meter provides a precise number, which might be inaccurate.
• Using the meter is far easier and faster than using the pH soil test kit.

The test kit is dearer than the meter, and only lasts until the liquid or powder runs out.

Costing just $13, the Gardman pH meter may be a gem, or it may not be totally accurate.  I am going to use the pH-meter from here on, in preference over the test kit, but will be on the lookout for another pH-meter that has proven reliability.

Changing pH

Changing the pH of soil is a process that requires patience. You can’t quickly change the pH for a particular crop you are going to grow next, and then change it again for the crop thereafter. Aim for a certain pH for the various areas of your food garden and gradually work towards that. 

Most Tasmanian soils are naturally acidic. Soil will gradually become more acidic if you never add lime and regularly add organic matter, such as manure, worm castings, biochar, unfinished compost and mulch.


Working towards less acidic soil = increasing the pH

Agricultural lime is the best way to make soil less acidic. The best time to apply it is when good rains are forecast. If you lime in autumn, soil pH may have changed by spring. To increase the pH, let’s say from 5.0 to 6.0, apply 100 grams of lime per square metre on sandy soils, and 400 grams of lime per square metre on clay soils. Do not apply lime at the same time as manures and fertilisers that contain nitrogen. They react against each other. 

Dolomite is a lime that also contains magnesium carbonate. The extra trace element may seem a bonus. However, continued use of dolomite can creates excess magnesium in your soil, which makes clay sticky and tightly packed (Steve Solomon – The Intelligent Gardener page 79). In some places in my garden soils have been a bit hard in the past. I now use lime in preference over dolomite.  

Wood ash contains phosphorous, potassium, calcium, boron and other elements that growing plants need. It can be used to make soil less acidic, but it is very alkaline. For that reason, it is best used sparingly, in compost heaps. If used directly on soils, it may well kill bacteria and worms.  

Builders’ lime can be used in food gardens. It is much finer than agricultural lime and acts twice as fast as agricultural lime. It can irritate skin and eyes. Certified organic growers are not allowed to use it.

Working towards less alkaline soil = decreasing the pH

Not many Tasmanian soils are naturally alkaline. Irrigation with ground water that contains lime or sodium can gradually make soils alkaline. Many farmers who use bore water for irrigation have dams they pump their ground water in to. Lime, sodium and other substances in the bore water are then given time to sink to the bottom, and then water is used for irrigation. However, soils can still gradually become alkaline as a result of irrigation with bore water. 

Compost and soil bought from commercial operators can be alkaline if it is the result of a composting process that was too hot. In large compost heaps organic material may burn and leave ash, which can result in compost or soil that is rather alkaline.

Sometimes soil is made alkaline by accident by putting soil right next to new concrete or cement. Over a period of time the lime leaches out, and makes the soil alkaline.

For Blueberries and other acidity-loving plants you may want to make a moderately acidic soil more acidic. 

Agricultural sulphur is the most effective way to make soil less alkaline. To make soil less alkaline, let’s say, take the pH from 8.0 to 7.0, apply agricultural sulphur from natural sources at 25 grams per square metre for sandy soils and 100 grams per square metre for clay soils.

For more info about agricultural sulphur see article Powdered Sulphur on the ABC Gardening Australia site. Make sure not to apply too much sulphur as a concentrated over-supply can kill plants. 

Gypsum (discussed below) contains sulphur. Regular application of it to alkaline soil will over time make that soil less alkaline, but this will not be a quick process.

Working towards making soil slightly alkaline = raising it slightly above 7.0  

Some crops, for example asparagus, prefer a pH of just above 7.0. It is a perennial crop, so you can work towards gradually increasing the pH to neutral and then just above 7. 

Agricultural lime is used to make acidic soil less acidic. If applied to neutral soil, it will make that soil alkaline.


Very little or no influence on pH have

Pine needles have a pH of around 3.5 when fresh, but, when composted that pH becomes near neutral. I used a pine needle mulch in my blueberry bed for quite a few years because I wanted the pH to go down a bit, but is has not! 

Gypsum is a kind of lime, but adding it to soil has little effect on its pH (source: Steve Solomon – The Intelligent Gardener). The gypsum you buy can be a by-product of chemical processes or it can be a naturally-occurring mined rock. Gypsum that is a naturally occurring mined rock is approved for certified organic farms. Gypsum’s main benefit is that it helps breaking up clay soil. 

Coffee grounds have a pH of around 6.5 and are not effective in changing soil acidity. Coffee grounds are best used on your compost heap.  Using them directly around crops can have a negative impact on them. See ABC video The right way to use old coffee in your garden.

Seagrass and seaweed are good sources of trace elements. If applied as a mulch or worked into soil over a long period of time, traces of sodium will influence pH, but adding them to soil is not a proven way to raise pH.

Happy pH-ing!