Wally Richards – getting the pH right in your garden

How do you know if your garden’s pH is too high or too low and what can you do about it?

Every so often I will get a call from a gardener who tells me their pH is too high or too low and what can they do about it.

The conversation will go along these lines: “Hi Wally I bought this pH metre and then I went around measuring the pH in my gardens. Shock, horror the pH is well out of satisfactory growing range and all the gardens should be looking pretty sick or dead.”

Of course that is not the situation at all, the plants are generally quite well and happy and it’s only the gardener having heart trepidations thinking the soil pH is all over the place.

The only problem is the loss of money buying a pH metre that gives false readings. Now if you want a pH metre that accurately measures the acidity or alkalinity of your soil, water or spa pool then you need to buy a unit that can be calibrated with the aid of Buffer Solutions.  The pH readings taken at any time are affected by temperature so you need to calibrate the meter prior to every reading taken.

This is done by placing the probe(s) into a solution that has a known pH and adjusting the dial to that setting. There are two Buffer Solutions commonly used: pH 4 and pH 10. It is even better to have a third solution with a pH of 7.
The price of this meter will range from a few hundred dollars to over a thousand, likely a bit too expensive for most gardeners.

If you are concerned about pH then the first thing to realise is most New Zealand soils tend to be a little on the acid side unless you are in limestone country (then it’s alkaline).

Over time the soils tend to become more acid and that is why a regular amount of soft lime is applied to vegetable gardens and to areas where alkaline loving plants are growing.  Also if you continually apply manmade fertilisers you will be increasing the acidity of the soil and this will affect a number of plant’s growth. In some cases nothing grows, no matter how much fertiliser is applied.

A simple test can be done using litmus paper which is readily available in kits from places that sell spas and swimming pools. You also need distilled water which is pH neutral.  Take a small sample of soil and place in a clean glass jar that has a lid. Add the distilled water to the jar covering the soil. Then shake the contents for several minutes.  Then place your litmus paper into the jar and compare the colour it turns to the colour chart supplied.
Now you have a good indication as to what your pH is in that spot where the sample was taken.

Remember that certain factors may have an influence on that spot such as concrete near by, past history and drainage. Thus you need to repeat the same test with fresh samples etc in other parts of your gardens.

For the home gardener an easy test in the vegetable garden can be done by planting two short rows of peas about 30cm apart. On one row sprinkle a soft lime and none on the other row. If the peas in the lime row grow better then you need to lime those areas where alkaline loving vegetables are to grow (but not where potatoes and tomatoes are going to grow).

I keep saying soft lime and there is a good reason for this. Soft lime is readily available to plants and the soil and if you wet your finger and rub some lime between the fingers, soft lime will make a soft slurry. Hard lime, coming from limestone, will be gritty.  Hard lime can take up to 10 years to become available to plants which is ok if you have been applying it once or twice a year for the last ten years.  Check the lime you are going to buy to ensure it is soft and thus it will be quickly available to your garden.

If you have a area which is alkaline, or you want to grow acid loving plants then dissolve say about 50 to 100 grams of sulphate of iron in water then add to about 5 litres of water, in a watering can and apply this over the area.
If you find your potatoes have scab then the above will assist in reducing that problem.

The correct pH for plants allows better absorption of elements making for better growth and health. Plants, like us, need calcium in their diet for their good health. In plants, calcium once fixed is not mobile in the plant. It is an important constituent of cell walls and can only be supplied in the xylem sap. Thus, if the plant runs out of a supply of calcium, it cannot remobilize calcium from older tissues.  If transpiration is reduced for any reason, the calcium supply to growing tissues will rapidly become inadequate. A more common problem caused by this is blossom end rot in tomatoes.

Without adequate amounts of calcium, plants experience a variety of problems, symptoms of which in crops are often called physiological disorders. The symptoms of calcium deficiency are: Necrosis at the tips and margins of young leaves, bulb and fruit abnormalities, deformation of affected leaves, highly branched, short, brown root systems, severe, stunted growth, and general chlorosis. Chlorosis is a yellowing of leaf tissue due to a lack of chlorophyll. Calcium deficiencies reduces the uptake of some minerals such as magnesium.

It must be remembered that these problems are caused by an inadequate supply of calcium to the affected tissues. These deficiencies can occur even when the soil appears to have an adequate presence of calcium. Garden Lime (A common supply of calcium) is often derived from lime stone which is hard and can take between 3 to 10 years to become available to plants after application to the soil. Calcium is found in many minerals in soil, but is relatively insoluble in this state. Calcium is not considered a leachable nutrient. Many soils will contain high levels of insoluble calcium such as calcium carbonate, but crops grown in these soils will often show a calcium deficiency.

High levels of other cations such as magnesium, ammonium, iron, aluminum and especially potassium, will reduce the calcium uptake in some crops.

A common misconception is that if the pH is high, adequate calcium is present. This is not always true. Calcium plays a very important role in plant growth and nutrition, as well as in cell wall deposition. The primary roles of calcium are: as a soil amendment, calcium helps to maintain chemical balance in the soil, reduces soil salinity, and improves water penetration. Calcium plays a critical metabolic role in carbohydrate removal. Calcium neutralises cell acids.

Another aspect that is often overlooked is that calcium is the fuel that feeds the soil life.               And that my gardening friends, is a very very important aspect indeed.

One comment on “Wally Richards – getting the pH right in your garden

  1. Mairi Isabeth on said:

    Hi, How do you get Calcium to them if they are Acid lovers, I have some banksia trees and I deep mulched when I came here but they all look as if they are dying I.E yellowing leaves and the leaves are dropping off.Taken a branch to several nurseries and garden centres but they have all told me they dont know.Mairi

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