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This is the first of what is hopefully going to become a regular section in our magazine.

The articles will cover in general terms aspects of the science of horticultural production. For those who are familiar with this subject this will serve as a refresher, and for others an insight of what might be occurring in your crop. In this first article we look at the subject of pH which is often over looked and underestimated in its effect on plants.


The pH story
pH is the term used to describe or measure the acidity or alkalinity of a soil or solution. The full measure or scale is 0-14 with 0-7 being acidic and 7-14 alkaline. 7 being in the middle of the scale is neutral, 0 being the extreme in acidic and 14 the

extreme in alkaline or caustic. In agricultural soils or waters the range is usually between 5 and 8.5.

In scientific terms pH is a measure of the concentration of hydrogen ions (H+) with the higher the concentration being reflected by the stronger acidic values, ie the lower the pH reading.

Most people only measure the pH of the soil growing the crop, however in horticulture where regular irrigation is part of the production process, water pH can play a significant role. Plants don't just grow in soil, they grow in a solution combination of soil, water, air and cultural additives (namely fertilisers) and it is the resultant combination of pH that we need to be paying the attention to.

 As an example if we have a soil with a pH of 6.5 a water source with a pH of 7.6 and an alkaline fertiliser is used then

Back to basics...

the resulting soil solution could easily remain alkaline, which is not ideal for maximising horticultural cropping potential. For nearly all of the crops we grow the desirable range for pH is 6 - 6.5 (see table below for desirable crop pH ranges).

 Another significant factor for trying to keep your combined soil solution in this range is that at these levels all of the necessary nutrients are available to the plant. If our soil solution becomes more acid or alkaline the availability of our plant foods is decreased or in some extreme cases can be made available but in toxic quantities. Looking at the pH chart on the left, you can see the widths of the bars indicating availability of the nutrients at the various pH levels, wider being more available and narrowing being less available. If you draw a line down the 6.5 value you can see that nearly all nutrients are of their maximum availability - TOGETHER. Welcome to the balancing act.

Balancing takes experience but by getting it right you can prevent a lot of problems from developing and make it that much easier to maximise crop potential. In summary pH is rarely a fixed point with any farming operation so it is wise to set up procedures where this value is monitored at regular intervals within a cropping season. The main points again:

Measure of acidity (0-7) or alkalinity (7-14).

Crops grow in the combined soil solution of soil, water and additives.

Each crop has its own desirable pH range.

pH can influence the availability and uptake of nutrients.

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