Whether you’re up to your armpits in compost or are a member of the “throw on more super and she’ll be right” brigade, sooner or later you come across the idea of soil pH (called “soil reaction” by the old hands). At this point many people throw up their hands in disgust and say “This is all too technical for me, I’m a farmer/gardener/nice person not a chemist!”. The concept of pH is not that difficult to grasp though (if I can understand it you can!) and it can be vital if you want healthy plants and bumper crops.
What is pH
In its most technically correct definition pH is the negative or minus logarithm of the hydrogen ion concentration in a solution, in this case the soil water solution. This probably sounds like bafflegab to you but that doesn’t matter, the important thing to remember is that the pH value represents how acid or alkaline a soil is it is expressed as a number between 1 and 14 where a pH of 1 is extremely acid, like battery acid and a pH of 14 is extremely alkaline like a soapmaking lye solution. A pH of 7 is neutral ie neither alkaline nor acid like pure water. The pH of soil can vary between 3 and 9 but in Australia most of our soils are acid and it is rare to see a soil pH outside the range of 4 to 7. The ideal pH range for most plants we want to grow – veggies etc – although some plants such as rice and potatoes do well down to a pH of 5 and barley and apples can tolerate a mildly alkaline pH ie 7.5 -8.0.
Why is pH of the soil important?
As mentioned in the previous paragraph a soil pH of 6 to 6.5 is required to get the best out of our fruit and veggies, but as the good professor said – “Why is it so?”. To answer this we need to understand what plants require from soil to help them grow, because plants need certain essential nutrients to grow just as we do. Plants absorb water from the soil through their roots and dissolved in the water are chemical nutrients which the plant uses to build up its tissues. Many people will be familiar with the three major nutrients; nitrogen, phosphorous and potassium also referred to as NPK but there are also three minor nutrients: calcium, magnesium and sulphur as well as eight trace nutrients. These trace nutrients are only required in very small amounts but if they are missing or unavailable from the soil you get very sick plants.
Now what has all this got to do with pH you may ask? The “availability” of a plant nutrient in the soil is tied up with pH. The soil may have all the good nutrients in sufficient amounts but the wrong pH may cause them to be in a form that is insoluble in water. If the nutrients are not soluble in the soil water the plant cannot take them up and use them; so far as the plant is concerned they don’t exist and it starves to death. At the optimum pH of 6 to 6.5 all the nutrients are in available form, but as you move away from that pH either way, some are locked up in the soil and others become available in excess eg iron and aluminium at low pH to the point where plant may be poisoned.
The soil pH can also affect the types of micro-and macro-organisms which can survive in the soil. Earthworms are usually absent from very acid soils but fungi prefer acidic conditions and bacteria prefer a neutral soil. This last point can be very important when growing peas and beans (legumes) because if the soil is too acid the rhizobium bacteria which fix nitrogen for the legumes will not be able to thrive, so neither will the legumes, so that a vital part of the crop rotation may be lost. Another example of pH affecting plants through soil microorganisms is the slime-mould which causes club root, a disease of crucifers, because it only thrives in acid soils and raising the soil pH can reduce or eliminate this problem.
