How pH Affects Fertilizers and Plant Foods

At very low pH levels, aluminium and manganese are dissolved by acids and escape into the soil. These are poisonous to some plants.

On the other hand, plants growing in a 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.

Iron Deficiency

With Iron deficiency, the leaf veins usually remain a deep green.

Boron deficiency shows up as twisted, distorted growth and often the terminal bud dies.
In turnip and beetroot, hollow, brown areas develop.

Manganese Deficiency

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.

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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.

pH Plant Preference Lists

Food Plants |  Ornamental Plants |  Trees |  Shrubs |  Potted Plants

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