Impact of soil pH on nutrient availability

According to the Potash Development Association (PDA), soil pH must be considered the all-important starting point of any crop fertiliser plan.

This is because the availability of all nutrients is affected by the pH of the soil. The optimum availability of most plant nutrients occurs around a neutral pH of 6.5-7.0.

However, the optimum pH for soils will vary depending on the soil type and the cropping. For most mineral soils, it is 6.0 for permanent pasture, through to 6.5 for continuous arable cropping.

A pH of at least 6.5 should be considered where acid sensitive crops are grown in the rotation, such as sugar beet.

Clovers also require an adequate pH and will not thrive at the lower pH tolerated by grass; mixed grass / clover swards should be maintained between pH 6.0 and 6.5.

At low soil pH, clovers may not nodulate effectively and their ability to fix nitrogen is reduced; at very low pH levels, clovers may fail to nodulate completely.

Soil pH

Since soil pH is measured on a logarithmic scale, a drop of just one pH value increases the level of acidity 10-fold.

For this reason, any drop in pH from the optimum range can have a large impact on the availability of some nutrients.

Research has confirmed that the availability of both phosphate and potash will fall-off as soil pH values drop below a value of 7.0 (increasing acidity).

Phosphate availability appears to be affected the most. But even potash is only around 50% available below a pH of 5.5. Vast quantities of potash are required by crops, up to 500kg/ha in some cases.

So, any reduction in the availability of potassium will have a significant impact on the crop’s ability to utilise soil reserves – regardless of any additions by way of fertiliser.

Soil sampling

There can be large variation in soil pH across a field, so the accuracy of the soil sampling is also important to ensure that the results are truly representative of the area measured.

Rainfall, which is naturally slightly acidic, ammonium nitrogen and root exudates will all contribute to the natural acidification of soil, but not to the extent of reducing the pH of high pH soils.

These factors are more likely to be detrimental to naturally acidic and neutral soils than they are beneficial in naturally alkaline soils.

It is not difficult to raise the pH of acidic soils; this can be achieved using liming materials. Those most often used are ground calcitic or magnesian (Dolomitic) limestone, chalk or sugar beet lime etc.

But as these materials often supply large quantities of either calcium or magnesium, the choice of product should also be influenced by the magnesium status of the soil.

Nutrients

The availability of nutrients from the soil reserves will be reduced if the pH is sub-optimal, with this problem being more serious if nutrient levels are below Index 2.

On soils with low nutrient reserves, especially where they are also acidic, if nutrient applications are being reduced or omitted, yields are going to suffer as a result, increasing the cost of production in a time of increasing input costs and lower returns.

Correcting a nutrient deficiency is challenging to achieve in the short-term, however it can be even more costly if not addressed.

Fertiliser recommendations for phosphate and potash are built up from immediate removal by the crop being grown (a maintenance or removal figure) plus an adjustment for soil index.

The first step to ensure crops receive optimal nutrition, and thereby helping to reduce the unit cost of production, is to ensure the correct soil pH.

Once this is corrected, achieving target soil index is the next consideration.