Fertilizer acidity: How it can ruin your soil

Fertiliser acidity has an impact on crop development and should always be kept in mind when scheduling applications. In general, any addition to the soil will modify its characteristics in one way or another. Let's see how different fertilisers affect the soil to understand the consequences of their use.

It should not be thought that directly when we use an alkaline fertiliser the Soil pH will go up. The modification of the pH of the soil depends on many factors. Natural soils generally have a high buffering capacity, so that in conventional applications the pH will not change in the short term.

The soils with the least impact on pH due to fertiliser use are organic soils, followed by clay, loam and finally sandy soils.

Spain is divided according to its acidity into two halves, a high pH eastern half (the orange one in the figure below) and a low pH eastern half (the bluish one). On the eastern slope the geology has a calcareous nature which in its composition produces a basic soil morphology. On the other hand, the soils of the western peninsular have a ganitic geological origin, and the weathering of these rocks produces more acid soils.

It is much more common to limestone applications in western soils with the aim of modifying their pH.

A fertiliser has an acidic impact when, during its decomposition, it releases hydrogen ions (H⁺). And alkalinity occurs when the applied fertiliser has a hydroxide ion (OH‾) as a by-product.

• Application of urea CO(NH2)2

Urea is the fertiliser with the highest content of nitrogenbut its main problem is that it is very soluble.

CO(NH2)2 + 2H2O ⇔(NH4)2CO3

The first part of the reaction is hydrolysis, which can take less than four days in a normal situation.

(NH4)2CO3 ⇔ 2 NH4⁺+ CO3²‾

Once the urea is hydrolysed, the dissociation between ammonium and carbonate is immediate in contact with water. The end result is the acidifying effect of the remaining hydrogen.

• Application of calcium carbonate CaCO3

This application is often used to raise the pH of acid soils. It can also be used to increase the calcium concentration in the soil. saturated extract or to fight against sodification.

CaCO3+H2O ⇔ Ca2⁺ + CO3H‾ + OH‾

You can find out the reaction of any fertiliser by a quick search on the internet.

• Potassium chloride (KCl)

The salts dissociate in the saturated soil extract, giving on the one hand potassium ion which is easily absorbed by the plant. On the other hand, chlorine can react with calcium, which will lead to a reduction in calcium and a decrease in buffering capacity. This effect can be pronounced in calcium-poor soils.

• Potassium sulphate (K2SO4)

It is an alternative to potassium chloride as the CASO4 formed is less soluble than calcium chloride. But sulphate fertilisers have a disadvantage for lawns or crops that are not tilled for aeration. The accumulation of sulphates coupled with a low infiltration capacity and aeration will cause black layer.

•  Ammonium sulphate ((NH4)2SO4)

As is the case with all fertilisers with ammonium in their compounds, it is acidifying. Sulphate also reacts easily with calcium, retaining it and allowing it to solubilise quickly with rainfall.

(NH4)2SO4 ⇔2 NH4⁺+ SO4²‾
Ca2⁺ + SO4²‾⇔ CaSO4

• Anhydrous ammonia (NH3)

This is an example of how a fertiliser has an alkalising impact on the medium, ammonia applied to the soil will have the ability to increase the pH of the soil.

NH3 + H2O⇔NH4⁺ + OH‾

Knowing a fertiliser correctly will help to avoid major problems in the future.

There are a multitude of fertilisers and each one has a different reaction in the soil, so I invite you to leave in the comments some feedback if you have or if you know how a fertiliser reacts in the soil.

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The SolStick pH meter is a perfect tool to measure the pH of soils very quickly, accurately and efficiently.