• Ammonium fertilizers are not recommended for acid soils because they increase soil acidity through a process called nitrification. When ammonium (NH₄⁺) is added to the soil, soil bacteria convert it into nitrate (NO₃⁻) and release hydrogen ions (H⁺) during this process:
• NH₄⁺ → NO₂⁻ → NO₃⁻ + 2H⁺
• The released H⁺ ions further lower the soil pH, making the soil more acidic. In already acidic soils, this can:
• Increase the solubility of toxic metals like aluminum (Al³⁺),
• Decrease the availability of essential nutrients (e.g., phosphorus, calcium, magnesium),
• Harm root development and overall plant health.
• Therefore, using ammonium fertilizers in acid soils worsens the acidity problem and negatively affects crop productivity. Gifole Gidago Wabalo

Gifole Gidago Wabalo

Applying ammonium-rich fertilizers to acidic soils is generally discouraged because it can exacerbate soil acidification, which negatively impacts soil health and nutrient availability for plants. This is due to the process of nitrification, where ammonium is converted to nitrate by soil microorganisms, releasing hydrogen ions (H+) into the soil, further lowering the pH.

Thanks!

You're right, it's generally not recommended to apply fertilizers with high ammonium content to acid soils. Here's why and how it can affect soil health:

Why High Ammonium Fertilizers are Problematic in Acid Soils:

The key issue lies in the nitrification process and its effect on soil acidity. Here's a breakdown:

How it Affects Soil Health:

Lowering the soil pH further in already acidic soils can have several negative impacts on soil health:

• Nutrient Availability: Soil pH significantly affects the availability of essential plant nutrients. In very acidic conditions: Availability of macronutrients like phosphorus, potassium, calcium, and magnesium decreases. Phosphorus, in particular, can become "fixed" by aluminum and iron ions, forming insoluble compounds that plants cannot readily absorb. Availability of micronutrients like iron, manganese, zinc, and copper increases, potentially to toxic levels for plants. Aluminum toxicity is a major concern in very acidic soils, inhibiting root growth and nutrient uptake.Molybdenum availability decreases, which is essential for nitrogen fixation by symbiotic bacteria in legumes.
• Microbial Activity: Soil microorganisms play a vital role in nutrient cycling, organic matter decomposition, and overall soil health. Acidic conditions can: Inhibit the activity of beneficial bacteria, including nitrogen-fixing bacteria (like Rhizobium) and those involved in the decomposition of organic matter.Favor the growth of fungi, which may not be as efficient in nutrient cycling as bacteria in many agricultural systems. Reduce the activity of earthworms, which are important for soil aeration, drainage, and nutrient mixing.
• Soil Structure: While the direct impact of ammonium on soil structure is less pronounced than the effects of pH, the indirect consequences can be significant. Reduced microbial activity and changes in root growth due to nutrient imbalances can negatively affect soil aggregation and stability.
• Increased Leaching of Nutrients: In highly acidic soils, there can be increased leaching of essential cations like calcium, magnesium, and potassium, as they are displaced by the excess hydrogen and aluminum ions.
• Reduced Crop Yields: Ultimately, the combined effects of nutrient imbalances, toxicities, and impaired biological activity can lead to reduced crop growth and yields.

In summary, applying high ammonium fertilizers to acid soils can create a vicious cycle, further acidifying the soil, disrupting nutrient availability, harming beneficial soil organisms, and ultimately compromising soil health and productivity.

Alternatives and Management Strategies:

For acid soils, it's generally recommended to:

• Use nitrate-based fertilizers (though these can also have their drawbacks with leaching).
• Apply lime to raise the soil pH to a more optimal range for nutrient availability and microbial activity.
• Incorporate organic matter, which can buffer soil pH changes and improve overall soil health.
• Consider slow-release nitrogen fertilizers to minimize the rapid release of ammonium.
• Employ nitrification inhibitors in some cases to slow down the conversion of ammonium to nitrate and thus reduce the rate of hydrogen ion release.

By understanding the chemical processes involved, we can make more informed decisions about fertilizer application and soil management in acidic environments.

Ammonium fertilizers are not recommended for acid soils because they can further increase soil acidity. Here's why:

• When ammonium (NH4) is converted to nitrate (NO3) by soil microbes through nitrification, it releases hydrogen ions (H⁺), which acidify the soil further.
• Acid soils already have a high concentration of H⁺ ions, so adding ammonium accelerates nutrient leaching, reduces microbial activity and makes essential nutrients like phosphorus and molybdenum less available.
• It can also increase the solubility of toxic metals like aluminum (Al³⁺) and manganese (Mn²⁺), harming plant roots.

Applying fertilizers with high ammonium content (such as ammonium sulfate or urea) in acid soils is generally not recommended due to several reasons that can negatively impact soil health:

1. Further Acidification of Soil

Ammonium (NH₄⁺) is converted to nitrate (NO₃⁻) by soil microbes through nitrification, a process that releases hydrogen ions (H⁺):

NH₄^+ + 2O₂ → NO₃^- + 2H⁺ + H₂O

2. Aluminum and Manganese Toxicity

As soil pH drops below 5.5, aluminum (Al³⁺) and manganese (Mn²⁺) become more soluble and can reach toxic levels for plant roots, inhibiting root growth and nutrient uptake.

3. Loss of Beneficial Microbial Activity

Acidic conditions can suppress beneficial soil microbes, such as nitrogen-fixing bacteria and decomposers, which play critical roles in nutrient cycling.

The most commonly used ammonium mineral fertilizers are three: Ammonium chloride (NH4Cl); Ammonium sulfate [(NH4)2SO4] and ammonium nitrate (NH4NO3). All of them have a certain, but not equally strong acidifying effect on the soil.

The simplest explanation for this fact is that all three compounds are hydrolytically acidic salts, because they hydrolyze according to the schemes:

NH4Cl + H2O a HCl + NH4OH

(NH4)2SO4 + 2 H2O a H2SO4 + 2NH4OH

NH4NO3+ H2O a HNO3 + NH4OH

the hydrolysis of the three salts are strong acids – HCl, H2SO4 and HNO3 and the relatively weaker base – ammonium hydroxide. This type of acidic effect of ammonium fertilizers is called hydrolytic.

The second mechanism of the acidifying action is the physiological one. It consists in the fact that when plants absorb the ammonium form of nitrogen, the corresponding acids remain in the soil. In this sense, the acidifying effect of ammonium nitrate on the soil is more special - its acidifying effect decreases relatively when the soil has conditions for more intensive absorption of the nitrate part of its molecule.

The three formulations of ammonium fertilizers have different acidifying effects. The acidifying effect of ammonium chloride is the strongest, followed by ammonium sulfate and finally, in this order, with the relatively weakest acidifying effect is ammonium nitrate.

Thus, to neutralize the acidifying effect of 100 kg of NH4Cl introduced into the soil, 128 kg of CaCO3are needed; for 100 kg (NH4)2SO4 – 110 kg CaCO3and for 100 kg NH4NO3 – 67 kg CaCO3

Of course, the interactions in the soil are far more complex, but with regard to the acidifying effect of ammonium fertilizers, the interaction of the applied fertilizers with the absorption complex in the soil is most important.

By themselves, the hydrogen cations obtained as a result of acid hydrolysis do not remain free in the soil, but after some complex interactions with the soil sorption complex, they cause the removal of aluminum cations (Al3+) from the crystal lattice of clay minerals into the double electric layer of mineral colloids. These aluminum cations are the cause of the toxic effect of soil acidity on plants.

The microbiota has a much more limited importance on the processes we are talking about

The general conclusion is that in acidic soils, the use of ammonium fertilizers should be avoided.

Instead of ammonium nitrate, if possible, it is better to use lime-ammonium nitrate. This fertilizer does not have an acidifying effect on the soil.

Simple answer is ammonium fertilizers are acid forming in nature which may aggravate acidity.

Como ya lo mencionaron todos los colegas, el amonio se usa en compañia del sulfato para la fertilizacion de los cultivos de arandano en mi pais como funte de nitrogenoy tambien porque permite acidificar el medio o sustrato y mantener el pH entre 4.5 a 6.0 que es el medio que le gusta al cultivo,en vez de usar como antes el acido sulfurico tan daniño para el medio ambiente.

Thank you all very much for your good explanation. I am highly satisfied with the explanation of why ammonium fertilizers are not recommended for acid soils.

because amonium sulfat decrease the soil pH, more than Urea

Thank you very much for the answer, Lila yao Jia.