The effect of fertilizers on soil pH

Fertilizers can be acid, basic, or neutral in their effect on soil pH:

• All ammonium N fertilizers (except ammonium nitrate with lime) have a gradual acidforming effect. That's because the conversion of ammonium (NH.) to nitrate (NO3) releases acid-forming hydrogen ions (H+). The same applies to urea and most NP and NPX fertilizers. (See Table 9-1.)

• Large applications of manure or compost also have a gradual acid-forming effect.

• Nitrate N fertilizers that have their nitrate combined with a strong base have a slightly basic effect (i.e. calcium nitrate, potassium nitrate, sodium nitrate).

• The straight P or K fertilizers have no effect on soil pH. Examples: potassium chloride, potassium sulfate, and the superphosphates.

The Practical Implications of Acid-Forming Fertilizers

Continued use of acid-forming fertilizers over the years will eventually lower soil pH enough to require liming, unless the soil is very alkaline. The rate that soil pH will fall depends on the kind and amount of fertilizer applied and the buffering capacity (negative charge, C.E.C.) of the soil (see Chapter 6). Since clayey soils or those high in organic matter tend to have more buffering capacity, they're usually more resistant to pH change than sandy soils.

So why use acid-forming fertilizers?: They're usually the most available and economical; on alkaline soils, they can actually be beneficial.

TABLE 9-1 Relative Acidity of Acid-Forming Fertilizers

Why not add lime to acid-forming fertilizers?: Some fertilizer labels state the amount of lime required to neutralize the acidity produced per 100 kg of the fertilizer, but this is just a legal requirement. Mixing in lime with such a fertilizer will convert much of its ammonium into ammonia gas which is then lost to the air. Don't add lime to the soil after each fertilizer application, either; it's unnecessary and time consuming. At any rate, most limited-resource farmers won't be applying high enough rates to markedly lower the pH in a year or two.