Fertilizer Source Impacts on Soil pH
Basics of soil acidification
Understanding soil pH is critical for maintaining healthy turf and maximizing plant health. The best way to determine the pH of the soil is to perform soil tests at various locations at depths not exceeding current rhizosphere. The soil pH (power of hydrogen) determines the alkalinity or acidity of the soil and is critical for nutrient availability, microbial activity, pesticide efficacy and overall root and plant health. The pH scale ranges from 0-to 14 and is based on a negative logarithm of hydrogen in soil solution. A soil pH of 7 is neutral and above indicates an alkaline soil in which hydroxide ions are greater than hydrogen (OH- > H+), and below pH of 7 would indicate acid soil with hydrogen ions greater than hydroxide (H+ > OH-).
Soil pH is a major factor affecting nutrient uptake and overall plant health and needs careful analyzation when choosing fertilizer sources. Soil acidification occurs naturally in areas with excess rainfall which causes breakdown of organic matter and slowly dissolving basic minerals such as calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na). A complete understanding of different fertilizer sources and the effect they have on soil pH could contribute to the overall success of growing pristine turf.
pH ranges and how nutrient availability is affected
Methods for acidifying
Soil pH is reduced by increasing hydrogen (H+) ions and is accomplished by supplementing with elemental sulfur or acidifying fertilizers as represented in the table below. However, it’s important to first have a basic understanding of the specific soil properties, climate and turf type and use when choosing which amendment is best.
| Amendment | Pounds CaCO3 Neutralized by 100 lbs of Amendment |
| Sulfur | 312 |
| Sulfur Coated Urea | 118 |
| Ammonium Sulfate | 110 |
| Sulfuric Acid | 100 |
| Urea | 84 |
| Diammonium Phosphate | 70 |
| Urea Formaldehyde | 68 |
| Monoammonium Phosphate | 65 |
| Ammonium Nitrate | 59 |
| Aluminium Sulfate | 45 |
| Organic Material | 0-10 |
A basic understanding of how acidification takes place with the use of certain amendments is important. The first thing to remember is an increase in hydrogen ions decreases soil pH; this means that the amendment that is used must do just that. Please note that sulfur in SO4- does not directly reduce soil pH, but is the only form that is absorbed by the plant. Elemental sulfur on the other hand is not readily available to the plant and must first be converted by microorganisms (Thiobacillus spp.) to sulfuric acid (H2SO4-) before eventually becoming available as SO4-. This conversion is temperature-dependent and is slower in cooler soils. When elemental sulfur is converted to sulfuric acid (H2SO4-) the pH of the soil is then reduced. Incorporating elemental sulfur into the soil will have a greater effect versus surface applied.
The use of ammonium (NH4+) and urea (CO(NH2)2) based fertilizers reduces soil pH because of the increase in hydrogen ions (H+). Urea converts into ammonium through hydrolysis. Ammonium contributes to the overall soil acidity by several methods:
- Acidification occurs when ammonium is absorbed by the plant and hydrogen ion (H+) is released into the soil for maintaining plant neutrality.
- Acidification occurs when ammonium undergoes nitrification by bacteria (Nitrosomonas spp.) releasing hydrogen and causing an increase in overall hydrogen content in the soil and causing a decrease in soil pH.
It is always a good idea to use best management practices and periodically check soil pH if using a new amendment to gauge the overall effects on acidification. Depending on soil conditions or water quality, the overall effect on pH could vary drastically.
For acidifying Yara Fertilizers, check out the following:
For more information, please contact your local extension service, distributor or Yara Sales Agronomist.
Sources:
https://hortnews.extension.iastate.edu/1994/4-6-1994/ph.html
Lowering Soil pH for Horticulture Crops (purdue.edu)
https://www.canr.msu.edu/uploads/files/Lowering_Soil_pH_with_Sulfur.pdf
https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/pnw599.pdf
Neil Mayberry - Crop Manager, Turf & Ornamental
