The pH Factor – Part II

A Basic Understanding of pH in Soil

Last month we defined the term ‘pH’ and discussed the factors that impact the pH of water in the spray tank. This month we will discuss manipulating pH in the soil, a more complicated issue because soil pH is due to the integrated effects of parent material, rainfall/irrigation, organic matter and texture among others.

Soil Reaction: Acidity and Alkalinity

Soil acidity is determined not just by hydrogen cations (H⁺), but by aluminum cations (Al³⁺) as well, with aluminum playing the dominant role. Under acid conditions, the major source of soil acidity is the hydrolysis of aluminum cations:

Soil pH < 5.0: Al³⁺ + H₂O Þ Al(OH)²⁺+ H⁺

Soil pH 5.0-6.5: Al(OH)²⁺+ H₂O Þ Al(OH)₂⁺+ H⁺

Exchangeable H⁺ cations are a limited source of soil acidity in acid soils and play a small role in contributing to soil pH.

Neutral to alkaline soils (pH 7.0 and above) are not dominated by either aluminum or hydrogen cations. Instead, H⁺ and Al³⁺ cations are replaced by exchangeable calcium, magnesium and other base-forming cations (Ca²⁺, Mg²⁺, Na⁺, etc.). Most of the remaining aluminum converts to insoluble compounds like gibbsite [Al(OH)₃] and many of the remaining H⁺ cations react with OH^- to form water.

Raising Soil pH

Soil acidity typically results where rainfall is plentiful enough to leach exchangeable base-forming cations (Ca²⁺, Mg²⁺, Na⁺, etc.) from the soil, from certain types of parent material and/or organic matter decomposition, from soil erosion and where harvest of crops removes significant amounts of Ca and Mg from the soil.

It is generally recognized that soil pH’s between 6.0 and 6.5 are optimal for the growth of most plants. Whenever the pH drops below this range, the normal procedure is to incorporate agricultural limestone into the soil periodically over time. Agricultural limestones are typically of two types: 1) calcitic: mostly calcium carbonate [CaCO₃]; and 2) dolomitic: calcium carbonate containing varying proportions of calcium-magnesium carbonate [CaMg(CaCO₃)₂]. If the limestone is almost entirely calcium-magnesium carbonate, it is referred to as dolomite.

Limestone raises soil pH when the Ca²⁺ and Mg²⁺ cations contained in the lime replace the hydrogen and aluminum cations on the cation exchange complex through a series of reactions with carbon dioxide and water. For example:

CaCO₃+ H₂O + CO₂ Þ Ca(HCO₃)₂

2 H⁺- Colloid + Ca(HCO₃)₂Þ Ca²⁺-Colloid + 2 H₂O + 2 CO₂

Adding powdered agricultural limestone (the most common form of limestone) to soil can be a very tedious procedure. Instead, some applicators have switched to granular (pelletized) and flowable limestones as a cleaner, easier procedure. Although it would take a number of years to thoroughly correct pH with flowable limestone, the fine particle size of flowable limestone can immediately correct the pH in the thatch and the top ¼ to ½ inch layer of soil. An improved pH environment favors an increase in microbial populations (reducing thatch) and better nutrient uptake by the lateral roots of the plant.

Cleary’s Limestone F can be used to manipulate soil pH in turf using the following table as a guide:

	LIMESTONE F (gallons/1000 ft²)
	SAND		LOAM		CLAY
Present Soil pH	To pH 6.0	To pH 6.5	To pH 6.0	To pH 6.5	To pH 6.0	To pH 6.5
4.8	1.00	1.42	1.67	2.33	2.33	3.33
4.9	0.92	1.33	1.58	2.17	2.08	3.08
5.0	0.83	1.25	1.42	2.08	1.92	2.92
5.1	0.75	1.17	1.33	1.92	1.67	2.67
5.2	0.67	1.08	1.17	1.83	1.50	2.5
5.3	0.58	1.00	1.08	1.67	1.25	2.25
5.4	0.50	0.92	0.92	1.58	1.08	2.17
5.5	0.42	0.83	0.83	1.42	0.92	1.83
5.6	0.33	0.75	0.67	1.33	0.75	1.67
5.7	0.25	0.67	0.50	1.17	0.58	1.50
5.8	0.17	0.58	0.33	1.08	0.42	1.33
5.9	0.08	0.50	0.17	0.92	0.25	1.17
6.0	-	0.42	-	0.83	-	1.00
6.1	-	0.33	-	0.67	-	0.83
6.2	-	0.25	-	0.58	-	0.67
6.3	-	0.17	-	0.42	-	0.50
6.4	-	0.08	-	0.25	-	0.33

Lowering Soil pH

Soil alkalinity typically results when there is a high percentage of base-forming cations (Ca²⁺, Mg²⁺, Na⁺, etc.) in the soil. This is often the case in soils where there is limited rainfall, alkaline parent material and/or irrigation with hard water containing sodium carbonate and bicarbonates (found in some river and well water). Lowering soil pH can be accomplished by a number of methods, although none is as cheap and efficient at lowering soil pH as limestone is at raising soil pH.

To lower soil pH:

1) Use acidifying nitrogen fertilizers - Most nitrogen fertilizers acidify the soil, even though the nitrogen itself may be chemically alkaline (e.g. ammonium - NH₄⁺). Through bacterial and enzymatic action, ammonium in the soil combines with oxygen to form nitric acid, which acidifies soil. You could, in theory, acidify your soil wholly with nitrogen fertilizer if you could stand the growth that would result. Nitrogen fertilizers that contain ammonium or hydrolyze in soil to form ammonium (e.g. urea) decrease soil pH by the following process:

NH₄⁺ + 2 O₂ Þ NO₃^-+ H₂O + 2 H⁺

2) Add sulfur – Sulfur is one of the six plant macronutrients. By itself, it is inert and non-phytotoxic. It acidifies soil by slowly oxidizing to form sulfuric acid.

2 S + 3 O₂ + 2 H₂O Þ 2 H₂SO₄

The most popular sources of sulfur are: gypsum (CaSO₄), sulfur coated urea and elemental sulfur. Of the three, elemental sulfur is the most efficient and practical to use when pH manipulation alone is your goal. Gypsum would be an appropriate choice if part of your goal is to reduce sodium content of the soil. The calcium cation supplied by the gypsum molecule displaces sodium on the cation exchange complex. However, the calcium cation also partially neutralizes the sulfur, making it less efficient at acidifying soil. Incorporating sulfur coated urea into your nitrogen fertility program to another option to manipulate soil pH. Not only will the sulfur oxidize to form acid, but the urea will hydrolyze to release H⁺cations.

3) Add organic matter – Organic matter decomposition commonly forms organic and inorganic acids which acidify soil slowly over time. This is especially effective if the organic substrate contains a low percentage of base-forming cations (pine needles, peat moss, etc.)

4) Add acid – This can be a very caustic and costly solution, but under certain circumstances, it can be justified.

A final thought

Always remember that whether you are trying to decrease or increase your soil pH, you are almost always working against the natural state of that soil. Soil pH is what it is for a reason: the integrated effects of parent material, texture, climate, organic matter content and a host of other factors over which you have little or no control. Add to that the amazing ability of soils to buffer themselves against change and you have a recipe for an uphill battle. Soil pH management is not a one-time proposition, but an ongoing commitment. Regular soil testing and a basic understanding of soil science are critical to your success.

BACK to Newsletter