Introduction

Soil pH provides various clues about soil properties and is easily determined. The most accurate

method of determining soil pH is by a pH meter. A second method which is simple and easy

but less accurate then using a pH meter, consists of using certain indicators or dyes. Soil acidity is

a major environmental and economic concern. If untreated, acidity will become a problem in the

subsurface soils, which are more difficult and expensive to ameliorate. It is estimated that 12 to 24

million ha is extremely too highly acidic with pH values less than or equal to 4.8.Acidic soils cause

significant losses in production and where the choice of crops is restricted to acid tolerant species

and varieties, profitable market opportunities may be reduced. In pastures grown on acidic soils,

production will be reduced and some legume species may fail to persist.

Degradation of the soil resource is also of wider concern and off-site impacts must be considered.

Off-site impacts mainly result from reduced plant growth. Deep-rooted species required to increase

water usage may not thrive, increasing the risk of salinity. Increased run-off and subsequent

erosion has detrimental impacts on streams and water quality. Increased nutrient leaching may

pollute ground water. Alkalinity problems are more common in clay soils than in soils with low

colloid content. As a consequence of the relative preponderance of sodium on exchange sites of

colloids, the alkaline reaction of the liquid phase and the swelling/shrinking clay minerals, the low

fertility of these salt-affected.

Soil pH

Soils support a number of inorganic and organic chemical reactions. Many of these reactions are

dependent on some particular soil chemical properties. One of the most important chemical

properties influencing reactions in a soil is pH . Soil pH is primarily controlled by the concentration

of free hydrogen ions in the soil matrix. Soils with a relatively large concentration of hydrogen

ions tend to be acidic. Alkaline soils have a relatively low concentration of hydrogen ions.

Hydrogen ions are made available to the soil matrix by the dissociation of water, by the activity of

plant roots, and by many chemical weathering reactions.

Soil fertility is directly influenced by pH through the solubility of many nutrients. At a pH lower

than 5.5, many nutrients become very soluble and are readily leached from the soil profile. At high

pH, nutrients become insoluble and plants cannot readily extract them. Maximum soil fertility

occurs in the range 6.0 to 7.2. Oil acidity is measured in pH units. Soil pH is a measure of the

concentration of hydrogen ions in the soil solution. The lower the pH of soil, the greater the acidity.

pH is measured on a logarithmic scale from 1 to 14, with 7 being neutral. A soil with a pH of 4

has 10 times more acid than a soil with a pH of 5 and 100 times more acid than a soil with a pH of

6.

Soil pH or soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH

units. Soil pH is defined as the negative logarithm of the hydrogen ion concentration. The pH scale

goes from 0 to 14 with pH 7 as the neutral point. As the amount of hydrogen ions in the soil

increases the soil pH decreases thus becoming more acidic. From pH 7 to 0 the soil is increasingly

more acidic and from pH 7 to 14 the soil is increasingly more alkaline or basic.

Descriptive terms commonly associated with certain ranges in soil pH are:

Extremely acid: < than 4.5; lemon=2.5; vinegar=3.0; stomach acid=2.0; soda=2–4

Very strongly acid: 4.5–5.0; beer=4.5–5.0; tomatoes=4.5

Strongly acid: 5.1–5.5; carrots=5.0; asparagus=5.5; boric acid=5.2; cabbage=5.3

Moderately acid: 5.6–6.0; potatoes=5.6

Slightly acid: 6.1–6.5; salmon=6.2; cow's milk=6.5

Neutral: 6.6–7.3; saliva=6.6–7.3; blood=7.3; shrimp=7.0

Slightly alkaline: 7.4–7.8; eggs=7.6–7.8

Moderately alkaline: 7.9–8.4; sea water=8.2; sodium bicarbonate=8.4

Strongly alkaline: 8.5–9.0; borax=9.0

Very strongly alkaline: > than 9.1; milk of magnesia=10.5, ammonia=11.1; lime=1

Soil acidification

Soil acidification is the buildup of hydrogen cations, also called protons, reducing the soil pH. This

happens when a proton donor gets added to the soil. The donor can be an acid, such as nitric acid

and sulfuric acid (these acids are common components of acid rain). It can also be a compound

such as aluminium sulfate, which reacts in the soil to release protons. Many nitrogen compounds,

which are added as fertilizer, also acidify soil over the long term because they produce nitrous and

nitric acid when oxidized in the process of nitrification.

Acidification also occurs when base cations such as calcium, magnesium, potassium and sodium

are leached from the soil. This leaching increases with increasing precipitation. Acid rain

accelerates the leaching of bases. Plants take bases from the soil as they grow, donating a proton

in exchange for each base cation. Where plant material is removed, as when a forest is logged or

crops are harvested, the bases they have taken up are permanently lost from the soil.

Causes of soil acidity

Soil acidification is a natural process accelerated by agriculture. Soil acidifies because the

concentration of hydrogen ions in the soil increases. The main cause of soil acidification is

inefficient use of nitrogen, followed by the export of alkalinity in produce.

Ammonium based fertilisers are major contributors to soil acidification. Ammonium nitrogen is

readily converted to nitrate and hydrogen ions in the soil. If nitrate is not taken-up by plants, it can

leach away from the root zone leaving behind hydrogen ions thereby increasing soil acidity.

Most plant material is slightly alkaline and removal by grazing or harvest leaves residual hydrogen

ions in the soil. Over time, as this process is repeated, the soil becomes acidic. Major contributors

are hay, especially lucerne hay and legume crops. Alkalinity removed in animal products is low,

however, concentration of dung in stock camps adds to the total alkalinity exported in animal

production.

Effects of soil acidity

Plant growth and most soil processes, including nutrient availability and microbial activity, are

favoured by a soil pH range of 5.5 – 8. Acid soil, particularly in the subsurface, will also restrict

root access to water and nutrients.

Aluminium toxicity-When soil pH drops, aluminium becomes soluble. A small drop in pH can

result in a large increase in soluble aluminium (figure 1). In this form, aluminium retards root

growth, restricting access to water and nutrients (figure 2).

Poor crop and pasture growth, yield reduction and smaller grain size occur as a result of inadequate

water and nutrition. The effects of aluminium toxicity on crops are usually most noticeable in

seasons with a dry finish as plants have restricted access to stored subsoil water for grain filling.

Nutrient availability-In very acid soils, all the major plant nutrients (nitrogen, phosphorus,

potassium, sulphur, calcium, manganese and also the trace element molybdenum) may be

unavailable, or only available in insufficient quantities. Plants can show deficiency symptoms

despite adequate fertiliser application.

Microbial activity-Low pH in topsoils may affect microbial activity, most notably decreasing

legume nodulation. The resulting nitrogen deficiency may be indicated by reddening of stems and

petioles on pasture legumes, or yellowing and death of oldest leaves on grain legumes. Rhizobia

bacteria are greatly reduced in acid soils. Some pasture legumes may fail to persist due to the

inability of reduced Rhizobia populations to successfully nodulate roots and form a functioning

symbiosis.

Management of Acidic Soils

Soil testing-Knowledge of how soil pH profiles and acidification rates vary across the farm will

assist effective soil acidity management.

Ideally, soil samples should be taken when soils are dry and have minimal biological activity. It is

standard to measure pH using one part soil to five parts 0.01 M CaCl2. Soils with low total salts

show large seasonal variation in pH if it is measured in water. pH measured in water can read 0.6

– 1.2 pH units higher than in calcium chloride (Moore et al., 1998).

Soil sampling should take paddock variability into consideration. For example, clays have greater

capacity to resist pH change (buffering) than loams, which are better buffered than sands. Samples

should be taken at the surface and in the subsurface to determine a soil pH profile. This will detect

subsurface acidity, which may underlie topsoils with an optimal pH.

Samples need to be properly located (e.g. GPS) to allow monitoring. Sampling should be repeated

every 3 – 4 years to detect changes and allow adjustment of management practices.

Interpreting pH results-Depending on soil pH test results, agricultural lime may need to be

applied to maintain pH, or to recover pH to an appropriate level. If the topsoil pH is above 5.5 and

the subsurface pH above 4.8, only maintenance levels of liming will be required to counter on-

going acidification caused by productive agriculture.

If the topsoil pH is below 5.5, recovery liming is recommended. Keeping the topsoil above 5.5

will treat the on-going acidification due to farming and ensure sufficient alkalinity can move down

and treat subsurface acidity.

Liming is necessary if the subsurface pH is below 4.8, whether or not the topsoil is acidic. If the

10 – 20 cm layer is below 4.8 but the 20 – 30 cm layer above 4.8, liming is still required. In this

case the band of acidic soil will restrict root access to the more suitable soil below.

Liming-Liming is the most economical method of ameliorating soil acidity. The amount of lime

required will depend on the soil pH profile, lime quality, soil type, farming system and rainfall.

Limesand, from coastal dunes, crushed limestone and dolomitic limestone are the main sources of

agricultural lime. Carbonate from calcium carbonate and magnesium carbonate is the component

in all of these sources that neutralises acid in soil.

The key factors in lime quality are neutralising value and particle size. The neutralising value of

the lime is expressed as a percentage of pure calcium carbonate which is given a value of 100 %.

With a higher neutralising value, less lime can be used, or more area treated, for the same pH

change. Lime with a higher proportion of small particles will react quicker to neutralise acid in the

soil, which is beneficial when liming to recover acidic soil.

Complementary management strategies-If soil pH is low, using tolerant species/varieties

of crops and pasture can reduce the impact of soil acidity. This is not a permanent solution because

the soil will continue to acidify without liming treatment.

A number of management practices can reduce the rate of soil acidification. Management of

nitrogen fertiliser input to reduce nitrate leaching is most important in high rainfall areas. Product

export can be reduced by feeding hay back onto paddocks from where it has been cut. Less

acidifying options in rotations will also help, e.g. replace legume hay with a less acidifying crop

or pasture.

Changes in Soil pH

Soils tend to become acidic as a result of: (1) rainwater leaching away basic ions (calcium,

magnesium, potassium and sodium); (2) carbon dioxide from decomposing organic matter and

root respiration dissolving in soil water to form a weak organic acid; (3) formation of strong

organic and inorganic acids, such as nitric and sulfuric acid, from decaying organic matter and

oxidation of ammonium and sulfur fertilizers. Strongly acid soils are usually the result of the action

of these strong organic and inorganic acids.Lime is usually added to acid soils to increase soil pH.

The addition of lime not only replaces hydrogen ions and raises soil pH, thereby eliminating most

major problems associated with acid soils but it also provides two nutrients, calcium and

magnesium to the soil. Lime also makes phosphorus that is added to the soil more available for

plant growth and increases the availability of nitrogen by hastening the decomposition of organic

matter. Liming materials are relatively inexpensive, comparatively mild to handle and leave no

objectionable residues in the soil.

Some common liming materials are: (1) Calcic limestone which is ground limestone; (2) Dolomitic

limestone from ground limestone high in magnesium; and (3) Miscellaneous sources such as wood

ashes. The amount of lime to apply to correct a soil acidity problem is affected by a number of

factors, including soil pH, texture (amount of sand, silt and clay), structure, and amount of organic

matter. In addition to soil variables the crops or plants to be grown influence the amount of lime

needed.

pH Affects Nutrients, Minerals and Growth

The effect of soil pH is great on the solubility of minerals or nutrients. Fourteen of the seventeen

essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must

be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid

soils than in neutral or slightly alkaline soils. Phosphorus is never readily soluble in the soil but is

most available in soil with a pH range centered around 6.5. Extremely and strongly acid soils (pH

4.0-5.0) can have high concentrations of soluble aluminum, iron and manganese which may be

toxic to the growth of some plants. A pH range of approximately 6 to 7 promotes the most ready

availability of plant nutrients.But some plants, such as azaleas, rhododendrons, blueberries, white

potatoes and conifer trees, tolerate strong acid soils and grow well. Also, some plants do well only

in slightly acid to moderately alkaline soils. However, a slightly alkaline (pH 7.4-7.8) or higher

pH soil can cause a problem with the availability of iron to pin oak and a few other trees in Central

New York causing chlorosis of the leaves which will put the tree under stress leading to tree decline

and eventual mortality. The soil pH can also influence plant growth by its effect on activity of

beneficial microorganisms Bacteria that decompose soil organic matter are hindered in strong acid

soils. This prevents organic matter from breaking down, resulting in an accumulation of organic

matter and the tie up of nutrients, particularly nitrogen, that are held in the organic matter.

Alkali soil

Alkali, or alkaline, soils are clay soils with high pH (> 8.5), a poor soil structure and a low

infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils

owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium

carbonate which causes the soil to swell and difficult to clarify/settle. They derive their name from

the alkali metal group of elements to which the sodium belongs and that can induce basicity.

Sometimes these soils are also referred to as (alkaline) sodic soils.

Alkaline soils are basic, but not all basic soils are alkaline, see: "difference between alkali and

base.They are not saline, i.e. the total amount of soluble soils, especially sodium chlorides, is not

excessive (ECe < 4 to 8 dS/m).

Causes of soil alkalinity

The causes of soil alkalinity are natural or they can be man-made. The natural development is due

to the presence soil minerals producing sodium carbonate upon weathering. The man-made

development is due to the application of irrigation water (surface or ground water) containg a

relatively high proportion of sodium bicarbonates.

The causes of soil alkalinity are natural or they can be man-made.

1. The natural cause is the presence of soil minerals producing sodium carbonate (Na2CO3)

and sodium bicarbonate (NaHCO3) upon weathering.

2. Coal fired boilers / power plants when using coal or lignite rich in limestone produces ash

containing calcium oxide (CaO). CaO readily dissolves in water to form slaked lime /

Ca(OH)2 and carried by rain water to rivers / irrigation water. Lime softening process

precipitates Ca and Mg ions / removes hardness in the water and also converts sodium

bicarbonates in river water into sodium carbonate. Sodium carbonates (washing soda)

further reacts with the remaining Ca and Mg in the water to remove / precipitate the total

hardness. Also water soluble sodium salts present in the ash enhance the sodium content in

water. The global coal consumption is 7700 million tons in the year 2011. Thus river water

is made devoid of Ca and Mg ions and enhanced Na by coal fired boilers.

3. Many sodium salts are used in industrial and domestic applications such as Sodium

carbonate, Sodium bicarbonate (baking soda), Sodium sulphate, Sodium hydroxide

(caustic soda), Sodium hypochlorite (bleaching powder), etc. in huge quantities. These salts

are mainly produced from Sodium chloride (common salt). All the sodium in these salts

enter into the river / ground water during their production process or consumption

enhancing water sodicity. The total global consumption of sodium chloride is 270 million

tons in the year 2010. This is nearly equal to the salt load in the mighty Amazon River.

Manmade sodium salts contribution is nearly 7% of total salt load of all the rivers.[3]

Sodium salt load problem aggravates in the downstream of intensively cultivated river

basins located in China, India, Egypt, Pakistan, west Asia, Australia, western USA, etc.

due to accumulation of salts in the remaining water after meeting various transpiration and

evaporation losses.

4. Another source of manmade sodium salts addition to the agriculture fields / land mass is in

the vicinity of the wet cooling towers using sea water to dissipate waste heat generated in

various industries located near the sea coast. Huge capacity cooling towers are installed in

oil refineries, petrochemical complexes, fertilizer plants, chemical plants, nuclear &

thermal power stations, centralized HVAC systems, etc. The drift / fine droplets emitted

from the cooling towers contain nearly 6% sodium chloride which would deposit on the

vicinity areas. This problem aggravates where the national pollution control norms are not

imposed or not implemented to minimize the drift emissions to the best industrial norm for

the sea water based wet cooling towers.

5. The man-made cause is the application of soft water in irrigation (surface or ground water)

containing relatively high proportion of sodium bicarbonates and less calcium and

magnesium.

Conclusion

So, soil pH does play a role in nutrient availability. Should you be concerned on your farm? Be

more aware than concerned. Keep the pH factor in mind when planning nutrient management

programs. Also, keep historical records of soil pH in your fields. Soils tend to acidify over time,

particularly when large applications of NH4+ based fertilizers are used or there is a high proportion

of legumes in the rotation.

Recent years have shown the pH decline occurring more rapidly in continuously cropped, direct-

seeded land. On the other hand, seepage of alkaline salts can raise the pH above the optimum

range. So, a soil with an optimum pH today may be too acid or alkaline a decade from now,

depending on producer land management.