Managing Soil Acidity: Practical Tips for Farmers

Introduction

This lecture focuses on practical approaches to managing soil acidity, particularly for farmers.
The primary goal of ameliorating acidic soils is to mitigate aluminium toxicity, which is the main constraint in such soils.

Understanding Aluminium Toxicity

As soil pH decreases (specifically measured in calcium chloride), aluminium concentration remains stable until pH reaches approximately 5.
Below pH 5, a slight increase in aluminium concentration occurs.
At pH 4.5, there is a sharp, substantial increase in aluminium concentration, making it a significant problem.

Amelioration Strategies

Several strategies can be employed to improve acidic soils:
- Adding organic material
- Applying liming materials
- Cultivating acid and aluminium-tolerant crop varieties

Organic Material

Focus is on decreasing the proportion of the soil exchangeable complex occupied by aluminium.
When organic material with low acid-neutralizing capacity is added, the exchangeable complex may still have a high percentage (e.g., 40%) occupied by aluminium.
As the acid-neutralizing capacity of the organic material increases, the amount of exchangeable aluminium decreases.
Ideally, the goal is to reach a point where there is no exchangeable aluminium on the cation exchange complex.
Note that changes in pH due to the addition of organic material are usually minimal.
Organic material works primarily by complexing aluminium, rendering it non-toxic, thus reducing its impact.
Different soils respond uniquely, with variations in the magnitude of response, although the pattern remains consistent.

Liming

Liming is an important method for managing acidic soils.
Acidic soils significantly impede root growth, which is a major issue.
Lime application enhances soil pH and promotes better crop growth.
In field situations, there is a notable difference in crop growth (e.g., barley) between limed and unlimed strips.

Field Example

Peter Settler applied substantial amounts of lime on a two-kilometer strip in a paddock.
Initially, neighbors were skeptical, but the benefits accrued significantly over the years.

The Chemistry of Liming

In acidic soils, a significant portion of the cation exchange complex is occupied by aluminium.
When lime is added, calcium replaces aluminium on the exchange complex.
This process forces aluminium into a solution with a higher pH, leading to its precipitation as aluminium hydroxide (Al(OH)_3), which is known as gibbsite.
Liming can result in a small emission of greenhouse gases.

Types of Liming Materials

Calcium carbonate (CaCO_3), or calcite, is the standard liming material against which all others are compared.
The neutralizing value of other materials is calculated relative to CaCO_3.
Some materials may have higher neutralizing values than calcium carbonate but might not be preferred due to cost.

Calcium Oxide

Calcium oxide (CaO) has a considerably higher neutralizing value than calcium carbonate and is more effective at neutralizing soil acidity.
However, it is generally too expensive for practical use.
The benefit of using calcium oxide would be lower transportation costs due to needing smaller amounts.

Neutralizing Values of Various Liming Materials

Calcium Carbonate (CaCO_3): 100% neutralizing value (the standard)
Calcium Oxide (CaO): Significantly higher neutralizing value than calcium carbonate.
Slaked Lime (Ca(OH)_2): Substantially higher neutralizing value than calcium carbonate.
Calcium Silicate (CaSiO_3): Important neutralizing value but not as high as calcium carbonate.
Dolomite (CaMg(CO3)2): Nearly as effective as calcite, sometimes slightly better.
Magnesite (MgCO_3): Slightly better than calcite.

Cost Considerations

Dolomite is typically more expensive than calcite.
Transportation costs play a crucial role in determining the choice of liming material.

Properties Affecting Liming Material Effectiveness

Several properties influence how effectively a liming material neutralizes soil acidity:
- Particle Size: Potentially the most important factor.
- pH Buffering Capacity of the soil: Soils with high buffering capacity may take longer to show changes, and require more lime.
- Rate of application: How much lime is applied.
- Water Availability: Crucial for the reaction.
- Soil Calcium Status: Influences cation exchange capacity and the balance between aluminium and calcium.
- Partial Pressure of CO2: High partial pressure can inhibit the liming reaction because CO2 emission occurs during the liming process.

Particle Size and Efficiency

Large particle sizes (e.g., 3 mm) result in low efficiency.
Smaller particle sizes increase effectiveness but also increase the cost.
Farmers must balance particle size and cost to select the most suitable option.

Impact of Lime Application Rate and Depth

Soil pH changes are influenced by the amount of lime applied and how deeply it affects the soil.
Small rates of lime (e.g., just over one tonne) primarily affect the topsoil with minimal effect on deeper layers.
Higher application rates increase pH in shallow layers and eventually lead to some increase in deeper layers.
Substantial lime rates are needed to affect deeper soil layers significantly.
Transport of alkalinity from surface-applied lime is slow and requires large amounts to reach deeper layers (around 20 cm, where the bulge of acidity often develops).
Applying very high rates (e.g., 10 tonnes of lime per hectare) can increase pH by about 0.5 pH units but is extremely costly.

Time and Water Requirements

Liming processes are not quick; they take years to show significant effects.
Solubilization of lime is slow without sufficient water.
More water leads to greater solubilization and a more significant pH increase.
Higher rainfall increases solubilization but may also lead to lime being consumed faster.
Once lime is solubilized, it is used up, and pH may decline over time, necessitating additional applications.
Therefore, while water is beneficial, one must account for the potential need for repeated lime applications.

Aluminium-Tolerant Cultivars

Using aluminium-tolerant cultivars is a simple and seemingly cheap option.
However, it is only a short-term solution because the soil will continue to acidify.
Eventually, only liming can remedy this continued acidification.
The Egret wheat variety, developed in the 1960s, was backcrossed with a Brazilian variety to introduce aluminium tolerance.
Tolerant genotypes exhibit much better root growth than sensitive ones under extremely acidic conditions.

Field Experiment: Long-Term Effects of Liming

Peter Sellars applied lime in long strips in 1984.
Years later, these strips were cross-limed to create four distinct soil profiles:
- No lime in 1984, no lime in 1999 (control)
- Limed in 1999 (topsoil amelioration only)
- Limed in 1984 (subsoil amelioration)
- Limed in both 1984 and 1999 (topsoil and subsoil amelioration)

Root Growth and Soil Profiles

In areas with no lime, the bulge of acidity develops around 20 cm depth, with high aluminium concentrations.
Where lime was applied in 1984, subsoil acidity was ameliorated, resulting in higher pH and minimal exchangeable aluminium in deeper layers.
Sensitive varieties show poor root growth beyond 35 cm in untreated acidic soils, while tolerant varieties perform better but are still affected.
When subsoil acidity is ameliorated, there is no significant difference in root growth between sensitive and tolerant genotypes.

Yield Results

When the entire profile is acidic, tolerant varieties yield slightly more than sensitive ones.
When subsoil is acidic, sensitive varieties are significantly affected, while tolerant varieties maintain higher yields.
When subsoil acidity is ameliorated, the yield difference between genotypes diminishes.
On average, tolerant varieties perform better across all soil conditions.

Different Site and Treatment: Wongan Hills Experiment

A "crazy" experiment was conducted to prove the point that subsoil acidity is the primary problem.
Topsoil was removed, lime was applied and mixed into the subsoil, and then the topsoil was replaced.
This isn't a practical farming technique but a way to isolate the impact of subsoil treatment.
Subsoil liming resulted in significant yield increases compared to topsoil liming or no liming, regardless of the crop or year.

Genotype Responses to Liming

Sensitive wheat genotypes benefit more from liming (whether topsoil or subsoil) than tolerant ones.
Liming assists tolerant varieties but not to the same extent as sensitive varieties.

Meriden Trial: Irrigation and Root Growth

An experiment involving different amounts of water was conducted to assess root growth and water access.
Wheat was irrigated in one go (wetting the entire profile) or in smaller portions (wetting only the upper profile).
The soil profile exhibited a decrease in pH and an increase in exchangeable aluminium with depth.

Soil Water Content and Root Distribution

Greater drying of the soil profile occurred in topsoil due to root uptake and evaporation.
Sensitive varieties did not extract much water from deeper layers, whereas tolerant varieties were more effective due to greater root growth.
Under dry conditions, sensitive varieties had few to no roots at 35 cm, while tolerant varieties maintained root growth.
With substantial irrigation, sensitive varieties could grow some roots, but tolerant varieties consistently performed better by extracting more water from deeper layers.

Conclusions

It's ideal to ameliorate both topsoil and subsoil acidity.
Liming is essential; using aluminium-tolerant genotypes alone is a short-term fix.
Continued soil acidification necessitates larger amounts of lime in the future, which is more costly.
Aluminium-tolerant genotypes in conjunction with liming provide the best solution.
Aluminium tolerance protects roots, allowing them to grow deeper and maintain the capacity to uptake nutrients and water.
Particularly in dry environments, roots need to seek for moisture in the lower soils.

Question and Answer

PAN refers to measuring evaporation of water in a pan. In the experiment, 0.3 or 0.6 of the pan level was used as the Irrigation level.
Aluminium toxicity affects root tips, leading to their death in sensitive genotypes. Tolerant genotypes maintain growth longer, leading to greater soil exploration. Shoots are protected over roots as an evolutionary advantage because food is provided for the whole plant from the shoots.