Question

The following are chemical properties of two Venezuelan surface soils. Predict their relative sensitivity to acidic inputs from rain or fertilizer and give reasons for your prediction. | | pH | OC | N | Clay | Ca | Mg | K | Na | Al | CEC//cmolkg^(-1) | BS//S | | | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :---: | | | | | | | | | | | | | | | | Machiques Barinas | 6.0 | 0.75 | 0.08 | 7.2 | 0.3 | 1.4 | 0 | 0.01 | 0.1 | 3.7 | 44 | |

Short answer

Machiques soil is relatively sensitive to acidic inputs due to low CEC and moderate base saturation.

Step-by-step solution

Evaluate Initial Soil pH

Examine the soil pH values. Machiques has a pH of 6.0, which is a mildly acidic to neutral pH level. This suggests a medium level of buffering capacity and a moderate sensitivity to acidic inputs.

Consider the Organic Carbon Content

Organic Carbon (OC) contributes to soil's buffering capacity. Machiques has an OC of 0.75. Higher OC would generally improve soil buffering due to higher cation exchange capacity (CEC).

Examine the Clay Content

Clay content affects soil structure and its ability to retain nutrients, impacting buffering capacity. With 7.2% clay, Machiques is likely to have a lower buffering capacity, making it more sensitive to acidification.

Review Base Cation Levels

Inspect base cations like Calcium (Ca), Magnesium (Mg), Potassium (K), and Sodium (Na) that contribute to soil buffering. Machiques has 0.3 Ca, 1.4 Mg, and very low K and Na, indicating limited buffering against acidification.

Analyze Aluminum Content

Aluminum (Al) becomes more soluble as pH decreases, potentially increasing soil acidity toxicity. Machiques has an Al content of 0.1, indicating some risk if acidification occurs.

Evaluate Cation Exchange Capacity (CEC)

CEC represents the soil's ability to hold onto essential cations and buffer against acidic inputs. Machiques has a CEC of 3.7 cmol/kg, a low value, indicating a higher sensitivity to acid inputs.

Calculate the Base Saturation (BS)

Base Saturation reflects the proportion of CEC occupied by base cations. Machiques shows a 44% BS, suggesting moderate vulnerability to cation leaching that will increase acidity.

Predict Sensitivity to Acid Inputs

Considering the pH, low OC, moderate clay content, low CEC, and moderate BS, Machiques is relatively sensitive to acidic inputs, as it may not buffer well against additional acidification from rain or fertilizers.

Key concepts

Chemical Properties of Soil

Soil is not just dirt – it's a dynamic and complex system with diverse chemical properties. These properties help define how a soil will react to environmental changes and additions like fertilizers. Important chemical properties include:

• Organic Carbon (OC): Key for soil fertility, high OC improves soil structure and water retention.
• Clay Content: Affects how soil holds onto nutrients and water, influencing its ability to resist pH changes.
• Base Cations: Elements like calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) that are vital for balancing soil acidity.
• Aluminum (Al) Levels: High aluminum levels can be toxic, particularly when soil becomes more acidic.

These components interconnect to determine how soils will interact with acidic inputs, affecting their sensitivity to changes.

Soil Buffering Capacity

The buffering capacity of soil is its ability to resist changes in pH when acids or bases are added. A higher buffering capacity means the soil can absorb more acidity without significant changes to its pH. This property is crucial in areas with acid rain or heavy fertilizer use.

Buffering capacity is affected by several soil properties:

• Organic Matter: Rich in organic matter, soils can better buffer against pH changes due to a higher Cation Exchange Capacity (CEC).
• Clay Content: Higher clay content generally increases buffering capacity, though Machiques soil is an exception with just 7.2% clay.
• Base Cation Availability: These nutrients help neutralize added acid, supporting the soil's ability to stabilize pH.

Although Machiques has a moderate pH of 6, its limited organic content and low clay and base cation levels imply a lower buffering capacity.

Cation Exchange Capacity

Cation Exchange Capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces. Essentially, it's the soil's ability to hold onto essential nutrients. A high CEC indicates robust nutrient availability, crucial for plant growth and soil health.

Factors impacting CEC include:

• Clay and Organic Matter: More clay and organic matter generally increase CEC because they have negative charges to attract cations.
• Soil Texture: Coarse soils (sandy) have lower CEC, while finer textures (clay) have higher CEC.

With a CEC of only 3.7 cmol/kg, Machiques soil struggles to hold onto base cations, making it more prone to nutrient leaching and acidification. This low CEC makes the soil sensitive to acidic inputs, evidenced by poor nutrient retention and altered pH balance.

Base Saturation

Base Saturation (BS) is the percentage of a soil's CEC that is occupied by base cations, like calcium and magnesium, which help neutralize acids. Having a high base saturation means the soil can better buffer added acids, reducing sensitivity to acidification.

A soil with low base saturation is more susceptible to becoming acidic as cations are leached away. Machiques soil has a 44% base saturation, indicating moderate vulnerability to cation loss. This means that over time, the soil's ability to buffer acid inputs decreases, potentially increasing acidity. Low base saturation is often a limiting factor in maintaining soil pH stability.

Soil pH Analysis

Understanding soil pH is critical for assessing soil health and its ability to support plant life. Soil pH reflects the acidity or alkalinity of soil, usually on a scale from 0 to 14, with 7 being neutral. Most plants prefer a pH between 6.0 and 7.5.

The pH level affects:

• Nutrient Availability: Certain nutrients become inaccessible to plants if the pH is too low or too high.
• Microbial Activity: Soil organisms that decompose organic matter and assist in nutrient cycling are sensitive to pH.
• Soil Structure: Extreme pH levels can lead to poor soil structure and reduced plant growth.

Machiques soil has a pH of 6.0, placing it on the mildly acidic side. Although this pH provides a somewhat balanced environment, the low buffering capacity suggests the soil's pH may become more acidic with continual acidic input, affecting plant health and soil sustainability.