In agriculture, lime (such as calcium oxide or calcium carbonate) is sometimes applied to soil mainly in order to achieve which effect?

Introduction / Context:
Soil chemistry plays a crucial role in agriculture because it affects nutrient availability and plant growth. Farmers sometimes add lime, usually in the form of calcium oxide or calcium carbonate, to their fields. This practice is known as liming. Understanding why lime is added helps connect basic acid base chemistry with practical farming methods. This question asks for the main reason lime is applied to soil, focusing on whether it increases acidity, improves porosity, restores nitrates or changes alkalinity.

Given Data / Assumptions:

• Lime refers here to calcium containing materials such as quicklime (calcium oxide) or limestone (calcium carbonate) used in agriculture.
• Many soils become acidic over time due to factors such as acid rain, fertilisers and leaching of basic ions.
• Plant nutrient availability and microbial activity are affected by soil pH.
• The options describe possible effects of adding lime: changing acidity, porosity, nitrates or nutrients like phosphate.

Concept / Approach:
Calcium oxide and calcium carbonate are basic substances. When added to acidic soil, they react with excess hydrogen ions and acidic components, thereby raising the pH and making the soil less acidic and more alkaline. This process neutralises soil acidity and makes many nutrients more available to plants. Although liming can influence soil structure and indirectly affect porosity and nutrient cycling, its primary chemical purpose is to correct soil acidity. It does not directly restore nitrate content, nor is it mainly used to supply phosphate. Therefore, the best description is that lime increases soil alkalinity and neutralises excess acidity.

Step-by-Step Solution:
Step 1: Recognise that lime used in agriculture is often calcium carbonate or calcium oxide, both of which are basic in nature. Step 2: Acidic soils have a low pH and contain excess hydrogen ions and acidic compounds that can limit plant growth and nutrient availability. Step 3: When lime is added to acidic soil, it reacts with these hydrogen ions, forming water and neutral salts, thereby raising the soil pH. Step 4: Increasing soil pH from strongly acidic to a more neutral or slightly alkaline range improves conditions for many crops and beneficial soil organisms. Step 5: The main purpose is not simply to make soil more porous; while structure can improve, this is a secondary effect. Step 6: Lime does not directly add nitrate ions to the soil; nitrates come from nitrogen fertilisers and microbial activity. Step 7: Lime also does not significantly supply phosphate; phosphate fertilisers are used when additional phosphorus is needed. Step 8: Therefore, the key effect of liming is to increase soil alkalinity and neutralise excess acidity.

Verification / Alternative check:
Agricultural extension manuals and soil science textbooks explicitly state that the primary purpose of liming acidic soils is to raise soil pH and neutralise acidity. Recommended lime application rates are often based on soil pH measurements and buffer capacity. Farmers observe improvements in crop yield when excessively acidic soils are corrected with lime. While some secondary benefits such as improved soil structure can occur, these are not the main reason for applying lime. The consistent emphasis on pH correction across multiple sources confirms that increasing alkalinity and neutralising acidity is the correct answer.

Why Other Options Are Wrong:
Option A, increasing soil acidity, is the opposite of what lime does; lime is basic and reduces acidity. Option B, making the soil more porous only, ignores the primary role of pH correction; any porosity change is secondary. Option C, restoring nitrates, is incorrect because lime does not supply nitrogen; nitrates are provided by nitrogen fertilisers or biological nitrogen fixation. Option E, supplying phosphate nutrients, is wrong because lime is not a phosphate fertiliser; compounds like superphosphate or diammonium phosphate are used for phosphorus. Only option D correctly describes the main purpose of liming as increasing alkalinity and neutralising excess soil acidity.

Common Pitfalls:
Students sometimes see lime mentioned in both construction and agriculture and may confuse its various uses, such as in mortar, with its role in soil. Another pitfall is to assume that any soil amendment automatically adds nutrients like nitrogen or phosphorus. To avoid these mistakes, remember that lime is primarily an acid neutralising agent. Its main job in soil is to adjust pH, not to act as a direct nutrient fertiliser. Associating the word lime with basicity and pH correction helps you correctly answer similar questions about soil management and agriculture.

Final Answer:
Lime is applied to soil mainly to Increase the alkalinity of the soil and neutralise excess acidity.