What amendments can be used to adjust soil acidity or alkalinity?

The correct choice focuses on compost or manure as effective amendments to adjust soil acidity or alkalinity. These organic materials can influence soil pH when incorporated into the soil. Compost, by enriching the soil with organic matter, contributes to microbial activity that can lead to changes in pH levels over time. Additionally, various types of compost can be produced from different feedstocks that may have specific pH levels, allowing for targeted adjustments to soil acidity or alkalinity when applied.

Manure also serves a similar function, often containing nutrients and organic materials that can change the soil's chemical properties, including its pH. The breakdown of organic matter in manure releases nutrients that are more readily available to plants, which can indirectly affect soil acidity and improve overall soil health.

Other options, while they serve important roles in soil management, do not primarily function to adjust pH. Fertilizers typically serve to provide necessary nutrients rather than directly altering soil acidity or alkalinity. Sand or gravel mainly aids in improving soil drainage and texture but does not have a significant impact on managing pH levels. Similarly, clay or silt can enhance soil structure and water retention, but they do not effectively lower or raise soil pH in comparison to compost or manure.