Which of the following is NOT a type of soil condition that can be improved?

Soil compaction is a condition that refers to the densification of soil particles, which can lead to reduced porosity and hinder root growth and water movement. Improving soil compaction typically involves mechanical methods, such as aeration or tilling, but also requires careful consideration of the soil type and moisture content to prevent further compaction.

The other conditions listed—soil structure, nutrient availability, and soil pH—are generally regarded as amendable through various agricultural practices. For instance, soil structure can be improved by adding organic matter, which enhances aggregation and porosity. Nutrient availability can be increased through fertilization and soil amendments capable of providing essential nutrients. Soil pH can also be adjusted with lime or sulfur applications, depending on whether the soil is too acidic or too alkaline.

In contrast, while soil compaction can sometimes be corrected, the term "improved" does not typically associate with soil compaction in the same way it does with the other options. Consequently, soil compaction stands out as the condition that is not commonly labeled as something that could be improved in a conventional sense.