What kind of fertilizer should be used to improve the soil?

To truly improve the soil (making it looser, more fertile, less compacted, and better at retaining water and nutrients), just using chemical fertilizers is not enough; organic fertilizers must be used instead. I will explain the effects from strongest to weakest for you: 

The best and most versatile: Composted farmyard manure 

Manure: Chicken manure, sheep manure, cow manure, pig manure (must be well decomposed) 
Function: Loosens the soil, improves soil structure, increases organic matter content, and nourishes the roots all in one step. 
Suitable for: All crops, compacted soil, saline-alkali soil, poor soil 
Usage: As a base fertilizer, 2,000 to 4,000 pounds per acre. 

2. High value-for-money ratio: Compost fertilizer/Biological organic fertilizer 

Clean, odorless, and no need for fermentation. 
- Can improve soil compaction and enhance the utilization rate of fertilizers 
One acre requires 80 to 200 kilograms. 

3. Quick Soil Tilling Aid: Humic Acid/Kalium Humate 

Specialized in treating soil compaction, discoloration (turning red, white or hard) 
Sprinkled with water for irrigation, it will become soft within a few days. 
Used in combination with fertilizers, the efficacy is doubled. 

4. Fertilization and prevention of re-planting problems: Microbial fertilizer (microbial agent) 

Increase beneficial bacteria in the soil, reduce dead plants and root rot. 
Suitable for consecutive cropping areas, old greenhouse sites, and areas with frequent diseases 

5. Crop residue returning to the field / Composting 

- The most natural and the most cost-effective 
Can significantly increase organic matter and make the soil more loose 

Brief Summary (Most Practical) 

- Want to completely transform the soil → Use well-rotted farmyard manure 
- Want to loosen the soil quickly? → Use humic acid. 
- Want to be clean and efficient → Use commercial organic fertilizer + microbial fertilizer 
- Relying solely on chemical fertilizers → The soil will only become increasingly compacted and deteriorate further