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

To truly improve the soil (make it loose, fertile, prevent it from becoming compacted, and retain water and nutrients), using only chemical fertilizers is not enough; organic fertilizers must be used. I'll explain to you clearly in order of effectiveness from strong to weak: 

The best and most versatile: well-rotted farmyard manure 

Manure: chicken manure, sheep manure, cow manure, pig manure (must be well decomposed) 
Function: Loosening soil, improving soil quality, increasing organic matter, and nourishing roots all in one step. 
Suitable for: all crops, compacted land, saline-alkali land, and poor soil. 
Usage: As base fertilizer, apply 2,000 to 4,000 pounds per mu. 

2. High cost performance: Organic fertilizer / Bio-organic fertilizer for agricultural use 

Clean, odorless, and no need for fermentation. 
It can improve soil compaction and increase the utilization rate of chemical fertilizers. 
80 to 200 kilograms per mu. 

3. Quick Soil Loosening Wonder: Humic Acid / Potassium Humate 

It is specifically designed to treat soil compaction, redness, whiteness and hardness. 
Apply it by dissolving in water and it will soften in just a few days. 
When used in combination with chemical fertilizers, the fertilizer efficiency doubles. 

4. Replenish beneficial microorganisms and prevent continuous cropping: Microbial fertilizer (microbial agent) 

Increase beneficial soil bacteria to reduce dead plants and root rot. 
Suitable for continuous cropping land, old greenhouse land, and land with many diseases. 

5. Returning straw to the field / Composting straw 

The most natural and the most cost-effective. 
It can significantly increase organic matter and make the soil loose.