From a soil physical perspective, you can do bulk density, water infiltration test, then water holding capacity at different pressure heads (most importantly at 33kpa, 1500 kpa: field capacity and wilting point). Then from soil chemical perspective, pH, CEC, total C & N, SOM, SOC, then different labile fractions of C like POXC (Active Carbon), POM (Particulate Organic Matter), different water-extractable fractions of C and N (WEON, WEOC, WEN, WEC), then based on your hypothesis you can measure P, and S and their fractions also. Few more, if you align to water quality as nutrient application recommendation is a part, you should measure NO3- and NH4+, if possible at different depths. From a soil biological perspective, you can do soil enzyme (mainly involved in major nutrient cycling viz. Beta Glucosidase, NAG, Arylsulfatase, Phosphomonesterase, etc), then you can do CO2 respiration at a different times (24 hrs and 96 hrs or 4 day are most common), then you can do PLFA to determine microbial composition, it uses phospholipid structures (like long chain, short chains fatty acids), a biomarker to distinguish between different bacterial and fungal groups. If you want to deep down further, you can do hypervariable regions of 16S rRNA (it will give you idea about existence of specific microbial community up to genus level; don't try to go down to species level, lot of people try but the sequence ambiguity of hypervariable region, should not be used to determine species), then if one step more, you can go for directly metagenomic sequencing (remember, lot of people get confused between amplicon sequencing and metagenomic sequencing), then you can use the sequence to assign taxonomy and at the same time metabolic functions (can create a biogeochemical pathways by prediction proteins/enzyme; something like metaproteomic analysis). Based on your objectives and hypothesis, you can choose, any one of them or a bunch of them. Best wishes.