There are a couple different things I can think of, that may or may not get at exactly you want. I assume that this will just be field observations, and not controlled experiments?

If there is an isotopic signature difference between the plant-based and mineral nutrients (for some nutrients plants create a small isotopic difference because of preferred uptake) then that could be used to partition the nutrient sources. Alternatively you could isotopically label organic matter and use that to trace the contribution. However that would be very tough in field settings due to what I assume would be high water flow through the rice system.

Alternatively you could perform decomposition experiments with the organic matter, and assume that the nutrients lost from your decomposition are added into the system for plant uptake, and therefore any additional uptake must either come from mineral weathering or outside nutrients. In a rice system I would think that the water flow would add a lot of P and K, and that there would also be some atmospheric deposition added the the system as well. I'd be happy to discuss with you more and I can talk to some colleagues who would know this answer better than me.

A rough estimation can be obtained by the contents of P and K in aerial and root biomasses of rice grown without fertilizer.

@ Noa Lincoln.

Dear Noa Lincoln thank you for showing interest in discussion and sorry for being a bit late on my part.

Actually i want to calculate the fertilizer dose for a rice variety based on partial ( less than full) maintenance of nutrient input-output balances. In this approach only a part of net loss of nutrients from the field during a season is made up through fertilizer applications. This provides us an opportunity to consider the supply of nutrients from soil reserves including contributions from biological processes. What in actually need to quantify are the limits of allowable drawdown of soil nutrient reserves(Ks & Ps) which as per literature could depend on processes like soil biological activity and soil characteristics like minerology. Below are given the equations

FK with limited K depletion= GY × RIEK - KCR – KW - KOM + KL+Ks

FP with limited P depletion= GY × RIEP - PCR – POM – Ps

where

FP / FK : Fertilizer P & K requirement to achieve target yield

GY : Target Grain yield

RIEP /RIEK : Reciprocal internal efficiencies of P and K respectively

PCR /KCR : K and P inputs with retained residues

KW : K input through irrigation water

POM/KOM : P and K input with organic matter

KL : K output through leaching

Ps/Ks : Threshold limit of P and K respectively.

May you provide your suggestion and suggestion of your friends please.