by determining the inputs, you might second guess the basis of the microbes you are breeding here. This is compoost, so essentially this is Actinobacteria that you see visibly here. This is a Nitrgoen fixing microbe, a Nitrogen mineralisaing
what you are missing is there is no count reduction per say, merely a change of line up. think about American football, you wouldnt leave you defense on the pitch if you where rushing to touchdown. higher levels of salts change the biome, not destroy the biome. it is true that we might overdo salts and so reduce the numbers of plant beneficial microbes, and yes its true that decaying microbes would struggle in areas of high salt, hence the preservative properties, but count the halophiles and see whats up. Plants have also been around a while, they have also more than one trick to help them grow. indeed the very microbes which support them might also be responsible for the type of metagenomic activity necessary in a dynamic environment, it might just be this microbe line up change allows resonant plants to grow in otherwise less favorable locations but that this is some innovation of evolution.
Low K levels can lead plants to switch to Na to continue to function. is there a greater overhead for the plant, yes, but then life isnt always perfect right.
What is the cost for a plant and mycorrhiza in a low P environment? Do plants grown only with Mycos suffer yield losses compared to those grown in abundant P?
What about in acidic soils, what is the cost, We surely must appreciate that NH4+ is more abundant where acidic soil conditions prevail and so it would be NO3- we would struggle to achieve in the soil, but the plant has a strategy of Nitrogen reduction right, the overhead is higher but the plant would live.
if you put salt on ham and then treated the land with the same abandon, yes you would get issues, but a little salt never hurt anyone, including many of our microbes friends who because of the metagenomic ability to generate never before seen brand new enzymes and nucleotides, are far better at diet changes than we give them credit for.