TDS on a soil test is always total dissolved solids :-) And salts only build up with frequent or excessive fertilization and inadequate watering. Otherwise they are taken up by the plants or dissolved or leached into the soil.

Salts build up is most often encountered in container gardening or in very arid climates with minimal irrigation.

I mean it as solids, but doesn't that include salts?

I think that salts are only solids when they precipitate out. When dissolved in water, they are not solid. But I beg to be corrected. I'm over my head. Chemistry was 40+ years ago.

You are correct Sara.

This all depends on what 'salinity' means in your particular test. If it means 'sodium' then you simply have no sodium hanging around which is fine. But all the other minerals - carbonate, sulfate, chloride, nitrate - all contribute to TDS too.

The reading of 7.8 sounds more like a pH reading than alkalinity. Alkalinity is the total capacity to neutralize acid and is calculated by summing individual values for carbonate, bicarbonate, sulfate etc., giving each ion a weighting depending on whether it can combine with one hydrogen ion (bicarbonate) or two (carbonate). Most tap water is in the range of 100 mg/L (ppm) for total alkalinity. My public drinking water supply is 121 mg/L alkalinity, 29 mg/L sodium, pH 8.45 and TDS 293 mg/L.

Alkalinity and pH are interrelated but not the same thing.

https://ag.umass.edu/greenhouse-floriculture/fact-sheets/water-quality-ph-alkalinity

What happens to dissolved salts? The ones not deposited in soil or taken up by plants or held in solution in the soil's water content (called the pore water or soil solution), are carried downwards into the ground. Note, any water you pull up out of a well will have minerals dissolved in it - in other words, TDS. The mix will change on the way down as the water interacts with materials it's leaching through.

Probably more than you wanted to know...

Thanks, and yes, I should have said pH. I wrongly assume pH scale is representative of the amount of alkalinity.

... and no it isn't more than I wanted to know. It is much appreciated.

thx yes that was interesting.

Some soils retain salts better than others. A sandy soil would retain less.

With water having TDS of 435, I suspect the "zero salinity" is either relative to some known standard and there actually are some salts in there, or "salt" is more narrowly defined as sodium, or other group I elements. (potassium, namely) I'd always assumed salt in the context of soil meant any free Group I or II ions. Which would include calcium and magnesium.

Only the lab can tell you for sure what they meant.