How do acids help 'solubilise' compounds?
There are multiple ways in which plant root and root hair cells increase the availability of nutrients and positively affect the plant’s ability to absorb nutrients from the soil.  

One of these ways is by excretion of $\ce{H+}$ ions from malaic acid. This promotes a cation exchange process that results in cationic nutrients being brought into the cell in order to maintain charge balance.  This is the process to which you are referring in your question.

Additionally, enzymes, for example phosphotases, can convert chemically unavailable nutrients, like organic phosphorus, into bioavailable inorganic phosphates.  Phosphate uptake is a case in which acidic soils actually inhibit the availability of this nutrient to plants. (Annals of Botany, 112: 317 – 330, 2013)

There are also proteins excreted by root hairs that can complex metal ion nutrients like iron, facilitating the uptake of such nutrients.