Soil consists largely of mineral and organic matter, air, and water. Plants obtain nutrients from mineral and organic matter, oxygen from air, and they use water as a carrier of nutrients from the soil into the root and to the above ground portion of plants.
Since soil water functions as a carrier of nutrients from solid fractions of soil into and through plants, it plays a very important role in plant nutrition. Because of this importance, correct chemical balance of the soil solution is necessary for best crop performance. This means that pH of the solution should be in the range 6.0 to 6.6 and the solution should not contain high concentrations of dissolved solid materials (salts).
Evangelou, V. P., "Effect of Fertilizer Salts on Crop Production" (1983). Soil Science News and Views. 164.