High Salt Levels in Soil 
To understand why high salt levels in soils are detrimental to plants you first of all need to understand how osmosis in plants works; secondly physical damage from root scorch can occur.

Osmosis is a key function of plant growth see link. wkipedia -OSMOSIS

When salt levels are high in the growing media, plants have difficulty in taking in water and dissolved ions (plant food). When salt levels are very low plants starve, therefore it is very important to get the correct amount of nutrients in the soil.

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I use this meter and find a reading of 0.4 mS will give good growth for most plants including chrysanthemums. A reading above this figure can result in petals scorching because satisfactory osmosis cannot take place especially on hot days when water loss through transpiration is greatest.

Simple Osmosis
Osmosis is directly related to transpiration. The more water lost through transpiration the greater the need for unhindered osmosis.

In simple terms water cannot pass freely through a semi-permeable membrane (cell wall) from a stronger solution to a weaker one. It must be weaker in the soil and stronger in the plant to get a free movement of water into the plant. The hotter the weather the more demand for water as the plant transpires. That's why petal scorch is more likely to occur in hot weather.

However long before the plants have petals you might notice your plants are not growing properly. The temptation would be to increase feeding. This would only make things worse. In extreme circumstances leaf scorch can occur if the salt levels are very high.

Soil Test
A soil test in the spring with a conductivity reading will be helpful. This will tell you which nutrients are deficient if any. If the soil conductivity is high you will be advised to flush the soil with water. It goes without saying that glasshouse soils are more problematic regarding high salts levels than soils outdoors because of the fact that they do not get rain over the winter months.

High salt levels are a result of over fertilisation with fertiliser or manures.

As an example, greenhouse soil with no organic matter added may read 0.6mS. If you wheel barrow that soil outside and expose it to winter rain I would expect it to drop to 0.1mS or thereabouts. On the other hand if soil from the garden that has had copious amounts of organic matter added in the autumn was wheeled into the greenhouse in its place, in say January. This soil might read 0.1mS at that time but by May when it has warmed up and soil bacteria start to break it down it might read 0.6 to 0.9mS or thereabouts. In other words don't underestimate the fact that organic matter besides improving the soil structure will contribute towards the nutrient levels in the soil over many months. Only a soil conductivity meter can alert you to how high this might be. 

Apply dry fertiliser evenly
One other point I would like to make, I use dry fertiliser on my pots of late flowering chrysanthemums, and I water it into the compost. Or the rain washes it into the compost. It is important to apply this fertiliser evenly over the surface of the compost. This is because the roots take up ions along with water. If there is a very strong concentration of ions in one part of the pot the roots on that side of the plant will be hampered from taking up water, the xylem tissue from those roots go directly up the stem to the leaves and petals on that part of the stem. You may find petal scorch in just one segment of a bloom. Likewise iron deficiency symptoms on half the plant, perhaps due to poor roots on one side or high pH in one spot.