A soil conductivity meter is a useful
tool in helping the grower decide whether or not to add fertiliser. A
conductivity meter will not tell the grower if nitrogen is low or high
it will only act as a guide towards the total salt concentration. An
example of a soil conductivity meter is given here. Hanna-Direct_Soil_Conductivity_Temp_Tester
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.
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.
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
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.