Delphiniums like humus rich, fertile soil to grow in, and should be fertilised in spring and again 6 weeks later. Some yellowing of basal leaves is normal, they should be clipped off as they turn. Cut down completely at the end of summer and remove all dead leaves and plant debris. A mulch of garden or soil conditioning compost, or composted animal manure is beneficial - applied in autumn, also gives protection to the crowns. Wear gloves and protect your skin, the sap is very toxic after handling/cutting. You should also wash your clothing afterwards, if you have managed to get sap on it.

This comment interest me, Quote;Could it be lack of water?I also have clay soil.

Do you suspect they are lacking water?

I ask this because the symptoms you describe could also suggest they are too wet!

Add this to clay and your root system may be under stress.

As Bamboo says they want humus rich soil so I would suggest as this is happening each year you dig them up after flowering and enrich the area with plenty of organic matter then replant them.

Meanwhile for this year you could fork in a high nitrogen fertiliser such as Sulphate of Ammonia or Dried blood at about 1oz per sq yard (28gms / sm)

Note;the latter is faster acting

The reason I am suggesting a granular fertilser rather than a liquid one is; if the soil is saturated you could compound the problem by adding more liquid.

Then again if you think the soil is dry then by all means use a high nitrogen liquid fertiliser.

Very informative thanks you two,didn't know about the sap Bamboo I did put around quite abit of horse manure for the first time this year.......The reason I wondered about lack of water Teegee was because they are SOOO green and healthy looking this year and I thought it may be because we have had so much rain, and they are (usualy)in full sun all day and also in a mixed full boarder alongside the house wall, or maybe it's the manure??

Delphiniums do much better in partial shade - they do not like to dry out, so in full sun on clay, specially if you've not incorporated humus rich material on a yearly basis, they will need to be kept watered during dry spells.