There is no secret for a good soil mix that is universally good for all herbs. Diverse herbs require different types of soils. For example Mediterranean herbs like thyme and oregano require completely different soil requirements when compared to mint and lavender.
By the quality of soil, we mean the pH value, the ingredients and the texture of soil. You can feel the soil by examining the colour, the granularity and consistency of the soil.
Regarding soil types, peat mix and composts, you will find plenty of resources in library and internet. However; use your experience with the herbs while referring to the literature. Learning each herb’s needs is the key element in growing your herb garden successfully.
Marshy and wet earth is not good for herbs, avoid muddy soil that does not provide oxygen to the roots, the herbs need air and if they are left to rot in the wetness they may wither away. The absence of air in the soil will attract moss which is again not a great thing for growing herbs, you want to feed the nutrients to the herbs and not to the moss.
The dream of an herb gardener is to cultivate a soil that is enhanced and equipped to sustain herbs seasons after seasons. For this purpose, poor soil constituents deprived from essential nutrients will not give a good crop.
If you are growing herbs in containers, you really don’t need peat moss, natural compost with soil will work great. You should not overfeed manure and fertilizers to the soil, you can get outstanding results for the herbs that are grown in not too rich soil. Besides, adding grit and gravel does a good job for rosemary and oregano.
In fact, the Mediterranean herbs love this type of soil. Remember that Italian herbs do not like too wet soil, but chives and cilantro like to grow in moisture retentive soils. Mint spreads fast and grows well in a soil that can retain moisture too.
In general, herbs have a requirement for good draining and you need to make sure that there are no wet and soggy roots. Having said that, you also need to take care of the soil and make sure that the earth does not crack of scorching sun and heat.