Obviously you can improve your left hand technique by playing special exercises on the piano or the organ. A great resource for the left hand technique is Piano Music for One Hand: A Collection of Studies, Exercises and Pieces
It is important to understand that both hands should be developed at a similar level. My favorite collection of daily exercises which develops both hands equally is Hanon: The Virtuoso Pianist. This collection contains 60 exercises in three parts. The entire set takes about one hour to play in a concert tempo.
In the beginning, it may be enough for you to play just the first part – the basic 20 exercises which last about 20 minutes. Take one new exercise every week and master it. Practice it daily several times. With the following week practice 2 exercises without stopping, later 3, 4, and so on until after about 20 weeks you will know all 20 exercises. After that it will be sufficient to play the entire set of 20 exercises just once.
After they become easy, start adding more advanced exercises from the Part 2. I think you get the idea. When you reach the end of Part 2, your left hand technique will already be at the decent level. If you go even further and play the Part 3 (60 exercises total), you will achieve exactly what the title of this collection promises – the level of the virtuoso.
I have to admit that these exercises are a bit mechanical and it takes some willpower even to play through the Part 1, but the results are really great. It is a great way to develop your technique by only playing one hour a day. I can testify personally that whenever I have not enough time to practice my organ music, if only I continue to play through the set of Hanon exercises daily, my manual technique is in good shape.
I have to add that most of the time I practice them on my organ and not on the piano. Because organ mechanics might be lighter than the piano (at least on a small instrument), it is possible to add a coupler for more resistance. A great thing about practicing Hanon exercises on the organ is that you can turn the power off and play on the silent keyboard. That way you won’t annoy your family but the results will be the same.
A word of caution – always pay attention to how you are feeling. Some fatigue is OK but if you start to feel tension in your hands, you should stop at once and take some time to relax your hands and shake off the tension. This is really important if you want to reap the best results and not to hurt yourself.
Take a slow tempo at first (60, 50, even 40 beats per minute). Later you naturally start to play a little faster and faster until you reach 120 beats per minute.
If you continue practicing the exercises from Hanon: The Virtuoso Pianist regularly, I can guarantee that you will see the definite changes in your left hand technique already after a few weeks. Real pianists spend playing etudes and other piano music several hours a day just to maintaining their keyboard technique. With the Hanon exercises you will only need about one hour a day because of the diversity of the exercises. Every imaginable classic piano technique and figure is included in the set, including tremolos, octaves, and scales in double thirds and sixths.
You may be thinking that theses exercises are good only for pianists. However, just think how much the legato Romantic organ technique was influenced by the piano technique in the French symphonic organ school (Widor and Vierne, for instance) and you will have no doubt about the benefits of the Hanon exercises for the organists.
The good thing about this collection is that at the end of the second part there is a set of exercises devoted to scales and arpeggios. So people who are fond of scales and arpeggios will find benefit from this collection as well. By the way, I play scales and arpeggios on two manuals because compass of the organ manuals is a little too short for some exercises. I recommend playing the exercises from Hanon: The Virtuoso Pianist before your regular organ practice because it also serves as a fantastic warm-up.
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