Adeniums are heavy feeders. Most seedlings and cuttings do most of their growth in the first two years than all the rest of their life as a specimen combined. Seedling and newly rooted cuttings do best given a high nitrogen base fertilizer. This fertilizer should be a balanced or one with a slightly higher (nitrogen) first number in the three number series. Young plants benefit from a constant feed source, such as that from Osmocote, Nutricote and or a supplemental liquid fertilizer (i.e. Peter's Soluble Plant Food). Do not use Miracle Gro! Feeding should begin immediately once a newly planted plant has become established. Establishment can take the form of new growth or thicker stem development. If in doubt, liquid feed initially at half to quarter strength every other week. Saturate the soil entirely evenly around the plant. Alternate a good watering, of plain tap water, in between fertilizing to leach excess salts and nutrient toxicities.
An excellent timed release fertilizer is Osmocote. This fertilizer incorporates a osmotic resin to transfer nutrients when water and higher temperatures are optimal. This fertilizer is best used for plants growing as seedlings or rooted cutting starts. An even nutrient ratio is good for all growing plants. Avoid the bloom specials as they tend to accumulate pots of the water filled plastic balls over time. These balls tend to make the soil too wet and may lead to root rot and soil fungus activity. But in transplanting, they tend to just fall out and is not too harmful in small quantities. Nutricote is another timed release fertilizer and has an advantage of long-term release. But some types may be too long for our current needs. Use your discretion when selecting a fertilizer. Base it on how long you estimate having the plant in that size container and how often will you transplant it before setting it in its final specimen container.
Established plants can benefit with a frequent high phosphorus (higher middle number) liquid feed once per month. Peter's Blossom Booster or Variegated Violet Special will do the trick on reluctant blooming plants. Peter's should be used at half to full strength at about one to two week intervals during warm, sunny periods. The phosphorus will also increase stem thickening and increase caudex formation on mature plants. High light intensities plus a high phosphorus fertilizer source will definitely increase caudex formation if your plants seem lacking in their potential. This fertilizer source along with a good draining media and a confined pot all help in developing a large caudex. Even cutting-grown starts soon develop a large root caudex (see examples under Growing Techniques). MagAmp (7-40-6) in the coarse grade is also a good fertilizer. This fertilizer should only be added to matured established plants when transplanting it to its final specimen pot. MagAmp tends to thicken stem and roots and provide a good reservoir of phosphorus for long term needs. The Magnesium, hence "Mag"-Amp, also included in this fertilizer will add a deeper green coloration to leaves as this is absorbed initially. The phosphorus tends to break down very slowly and may take years if at all. It is best used at the root level nearest feeder roots for best results.
Again, performance will vary. Use a fertilizer that will be low in saturated salts and allow even distribution of nutrients. Fertilizers also vary in their performance based on hard water, dissolved minerals already present in your local water, flourine addition, temperature of your water, etc.... If you feel that you are not getting results with your fertilizer, please discontinue use and start with a new one. The best advice here is trial and error. What works well for one does not necessarily work well for someone else. Again, use your discretion.