Orchid’s root has lower efficiency of absorption compare to the other variety plants due to the Tilosome at the exodermis layer of root. It is a barrier to move the nutrient between exodermis and cortex layer. But CAM orchids have a high efficience absorption at leaf while stomata are opening. So, the fertilizer should contain Urea, it will easily be absorbed to the Cutin or wax layer of leaf.
The nutrient absorption of orchid’s root depends on the amount of carbohydrate in leaves. And The absorption of Nitrate will increase with the amount of carbohydrate in root. When the orchids don’t have enough light or the temperature is high, you can add some sugar in fertilizer to increase the carbohydrate in plant.
When you mix crystal fertilizer, you should add wetting agent or spreader but not Calcium (The solution will precipitate). For small and fast grow orchid which have good respond with Nitrogen such as Dendrobium or Cattleya. The recommend fertilizer grade is balanced grade (21-21-21 grade) and alternate with high N fertilizer grade (30-20-10 or 30-10-20 grade). When the weather is dry, use high N fertilizer grade for 2 times (1 time/week) and alternate with balanced grade for 1 time. In humid period, use high N fertilizer grade for 1 time alternate with balanced grade for 2 times or use 28-14-35 grade for all over the year.
For small and slow grow orchid which is easily to get rot such as Vanda and Rhy.Gigantea, when the weather is dry, use balanced grade for 2 times and alternate with high K fertilizer grade (16-21-27 or 13-27-27 grade) for 1 time. In humid period, use balanced grade for 1 time and alternate with high K fertilizer grade for 2 times.
For medium to mature size plant, you should use balanced grade alternate with high P fertilizer grade (15-30-15 or 10-52-17 grade). In dry period, use balanced grade for 3 weeks and alternate with high P fertilizer grade for 1 week. In humid period, use balanced grade for 2 weeks and alternate with high P fertilizer grade for 2 weeks or 28-14-35 grade for all over the year.
And for the mature plant that is easily to get rot. You should use balanced grade for 2 weeks and alternate with high K fertilizer grade (16-21-27 or 13-27-27 grade) for 2 weeks in dry period and for humid period, use balanced grade for 2 weeks and alternate with high K fertilizer grade for 3 weeks. (TIP: high Nitrogen amount can affect the size of stem and length of spike).
When water is salty (high conductivity value), you should divide the dose of fertilizer to 2-3 times/week instead of one. And when the weather is suddenly change from hot/dry to heavy rain. You should spray 0-52-34 (non-Nitrogen fertilizer grade), use it 3-4 weeks. It will prevent the plant from getting rot.
For the concentration of fertilizer in solution, it depends on the size, variety, temperature, frequency of use and humidity. The same variety but different in size. The absorption is also different. For the example, a small orchid has a better absorption of Ammonium than Nitrate, Phalaenopsis orchid in sphagnums has a higher growth rate when you spray fertilizer every 2 days instead of every 7 days. So, you should follow the labels at least 1 months and observe the change then adjusting the amount.
Moreover, there is a report that said when you water the orchids, it make them less absorb the fertilizer. So, you should reduce the water before fertilizing
REF: ว่าด้วย กล้วยไม้: ศาสตร์และศิลป์ เพื่อการพัฒนา