The best strategy to fertilize Haworthias is to supply a continual low concentration in every watering. By linking nutrient supply with water supply, a plant always gets adequate nutrients in proportion to the water uptake, regardless of its size and growth period. In addition, some micronutrients (e.g. Calcium and Silicon) are not mobile in the plant, so must be in the nutrient stream at all times to be effective.
Synthetic fertilizers are better choices than organic fertilizers. Most synthetic fertilizers are soluble in water and available for immediate uptake. This allows the nutrient supply to be accurately controlled. In contrast, organic fertilizers need to be decomposed before being usable, but the decomposition process is affected by many environmental factors (moisture, pH, temperature etc.) and can hence be unstable.
Besides the basic nitrogen, phosphorus and phosphorus (NPK), a plant also needs various micronutrient for optimal health. All plants need nutrients in roughly the same ratio. An unbalanced nutrient availability can lead to deficiency and even toxicity problems. Giving Nitrogen the value of 100, the ideal supplement ratio of essential elements are listed below as a weight percentage of N.
|Element||Ideal Range||Optimal Value|
Several fertilizers may be needed to complete the above list. As the base, use a NPK fertilizer with a ratio 3:1:2 (e.g. fertilizers with NPK rating 24-8-16, 12-4-8, 9-3-6 ratings), because its actual NPK contents are the closest to the above 100:16:62 ratio (how to calculate). Avoid fertilizers that contain urea, because the conversion of urea to nitrogen usable by plants are less efficient under low pH, which is preferred by Haworthias.
If tap water or well water is used, check the water quality report from the supplier or get the water analyzed in a laboratory. The water may already have some elements (often Ca and Mg) well represented.
Add Silicon to the Fertilizer Program
Silicon is not considered as an essential element, but experiments have shown that it improves plant growth, increases the tolerance to temperature extremes and drought, and also protects plants against pathogens and insects by making their cell walls stronger.
Using N as 100, the ideal ratio of Silicon (Si) is ~86.
For Haworthias, the concentration of nitrogen in the water solution should be ~0.003%, and the concentration of other nutrients should follow the ratios in the above table. Note that a fertilizer should be fully diluted in water before adding another, otherwise there can be undesired chemical reactions among concentrated fertilizers.
The acidity of the nutrient solution is a major determinant of nutrient uptake. If the pH goes outside the ideal range of between pH 5.0 and 5.5 (5.2 optimal), nutrient balance can break and lead to problems. Tap water and well water are most likely alkaline, and fertilizers are often not pH neutral as well. After diluting all fertilizers in water, use a pH meter to test the solution and add a pH adjustor until the pH falls in the ideal range. Hydrochloric acid is the best acidifier for lowering pH, as other acidifiers often result in chemical by-products which negatively affect the plant growth.