People often tend to look at Paphs as a homogeneous group of plants, but the fact of the matter is they come from a vast range of habitats, and it is difficult to make generalities as to culture. In the case of Paphs, many grow on or in close proximity to calcareous rock, but at least an equal number do not, and many of these would actually do poorly with a higher pH and calcium/magnesium supplement, so it is important first to know which Paphs tend to have a calcareous substrate in nature. How strongly calcicolous a Paph may be can be inferred to a certain degree from the nature of its relationship with the calcareous rock. Some may be found with their roots in direct contact. Others may grow in accumulated detritus or soils derived from the erosion of the rock, and others, while growing in leaf litter or humus, may still be greatly influenced if they are growing in cracks or crevasses in the calcareous rock, especially those growing below the summit on steep slopes and cliffs, with the water washing down over the stone and into the litter or humus. Plants growing in rapidly accumulating leaf litter on a level forest floor may not be significantly affected by an underlayment of limestone far below. Others are only occasionally associated with limestone (such as Paph. lowii, which is typically epiphytic), and in cultere we do not treat these as calcicolous.We have not determined whether the requirements are for increased calcium and magnesium, or simply the higher pH that results, or all of these factors, but do believe from experience that for the strongly calcicolous species dolomitic limestone supplementation is needed for the long term health of the plant. A few species are also associated with serpentine, which is a basic silicate of magnesium, iron, aluminum, nickel, zinc, and manganese, and a more basic pH is also a consideration for these species.

Secondly, whether to supplement or not depends on the nature of your water and fertilizer. If you already are dealing with high pH hard water, further supplementation would probably be counterproductive. The nature of your potting medium also comes into play. We have always preferred at least a part of the mix to be fir bark for the mixed Paph collection, providing a somewhat acidic base. We feel the ideal situation for the mixed Paph collection is to water with a pure water source supplemented with balanced fertilizer program containing about 40 ppm calcium and 20 to 30 ppm magnesium and a pH in the 6.2 to 6.6 range. This will satisfy the needs of the noncalcicolous Paphs (note there are a few species that will need to have even lower calcium levels, but these are less common). For the calcareous species you would then add, preferably as a top dress to the pot, either crushed oyster shell, pelletized Dolomitic limestone, or Dolomitic limestone chunks. Finer grades of Dolomitic lime or micronized dolomitic limestone are useful for quick corrections, but must be reapplied frequently if not followed by an appropriate supplement. We also prefer to top dress rather than incorporate it into the mix so that we can see when the supply has been depleted and reapply.

You will often hear conflicting views on addition of calcareous materials, but if you explore them they are usually formed based on improper or inappropriate use. On one internet forum recently one person posted that her Paph. delenatii did poorly until she added limestone, and another posted that he saw a bunch of delenatii plants that were covered in limestone and doing very poorly at a nursery. Well, even though a parvisepalum, Paph. delenatii is not a calcareous associated Paph. The first person who saw improvement may have been watering with extremely acid irrigation water, or very low calcium or magnesium levels, and in these cases of poor culture it may have given a boost. In the second case, I’ll bet the plants were in poor shape before the lime was added, and it is very unlikely that the lime helped them, except to a faster demise.

One note on making bark mixes slightly alkaline, they will tend to break down quite a bit faster.