(status. last rev. October 2019, believed finalized)
all stuff about NPK and friends ... Warning! active brain cells needed for stuff ahead!!
Fertilizing and Fertilizers for succulent plants.
Most succulents needs very little fertilizer and Lithops need virtually none. If one wants to fertilize Lithops to make them grow within acceptable bonds then use a fertilizer that has a very low % in Nitrogen.
An explicative note, Fertilizing and Fertilizers in a simplified nutshell. Here is all the techies stuff!
Fertilizers are defined by their N P K composition!
Those are the chemical symbols for Nitrogen, Phosphorus and Potassium(Kalium) which are the main nutrients for plants. Sometimes also a figure for Mg = Magnesium is given but not always (see the Epsom salts myth in gardening). Some of the better ferts also carry the mention "with micro elements". That means that some essential trace elements have been added to the fertiliser to make it a near perfect, full blown nutrient. (see Micro-Macro elements in google, and this Wiki entry Plant_nutrition).All commercial nutrient compositions are listing numbers for their NPK content, for example something like 5-7-6 or so. This means that the solution or powder you buy has %-ages of 5%, 7% and 6% of each compound. The inter ratio of the elements is an important factor for good growth.A good fertilizer for succulents however should be something like NPK(Mg) 6-12-36-3, that is very low in N (Nitrogen) but reasonably high in the other components. Such fertilizers are a bit difficult to find, but many combinations can be found on the market.
Most users just use "a splash" or "a pinch" of material dissolved in "some water" to use as a fertilizer claiming a 1/4 dilution from the recommended quantity. This will undoubtedly work but one has no refined control over the "how much is a splash".
 |
| A well documented fertilizer label |
It might, however, be much more rewarding to follow a really calculated and thus more standard procedure for fertilizing. It pays off because one has an ever lasting method for comparing!
So for those interested, read on ... As said the figures are % of compound in the "raw" fertilizer (liquid or solid). It is however much easier to express final concentrations in "ppm" aka parts per million. (Apology to the non metric users, google metric conversions is your friend).
Basically an amount of 1 milligram/milliliter per Liter of solvent (water) is a 1 ppm concentration. The simple concept is to take Nitrogen as a reference for calculations because it is a single element with no corrections needed so the calculation is just straightforward.
Leaving out all the "in between" maths one gets this basic rule: for a solution of 1 gram/or milliliter of fertilizer diluted into 1 Liter of water, one gets the associated number for N x 10 in ppm.
Example: suppose a 10-10-10 NPK fert. At the given concentration of 1gr/Liter the final solution is 10(%N) x 10 = 100 ppm in Nitrogen.
Does this calculation apply to the other elements as well? yes, but there is a catch. Concentrations of the other elements is not expressed as % of the pure element but as % of an analytical derivative, that is P2O5 for Phosphorus and K2O for Potassium and MgO for Magnesium. So these elements will have a lower ppm concentration but that is not a problem. see (*)
As stated above the inter ratio of elements is important so a 10-10-10 or a 20-20-20 or a 30-30-30 present no direct advantage over a 5-5-5 because it is all a matter of concentration ie the 20-20-20 at 1/4 strength is equal to the 5-5-5. (Except for the concentration of micro elements maybe). Only price and availability maybe a concern.
For cacti a final concentration between 80-90 ppm N (sustained fertilization) and 200 ppm N (growth boost) seems to work well. BUT a word of caution: I know by experience that many many people seem to have an emotional bond with the fertilizer bottle and they can't leave it alone (which for succulents is pretty useless). So one cannot use a sustained 80-100 ppm fertilizing with every watering if one is watering every week! You may start to grow a kind of inflated cactus pumpkin.
For Lithops one may suggest an upper limit of 30 ppm N, even preferably 20 ppm N. Lithops, in my limited experience, seem to react rather strongly to a Nitrogen fertilisation, and I saw clear indications of some "bloating" with a 50 ppm N.
With an NPKMg at 6-12-36-3 a concentration of 20 ppm N, is easily made by using a 1/3 of the "standard" concentration. The real caveat here is the weighing/measuring of the required quantity of fertilizer and many times the required final concentration needs a large volume of fertilizing liquid to be made. Unfortunately such watery solutions cannot be stored very well as they get "infected" very soon growing fungus and other "stuff". The other "boost" elements will still be there in more than sufficient concentration.
With other NPK concentrations, like an 6-8-7 for example, used in a "recommended" concentration, it is equally simple to use it, but P & K will drop terribly. (see (*) again). One may notice that "recommended" concentrations for the average commercial fertilizers will result in a 150-200 ppm N concentration, which is probably OK for house plants but not for succulents.
One last word of caution about using ferts on a very regular basis. This can cause "salting out" of the soil. Which means that the soil is becoming so high is "waste" salts that it becomes unusable/unfit for growing plants. Where Nitrogen is easily "washed out" the other elements, P, K, Ca, Mg are not easily washed away, especially as succulents and certainly Lithops do not very often come in a situation of "water wash through". These chemicals may form complex salts that are only very partially soluble and that are not washed out, aka they remain "bound" in the soil. The residual salts may impair the acidity (pH), especially Ca and Mg components by forming carbonates with CO2 from the air. Their ionic strength and correlated electro-conductivity (EC) of the soil, may also cause problems for growth.
And besides, a bad soil pH will influence the ability and availability of nutrients as some may become unavailable if the pH of the soil is wrong.
(*) For the mad scientists: P = 45,83%, K = 82,98% and Mg = 60,3% of the concentration on the bottle.