Quote:
Originally Posted by Ray
Per the manufacturer, 1.45g/gal will yield a 50 ppm N solution.
Translating mass to powder volume (especially heterogeneous ones like the Greencare MSU formulas) is damned difficult and inaccurate.
That said, dividing 4 by the %N on the label is a way to estimate the teaspoons/gal for 50 ppm N, so 1/8 tsp/gal of a 13%N formula is more like 15 ppm N.
Personally, I think the discussion of what TDS a plant can tolerate is of no value. These guys have evolved to expect very little food, provided in many, small doses, so it makes sense that mimicking that is the better approach.
Humans can "tolerate" a booze binge, but doing that periodically will be ultimately harmful, while a small amount, consumed regularly, has been shown to have health benefits.
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Fred meant 50 ppm total dissolved solids, not just N. He made this clear in the discussion.
I reported elsewhere here on Orchid Board Brandon Tam's lecture to our orchid society about how the Huntington grows Paphs.
---------- Post added at 09:13 PM ---------- Previous post was at 08:57 PM ----------
As Naoki pointed out, some nutrients are actively taken up by roots. That means the plant expends energy absorbing these nutrients. Water tends to flow passively, replacing that transpired when stomata are open.
Some nutrients - calcium is an example - are excluded from most plant roots, except at the narrow collar near the root tip. I don't know whether this calcium exclusion is found in orchid roots. Without this plants would be flooded with calcium, a common mineral in most ground water. This mineral plays a part in regulating enzymatic activity inside plants, just as in animals, so too much calcium would be fatal.
Root "burn" is often an osmotic issue. In other words, it is mainly related to the number of dissolved osmotically active particles in the solution - if the external water solution has a higher particle concentration than the plant, water flows out of the plant through the roots, or the plant is unable to take up water. This leads to root death.
Nutrition is different from water balance. What growers are interested in is providing sufficient nutrients to plants without causing an osmotic problem at the roots, or preventing adequate water uptake. Orchids don't need much nutrition, but they do need a lot of water.
Electrical conductivity measures the number of particles carrying an electrical charge in the solution. It can't be translated to TDS without knowing exactly what chemical species are in the solution, and in what amounts.
EC can be used to compare relative proportions of the exact same mineral mixture added to water, so it can be used to measure fractions or multiples of a known constant concentration of a particular fertilizer product of unvarying composition, when mixed in different volumes of water. EC can't provide much useful information for a solution of unknown composition.
It is possible to have a solution with very low EC that has so many osmotically active particles roots would be killed - an example would be a concentrated solution of table sugar.
During periods of high humidity, most fertilizer powders absorb water from the atmosphere, without much changing the volume in the container. That quarter-teaspoon will weigh more during humid times than in arid times.
Fortunately for us orchids don't need much fertilizer.
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