Fertilizing Alpines

Through the years of running our nursery, now approaching 25, I have always looked to improve the way we grow plants in the garden. Alpines are especially demanding, in that they are plants that do not thrive in lowland areas. However, conditions of culture are a continuum and what is lacking in one component, may be compensated with other factors. This need not be complicated; a few basic tenets will always remain:

1. Free draining soils: Wet and sodden soil lacks oxygen and encourages root and stem diseases.

2. Lot's of air movement: Again to dry the leaves and inhibit disease

3. Water that is near neutral or slightly acidic pH. It also is better if the TDS (Total Dissolved Salts) is below 200 ppm; better still below 100ppm _ a level that is generally OK for orchids. You need a greenhouse mineral test from a lab such as A&L to determine the pH and TDS _ only $20.

4. Appropriate light levels: In most cases, open light to morning sun, the eastern exposure is the most desirable. In the heat of the summer afternoon, most alpines prefer to rest. Note well that there are plants which tolerate/want a hot, baking site.

The one area where I had the most difficulty, especially growing in pots, was with nutrition. Liquid feed was the primary fertilizer we used. In spring, you can provide a bi-weekly feed of a balanced product at a low rate ~ 100 ppm of Nitrogen, which converts to roughly 50gm / 100 liters of water. Soluble fertilizer is basically salt and adding it to water drives up the TDS; not so good in the hot weather as the possibility of fertilizer burn increases with heat. Think how beneficial a thundershower is for the garden. Rainwater, which is very low in TDS, provides needed moisture, cools the environment and also washes the plants, diluting the effect of excess salts.

The problem with commonly used fertilizer salts is that they are geared to produce rapid growth of edible plants such as cabbages, not the low-level constant feed that most alpines prefer.

I have also tried a number of slow release fertilizers in pellet form. Of these, the one I came to like best was MagAmp, but it is no longer available. So it was with some interest that I spoke with a fellow selling organic fertilizer products to organic field crop producers in Southern Ontario. His most interesting product was Spanish River Carbonatite, a gritty sand that is applied much like a slow-release fertilizer. It has a favorable distribution of elements: Nitrogen 0.3%, Phosphorus 2%, Potassium 0.7%, Calcium 20%, Magnesium 1%, plus lesser amounts of trace elements. Carbonatites are of volcanic origin-magma flowing from the earth's core along a fault line. Vegetation growing on the carbonatite is very lush and is readily spotted from the air if flying over it. On the Spanish River site, there are white birches that are 90 cm (3') in diameter.

We have been using it now for 2 years. The photos of the Androsace and Saxifraga are examples of "before and after" applications. The sax cuttings were very pale when planted into the tufa. A little SRC in the transplant hole in the tufa provided enough nutrition to restore them to health _ this wouldn't have happened if just sand or tufa dust were used. With the Androsace barbulata, the SRC was top-dressed on the pots in July 2007 (very dry summer here). The leaves turned green within 2 weeks. We use it now in all the mixes for plants. Since it is not dose-specific, you can use as much, or as little as seems appropriate. Plants need a reserve of nutrition. Too much, too fast is dangerous. Results are never dramatic, but plants look healthier with fewer pest or disease problems. My speculation about its action is that it may promote a healthier mix of soil microorganisms, which in turn may make nutrients better available to plants. Probably a complex series of interactions that is not understood yet. Certainly a good candidate for some basic research. I find it useful to include in some of the very lean, mineral mixes I use for plants that dislike too much organic content. Dry-land plants from the West respond well. There are perhaps a very few things that don't tolerate the Calcium content.

A few points on SRC to consider:

• Flora is generally richer on limestone mountains

• There is a spike in the availability of nitrogen that occurs at the very end of snowmelt, when it is most concentrated, albeit in ice-cold water. Snowmelt plants have evolved to take advantage of this situation. The lesson for gardeners is to have the fertilizer in place at that time. Wheat crops are fertilized as soon as it is safe to drive on the field, sometime in early April.

• Now that we use SRC in the soil mixes, we use liquid feeds at half-rate.

• The use of SRC in mixes is similar to the use of a sterilized, fertile, garden loam.

• More info at www.carbonatite.com

(June 2008)