Nitrogen and plant food

[Mike]

A piece in the Cactus Talk section of the last BCSS journal referred to an opinion that added nitrogen (supplied as a ammonium nitrate 'pill') was all that our plants need in addition to the nutrients that are already present in tap or rainwater.

It also suggests that the nutrients in proprietary plant foods (such as I use, diluted, from time to time) can cause problems with a build up of unwanted salts.

Does anyone have experience with this? My plants all seem to have been growing well for the last two years with occasional diliute general feeding. I am really storing up problems for the future?

Mike

[Phil_SK]

I think the problem is if you only give them a bit of water or stand them in saucers the salts can't get away. As long as they get a few deluges during the year to was all the salts away I wouldn't have thought it'd be a problem.

[Roy]

I think regular repotting solves the build up of anything problem

[iann]

As I read the article, the tapwater or rainwater does not supply the other required nutrients at all. These would be supplied in time honoured fashion by an occasional dilute fertiliser. Or perhaps the article does make this claim, since it describes tapwater as containing 200-250ppm of dissolved solids. I would dispute any claim that tapwater would supply all the nutrient needs of a cactus, and certainly rainwater won't. Hard water will supply some nutrients but you won't know which ones and soft water will supply few nutrients of any kind. A loam-based soil will include enough of most nutrients (except nitrogen) to last for many months or a few years. That isn't an excuse to never feed but a reminder that fertiliser isn't needed every week.

The article describes that a very small amount of nitrogen ("prill", although the form should not matter) supplied at virtually every watering is a valuable nutrient that is not provided from the water or from the occasional standard fertilising. This makes sense since nutrients other than nitrogen are retained on clay particles in the soil, on peat, or on clay aggregate, whereas nitrogen is not. In habitat, nitrogen would be supplied by direct rainwater or from continuous mineralisation of nitrogen within the soil that doesn't take place in pots because they do not contain a full ecosystem.

The level of nitrogen described, and you really should get out the calculator and work out some ppm numbers, is about a tenth of a typical fertiliser application, so really very dilute.



Post Edited (10-20-06 16:40)