The facts about phosphates
While phosphorus is necessary, plants prefer it in small doses
‘If you want to get your plants to root and flowers to bloom, use a high-phosphate fertilizer.’
For years, my standard treatment for getting our greenhouse plants to develop vigorous roots and plenty of blooms was to give them a good ‘shot’ of a high-phosphate fertilizer.
Plant Starter was always my fertilizer of choice because it had the highest phosphate content (52 per cent) of any product on the market. After our seedlings were transplanted, I ensured that each and every one received a concentrated application of phosphate.
Sure enough, all of the treated plants developed strong roots and yielded plenty of gorgeous flowers at maturity. For years, no plant would ever find its way into our greenhouses without first receiving its prescribed dose of high-phosphate fertilizer.
But there was just one tiny problem with my high-phosphate feeding program. The correlation between great roots, bountiful flowers, and lots of phosphate was, unfortunately, mere coincidence.
A bit of science
The origin of the roots/ flowers/high-phosphate relationship was based on some old and rather poorly designed research. Later, when high-phosphate starter fertilizers were really put to the test, they provided little, if any, additional benefit to most plants compared to low-phosphate fertilizers.
Plants generally don’t need an inordinately large amount of phosphate to produce robust growth, and prefer to sip rather than guzzle it. Phosphate is not particularly abundant in most natural soil environments, yet the plants manage to grow just fine.
There is no doubt that phosphate is essential for growing healthy plants, and it is one of the ‘big three’ nutrients. On all fertilizer labels, it is always the middle number, bookended by Nitrogen (first number) and potash (last number).
Phosphorus (contained in phosphate) is a key component of the energy system of all plants. Those that don’t have enough ‘run out of fuel’ and often appear stunted, and frequently develop purplish leaves. I remember one of our cornfields that should have been emerald green, but instead was a brilliant purple — pretty, but not great for production of high-quality cobs.
What s hou ld you do?
Generally speaking, for home garden soils, drop the high middle-number fertilizers.
For plants growing in the garden, pots and hanging baskets, regular applications of fertilizers like 10-4-3 will provide all the phosphate that your plants need. However, if you have a very large garden and a soil test determines that your soil is deficient in phosphate, then applying a high middle-number fertilizer would be the most economical way to bump up the phosphate to a level that satisfies the needs of most plants.
Don’t forget that composts like Seas oil (composted fish waste and bark) and well-rotted manures provide a substantial amount of phosphate when blended into the garden or containers. These organic materials break down slowly over time and provide a nice supply of phosphate throughout the growing season.
Finally, keep in mind that prudent application of phosphate is important not just for plants, but for the environment as well. Phosphate contributes to algae blooms in waterways, and while it does tend to stay put when applied to soils, having it run down driveways and into sewer systems is a potential source of water pollution.
If you are still reluctant to reduce the amount of phosphate that you apply, I suggest that you do a few trials of your own. I’m sure you’ll find that less is more, and a little phosphate can go a long way.