Recently I read a book with the above title by Robert Pavlis, a well-known speaker and writer on gardening topics who owns a six-acre botanical garden in Guelph, Ontario. The book debunks several myths that I admit having accepted over the years and has changed some of my ideas, especially relative to fertilizing. The advice one gets from magazines and web magazines is very helpful on managing seeds, seedlings and plants with respect to water, light, diseases and pests, but has little to say about soil care, apart from recommendations to add organic material and fertilizer. The author’s background is in chemistry and as a chemist he explains things that are confusingly vague in most garden writing. The book is well worth reading completely, but here to whet your appetite I’ll share a couple of his insights that were thought-provoking for me.
For many years I have used “balanced fertilizer” containing ‘equal amounts’ of nitrogen, phosphorus and potassium (K). The amounts are not truly equal because the phosphorus content is actually the amount of phosphoric acid (P2O5) and the potassium is in the form of potash (K2O) , so the true N-P-K proportions in a 10-10-10 mix are 10 - 4.36- 8.3. I generally looked for the highest absolute amounts of each, like 15-15-15, mainly because that gives you the biggest bang for your buck, Of course, plants don’t use equal amount of each nutrient, but there is some comfort in believing that you’ve supplied an adequate amount of each.
Ai first glance, the fact that the fertilizers are not truly balanced would seem a mere semantic issue. A much more important problem is that the persistence of each component in your soil is quite different. The nitrogen compounds in most commercial fertilizers are rapidly soluble in water and so are quickly available to your growing plants. However, this solubility also means that they will rapidly be leached out of the vicinity of the roots with watering or rainfall and so must be applied more frequently to be effective.
It's more difficult to know whether potassium needs to be added to an established garden and, if so, how much. Potassium is an essential nutrient, and in its available form as the soluble ion it is readily leached from the soil. However, most soil contains slowly available insoluble forms and San Diego municipal water contains more than most tap water. There is certainly much more than needed in your ‘balanced’ fertilizer.
“Phosphorus is a critical part of plant growth”, being a component of DNA and RNA as well as ATP (adenosine triphosphate), “the substance of life”. Agricultural soils that have been harvested for many years may be deficient in phosphorus and the same may be so for gardens on land recently converted from farms to housing. These may need added phosphorus for a few years but little will be needed thereafter. “Stable phosphorus” is held strongly by the soil and can build up to toxic levels with repeated application. Pavlis summarizes, “The bottom line for home gardeners is simple: Too much phosphorus is bad for the environment and may harm soil organisms. Unless your soil test indicates a shortage of phosphorus, you should assume that your soil has plenty of it and don’t add more in the form of commercial fertilizers.”
Yipes, this is pretty alarming. I’ve been adding phosphorus with each new planting, sometimes three times a year, to my vegetable beds which are now 38 years old. The new plantings are flourishing and so I don’t seem to have done any damage so far, but I plan to be very sparing with everything but nitrogen in the future.
Well, maybe I should have the soil tested, but where should I select the samples? I have 20 vegetable beds, some like the asparagus and blackberry beds with permanent plantings. Some, shaded during the winter by the neighbour’s tall hedge to the south, are used only half of the year and are planted only once per season. Some get year-round full sun and may be freshly planted and fertilized up to four times a year. Pavlis discusses soil testing in much greater detail than I can go into here but summarizes, “A soil test can give you some quick insights into a new garden, but if you have been gardening in a place for a while or moved into a home with established gardens where things are growing well, you probably don’t need a soil test.”
So, that might leave you with the question, do I need to fertilize at all? Pavlis’ answer is that you don’t need to fertilize if you mulch with an organic material or add compost yearly, if you return plant material to the garden instead of sending it to the landfill, and if your plants are growing well. If they aren’t doing well, lack of nutrients is rarely the problem.
This probably doesn’t apply to a bed where you’re making repeated plantings of rapidly growing vegetables and are using the whole plant at the table or putting the inedible parts on the compost pile. The same may be true of roses if you’re pruning them properly, removing much of the new growth after each blooming cycle, and perhaps three quarters of the whole plant at the winter pruning. In the vegetable beds I plan to continue my current practice of adding compost every time I plant seeds or set out new seedlings. I will occasionally add a high nitrogen fertilizer to crops like corn and cold vegetables but will supply nothing but compost to legumes and root vegetables.