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Without knowing what is needed for good plant growth, we are largely tapping in the dark, following fertilizing schedules or others’ timeworn advice. That makes it impossible to estimate whether the supply of nutrients through fertilizer application is beneficial, harmful, or just contributes to runoff pollution. Regular soil testing provides the necessary guidelines for effective crop management. As my example illustrates, while my dahlia plot had plenty of compost, it also had an excess of nitrogen, phosphorus, and potash, all with detrimental effects on dahlias and the environment. With a pH of 5.4 the soil was too acidic to good dahlia development. All those results most likely were due to my habit of tossing high-value granular fertilizer in the planting hole before putting in the tuber —year after year. It could use more magnesium, it notes.
Whether one applies 6-1-1 liquid fish emulsion or (as I did) 16-16-16 granular product, a plant will take up only as much as it needs. That low-nitrogen fish concoction preferred by dahlia growers translates into very expensive stimulant, as opposed to high-percentage formula, most of which is wasted that way. Soluble nitrogen, as in urea, gives an immediate boost, while insoluble granular products like my fertilizer take a long time to break down. In fact, well-composted manure provides most of the nutrients a dahlia requires.
Recommended Video. Excess nitrogen just encourages more top foliage and weaker tubers. It also hinders nitrogen-fixing bacteria from their task of breaking down that essential chemical. Fish does offer nitrogen, but it also contains macro-molecules of oils and protein (neither of which is usable to plants but may benefit microbes). Phosphorus already is present in good soil, so more just inhibits iron and zinc needed by plants. It also leaches into waterways to kindle eutrophication. When the substrate is not topsoil but a potting mix or inert media, plan on adding fertilizer, even when potting soil includes slow-release fertilizer.
Let the soil test guide you in application of any amendments or fertilizer. Your county extension service may offer soil testing year-round (and often at no cost). Here in my area the KCDS office provides up to five free tests for residents. There also are commercial labs that can provide comprehensive testing at some cost. And yes, there are any number of soil testing kits in the marketplace and at nurseries, but the self-use tester needs to understand that results from these are quite unreliable. The only proven home sampling kit is the Mysoil kit ($32 from mysoiltesting.com) that takes your soil sample to analyze it in the company’s lab, then provides detailed results and suggestions.
No matter what soil you have for planting dahlias (friable loam, silty, sandy, or hardpan clay), the more you disturb the micro-organisms’ habitat, the less they are effective in supplying the nutrients to the plant roots. Although humans have tilled, plowed, raked, and double-spaded their plots for eons, Anne Biklé explained that – other than making soil more porous and suitable for seeds and plants - there is little benefit in upsetting the well-interconnected organic community below hard at work.
The entire growing substrate could not succeed without adequate water supply. A copious rainy season will (for some time) fill the open crevices between the soil strata, but for growing crops successfully - while taking advantage of the micro-nutrients deployed by critters in the soil - regular moisture is the very essence of life. This applies in particular to heavy-feeders like dahlias.
We’ll address this topic in another month.
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