The real key to maintaining the health and vigor of the plants in our gardens is to provide a consistent source of the nutrients each type of plant needs, at the most beneficial time.
There are major differences in how plants respond to different fertilizer formulations and how long the nutrients remain available to plants.
A few examples of well-known, popular brands of liquid and granular fertilizers may help explain some of the differences in formulations and nutrient availability. Liquid fertilizers, whether applied as foliar sprays to the leaves or applied as a soil drench to the root zones, are very rapidly absorbed and drawn up into plant tissues. Many liquid fertilizers contain high percentages of nitrogen. Miracle-Gro’s Water Soluble All Purpose Plant Food contains 24 percent nitrogen (24-8-15 – the percentage of nitrogen is always the first number, followed by the percentage of phosphorus, then potassium).
The label for this fertilizer recommends weekly or biweekly applications. The label for Dr. Earth’s organic Liquid Solution All-Purpose Fertilizer shows a 3-3-3 formulation with 3 percent each of nitrogen, phosphorus and potassium. Biweekly application is recommended.
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Many gardeners assume that rapid growth of green foliage is an indication of plant health. That’s not necessarily true. Slow, steady growth is a better marker of vigor and health. Plants respond quickly to higher nitrogen amounts (above 5 percent) with a strong flush of new green growth; the too-rapid greening and growth can actually stress the plant. Too much nitrogen and not enough phosphorus and potassium produces all-green growth with few fruits or flowers-like a bright green 12-foot tall tomato vine with just two tomatoes.
Liquid fertilizers dissipate quickly from leaf tissue and are quickly leached from the soil and must be replaced more often than granular formulations. Applying higher-nitrogen fertilizer formulations repeats the rapid growth cycle more often.
Lower-nitrogen liquid fertilizers generally provide sufficient nutrients for slow, steady growth. And they contribute less to the runoff of excess nitrogen that is infiltrating our water supplies and water ways.
Miracle-Gro’s Organic Choice All-Purpose Plant Food is a granular fertilizer with 7 percent nitrogen (a 7-1-2 formulation). Its label recommends application at planting time and every two months. Dr. Earth’s Total Advantage granular fertilizer contains 4 percent nitrogen (4-6-2) and its label recommends applications at planting time and again mid-season. It also advises feeding container plants every six weeks.
The recommended application timing for each of these products makes it obvious that liquid fertilizers need to be reapplied much more frequently than granular formulations in order to maintain available nutrient levels.
One other consideration regarding the choice of liquid/granular, high-/low-nitrogen fertilizers is that for at least six weeks during our very hot dry summers, most plants go into a state of semi-dormancy when growth slows and little fertilization is needed. Because the nutrients in lower-nitrogen, granular plant foods remain available for up to two months in the soil, plants will be able to very slowly draw up nutrients, even while semi-dormant.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).