This month, Master Gardener Ann Bartlett discusses restoring nutrients to the soil in your garden.
“I promise that even though we are dealing with chemistry, we will not delve into the nuts and bolts of the processes,” she says. “Our objective is to get a grip on the big picture so that we spend our fertilizer dollar wisely.”
In the wilderness, fallen leaves, dead plants, and bird and animal waste are continuously recycled by the myriad inhabitants of the soil. This ongoing process maintains adequate nutrient content in the soil. But Mother Nature does not practice intensive agriculture. In the early days of our nation some of the push westward was sparked by the exhaustion of the soils. Pioneers needed fresh land to farm in part because they did not know how to restore nutrients to the land they had wrestled from the forests.
Step into any garden center today and you will find plenty of restorative resources. Now the question becomes: What do I need? On every label are three numbers, which stand for the proportion of the three most used soil nutrients: nitrogen, phosphorous, and potassium (N-P-K). What do they do? Why, how often, and how much do they need to be replenished? Furthermore, are any other nutrients needed in the mix?
Three Essential Soil Nutrients
- N is for nitrogen, the soil element most used by plants. Essential for rapid growth, nitrogen leaches from the soil with every episode of rain or irrigation. Compensate by adding it every year to areas where annuals are grown. Perennials, shrubs, and trees do not want a rich diet and are quite content with the nitrogen recycled into the soil by decaying organic mulches or compost. Modern roses are known to be heavy feeders, so rose fanciers apply this nutrient several times during the growing season.
- P is for phosphorous, which plants need to flower and fruit. This element does not move much in the soil, but it can transform and become chemically unavailable to the plants. Beware of "super phos" products that promise super blooms, because phosphorous can build up to toxic levels. Because phosphorous does not wash out of the soil, this buildup is a difficult problem to solve.
- K is for potassium, second to nitrogen in the quantity used by plants. It is critical in sugar, starch, and protein formation (among other things). Potassium leaches from the soil and becomes fixated and unavailable to the plants.
Three Important Nutrients to Include in Fertilizer
- Ca is for calcium, vital for cell growth, nitrogen use, and root formation. Many old-time gardeners use an eggshell tea to add this nutrient. Calcium leaches from the soil and it must be replaced.
- Mg is for magnesium, essential to the formation of chlorophyl, fats and sugars, and phosphorous use. This is another one that washes out of the soil. Many folks go around with a bag of Epsom salt to replenish the supply available to their plants.
- S is for sulfur, which gives dark green color to leaves and stems. It is essential to the formation of vitamins and amino acids. Though it leaches from the soil, rain applies 10 to 20 pounds of sulfur per acre each year in this part of the world, so there is no need to apply more.
Soil Nutrition by the Numbers
A balanced, slow-release fertilizer is generally the best way to go in the vegetable or flower garden. These have label numbers such as 10-10-10 or 12-12-12. The slow-release factor means they will last all season. Many of these products contain calcium and magnesium as well as traces of other elements. Read the label for the amount needed to fertilize your garden area. Apply at planting time because nitrogen begins to wash away immediately.
Additional micro-nutrients occur in such abundance and are needed in such small amounts that they rarely need replenishing. That doesn’t mean the gardener never needs to worry about them. For example, soil in west central Nebraska is deficient in iron, which is critical to chlorophyl formation. Nebraskans add iron to the soil every spring, applying it with a hoe-end sprayer after a rare drenching rain. Fortunately, plant response to that product is very rapid.
As you can see from the chart above, the pH of the soil is critical to making all nutrients available to plants. The optimum range is from 6.2 to 7.3. A pH meter is a terrific investment in trouble shooting, because if pH is the root of the problem, all the fertilizer in the county will not improve your plants' nutrition.
Hamilton County Master Gardeners
Hamilton County Master Gardeners are here to provide you with the latest university research-based gardening information. Whether it’s plant or pest ID, sick plants or general gardening advice, we are here to help!
To reach us:
Send an email to mghchotline@gmail.com. If you can’t email, call (423) 209-8560 and leave your name, phone number, best time to reach you and a description of your gardening question. If you need help diagnosing a problem, include a few details:
- Damage you see and when it started.
- Parts of the plant affected (for example flower, leaf, stem or fruit).
- How much water the plant gets.
- Plant location (for example in a garden bed or in the lawn, next to a driveway or house, in full or part sun).
- (For emails) Photos of the plant or section of your yard in question. Photos of the area and the entire plant, including closeups, are helpful.