How and When to Fertilize Your Garden

I bet you thought fertilizing your garden was pretty basic – just dump a bunch around the plants and water it in. Unfortunately, it isn’t that simple and shouldn’t be done that way for a variety of reasons.

But first: Do your plants really need fertilizer?

Many plant problems cannot be solved with fertilizer – pay attention to your soil first – add organic material like compost regularly to improve soil tilth, drainage, and nutrition.

There are 18 essential nutrients that plants require to grow and survive. Carbon, Hydrogen, and Oxygen are needed in the greatest quantity and are obtained from air and water. The next 6 nutrients are considered macronutrients;…Nitrogen, Phosphorus, and Potassium…. Calcium, Magnesium, and Sulfur. The remaining 9 elements are considered micronutrients and include Chloride, Iron, Boron, Manganese, Zinc, Copper, Molybdenum, Nickel, and Cobalt… Each of these play an important role in the different structures and functions of the plant… [and] if one essential nutrient is deficient, plant growth will be poor even when all other essential nutrients are abundant. These nutrients primarily come from the soil; however, when the soil’s nutrient reserves become low, nutrients can be supplemented through the addition of… fertilizer, compost, and manure.

Starting a Garden: Fertilization, College of Agricultural, Consumer & Environmental Science, Illinois Extension

As we stress time and again, never fertilize your garden without doing a soil test first. In most soils, especially where compost is added regularly, there is enough fertility to grow the majority of plants. If soil is cared for correctly in the perennial garden where roots run deep, fertilizer will rarely be necessary (I can’t remember the last time I fertilized my flower beds).

Vegetable gardens should be tested every other year for fertility. Due to the high turnover of plants in veggie gardens, there MAY be a need for nitrogen, but rarely anything else. If you follow crop rotation basics, composting, mulching and cover cropping with legumes, the need for additional fertilizers is rare. When growing vegetables or flowers in containers however, plants will need to be fertilized regularly.

And just because the leaves are yellowing, vegetables are small, flowering is late or your plants “look like they need fertilizer”, fertility may not be the problem. Many factors influence a plant’s growth including soil pH (the measure of acidity or alkalinity), soil drainage, weather conditions, disease and soil temperature. Fertilizer can’t fix any of these.

What problems occur if I fertilize when my garden doesn’t need it?

• Most commercial fertilizers, especially synthetic fertilizers, are nitrogen-heavy (the first letter in NPK on the bag). When you over-fertilize with nitrogen, you end up with a lot of top growth on the plants and a reduced number and size of fruits and roots. Or none at all. Too much nitrogen can also cause insect and disease problems, according to Colorado State University Extension. Certain soils that are very porous however may need regular applications of fertilizer. Again, a soil test is necessary to know how much should be added.
• Adding too much fertilizer also causes water pollution. Phosphorous that isn’t used by plants is washed away by rain and into local waterways and onto rivers, lakes, and oceans. There, it causes problems like algal blooms that choke marine life. You may not believe that one gardener can make a difference – but consider how many gardeners there are in any region, multiplied by fertilizer runoff from farms and you can see why this is a big problem. Each action matters.
• Too much phosphorous can also cause chlorosis, a yellowing of leaf tissue, and kill off beneficial mycorrhizal fungi needed by the plants. If mycorrhizal fungi disappear, that plant will have difficulty taking up iron and other micronutrients.

Fertilizer nutrients required by vegetables in the highest quantity are nitrogen (N), phosphorus (P) and potassium (K). Other nutrients, including iron, copper, manganese and zinc are needed in much smaller amounts. With the exception of nitrogen and phosphorus, most of these nutrients are most likely available in the soil at adequate or even excessive amounts. Adding nutrients that are not needed can cause deficiencies of other nutrients and can cause an imbalance of nutrients. Nobody can tell you what your soil really needs without referring to a soil test conducted by an analytical laboratory.

Fertilizing the Vegetable Garden, Colorado State University Extension.

When shouldn’t I fertilize my garden?

• Do not fertilize with granular fertilizer before a heavy rain, because most will wash away.
• If you’ve had a long run of dry weather, especially in drought, don’t fertilize as it may do more harm than good. When water is restricted, the plant may be slowing its processes due to heat stress.
• If your area was recently flooded or had heavy saturating rain, wait until the soil is thoroughly dry before fertilizing.
• Do not fertilize plants when they are dormant, as the plant won’t use and doesn’t need the nutrients. The fertilizer will be washed away before the plant needs it.
• Fertilizing heavily when plants are young can result in a very “leggy” plant – too much stem and thin top growth and a small root mass that won’t be able to support later growth or fruits.

How to properly apply fertilizer

• If you’re preparing a garden bed for spring planting, broadcast the suggested amount of fertilizer across the entire garden bed evenly. Work it into the top 3 inches of soil with the back of a rake. Gently water it in to move the fertilizer into the soil. Adding granular fertilizer weeks before planting gives soil microbes time to convert it into the form of the chemical the plants can use.
• You can also side-dress your plants. Apply fertilizer 2 inches deep and 6 inches aside the row to be planted or beneath the roots. But make sure the newly planted seedling’s roots do not come into direct contact with the fertilizer or they may be burned. As the plant grows, this becomes less of an issue.
• If you’ve already planted, only add granular fertilizer around the root zone of the plant and gently work it into the soil with a hand trowel, cultivator, or other gardening tool that can get it beneath the soil. Try to work in the fertilizer 2 inches deep but don’t disturb the roots. Do not let the fertilizer come into contact with the stem or foliage as it may burn the plant. After working it in, gently water it into the soil.
• Compost is the best way to fertilize as it also adds organic matter to the soil which has a variety of benefits. But the compost needs time to break down – generally a few months. That’s why you should add compost early in spring and again in fall, 2 inches thick. You can also add it after planting to serve as a mulch – add it around root zones another two inches but do not contact the seedlings. There is no need to work compost into the soil, as bacteria, insects, and other microbes do the work for you.

Does it matter if I use liquid or granular fertilizer?

Liquid fertilizers make nutrients immediately available to plants. So if a soil test indicates an immediate need, liquid fertilizers are best. Granular fertilizers take time to break down in the soil into a usable form. They are best applied early in the season before planting or at planting time.

Resources: Do fertilizers help or hurt plants?, University of Minnesota Extension; Starting a Garden: Fertilization, University of Illinois Extension; Fertilizing The Vegetable Garden, Colorado State University Extension; Different ways to apply compost and commercial fertilizer, University of Saskatchewan.

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