All About Fertilizer

Your Plants Need to Eat, and the Soil Does Not Always Have Everything They Need

Liquid Fertilizer by Midwest Gardening.jpg

Every chance I get my soil is amended with compost and organic nutrients that will feed my plants.  This is easily accomplished in vegetable gardens and perennial gardens each spring.  In the perennial garden I can only incorporate large amounts when moving plants.  Top dressing with compost later in the season replenishes some of the nutrients that have been used up.  But roses and annual flowers in particular tend to be ravenous for certain nutrients and need supplemental fertilizing.  And certain nutrients are difficult for some plants to access without the aide of specific elements.  It is important to know what your plants really need before randomly applying general purpose fertilizer, over fertilizing can be more damaging to some plants than not feeding them at all.  Soil test kits are inexpensive and easy to use, and will give you a good idea of what is already in your soil.

Organic Fertilizers

Organic fertilizers contain only natural ingredients.  The materials release slowly into the soil as they are broken down by microorganisms as well as condition the soil.  Organic fertilizers feed the soil, so your soil can feed the plants most efficiently.  Nitrogen-phosphorous-potassium (NPK) ratios will generally be low, reflecting only nutrients made immediately available to plants.  The ingredients are naturally slowly released into the soil, so the ratio somewhat reflects a constant availability over a long period.  Many of the organic fertilizers are mixed for specific purposes such as feeding tomatoes, heavy bloomers, or roses.  These blends contain the basic organic material and nutrients that you soil will need for the specific application.  Organic fertilizers are available as dry granular or liquid.

Chemical or Synthetic Fertilizer

Fertilizer made from chemicals are generally water soluble, so they can be applied to the soil surface, allowing rain to wash it into the soil or dissolved in water to be applied to the foliage or soil.  Chemical fertilizers contain mineral salts to aid the roots in soaking up the fertilizer.  But these salts are not particularly good for plants, and they do nothing for the soil other acidify it.  The salts will also repel earthworms, which are very important to keeping soil healthy.  Gardens that are consistently treated with chemical fertilizers experience decline in soil structure and living organisms.  Water retention diminishes as the soil declines, and you will need more and more fertilizer all the time.  We each need to make choices as gardeners about our willingness to use chemicals.  If you feed your soil by adding natural amendments and compost as much as possible, chemical fertilizers can be used in moderation to supplement heavy feeders.

What’s in Fertilizer?

All basic fertilizers will contain Nitrogen, Phosphorous, and Potassium or N-P-K.  The ratio of these ingredients are expressed as 10-10-10 for example.  There are other trace elements added, often added unnecessarily or sometimes they are added to create a blend for a specific need.

Nitrogen aids the plant with foliage growth which is normally desirable in spring.  Nitrogen is depleted from soil very quickly so a slow release granular may be preferred, or natural organic materials that produce nitrogen and decompose slowly.  High nitrogen fertilizers are ideal for lawns since the grass is consistently growing new green leaves.  Bloodmeal is one example of a natural organic material that releases nitrogen rather quickly to the soil.

Phosphorus encourages root growth, seed development and fruit ripening.  Since blooms are technically fruit, blooming is encouraged with phosphorus.  Phosphorous is essential to young developing plants.  Compost is an excellent natural source of phosphorus that releases slowly in the soil.

Potassium, often referred to as potash, aides in the overall health and vigor of plants and the development of leaves and fruit (don’t forget, flowers are fruit).  Potassium will also aid in disease resistance.

There are often a combination of trace elements and minerals in most fertilizers, whether synthetic or organic.  They have specific purposes and may, or may not, be appropriate for the plants you are feeding.  For example a fertilizer formulated specifically for tomatoes will contain calcium which minimizes blossom end rot and neutralizes toxic materials.  Magnesium, sulpher and iron are good amendments for plants that require acid soil such as azalea, rhododendron, camelia and blueberries.  Magnesium is a component of chlorophyll.

Special Fertilizer Blends

Specially blended fertilizers are still quite “general purpose”.  The may be higher in one nutrient, lower in another, to very generally suit the needs of a type of plant or flower.  But we all know that individual varieties within a species may prefer entirely different nutrients and feeding schedules.  So special blend fertilizers are ok in a pinch, but it is very important to know your plant and your soil!

Rose fertilizer is formulated to benefit the blooming of most rose varieties.  Generally roses prefer a slow release, high nitrogen fertilizer.  BUT, not all roses like so much fertilizer.  Rugosa roses prefer one feeding in spring of slow release, not too high nitrogen.

Flower fertilizer may be granular or water soluble and is generally quite high in phosphorous to aid in bloom production.  This type works well for most annuals, but many perennials do well when starved of nitrogen and phosphorous. 

Vegetable fertilizers generally will be granular and slow release, and are most effective when mixed into the soil or watered deeply into the soil.  An application following directions is meant to last two or three months in the soil.  Once again, every vegetable does not use the same nutrients, but a general purpose vegetable fertilizer is uniquely blended to put a wide variety into the soil.  Your vegetables will generally use what they need.  The vegetable garden is one place to remember that food is produced with the nutrients in the soil.  Are you sure you want to eat tomatoes grown from chemicals?  The next two articles discuss composting and organic soil amendments.

Houseplant fertilizer considers that container bound plants often need more nutrients than they can get from the small amount of potting soil in the container.  Follow directions on the package, it is easy to over fertilize with a heavy dose.  Frequent feeding is often best.

All purpose fertilizer is formulated to feed flowers, trees, shrubs, houseplants and vegetables.  Better than nothing, but hardly plant specific.  If the all purpose fertilizer you choose is organic, then at least the nutrients are broken down slowly and made available to your plants if and when they need them.  A synthetic, or chemical fertilizer is taken up very quickly.  This can burn the foliage and damage plants.  Don’t get me wrong, I always have some balanced all purpose (5-5-5 if I can find it) on hand for a quick boost for plants, especially annuals or container plants.  But generally spring would be the best time to use synthetic fertilizers when the soil is too cold for microbial activity.  Until the soil gets warm, the organic nutrients you added last fall are not released for the plants.  But don’t worry too much about that, most plants don’t really begin growing until the soil is warm anyway.

What about pH?

Most plants prefer a neutral pH between about 6.0 and 7.0.  In a neutral range, nutrients are most easily absorbed by plants.  Minor adjustments to pH are possible with specific amendments, but keep in mind that soil will generally revert rather quickly to its’ natural state.  If you plants are growing well, they are probably well suited to your soil type and pH.  The article on soil amendments gives more information about organic materials to help adjust pH.

So test your soil, then provide the nutrients your soil needs for the plants you grow.  Investigate organic fertilizers, and read the next couple of articles about composting and soil amendments.

Know your soil and know your plants!  Happy growing!

Sharon Dwyer