Choosing fertilizers, soil amendments that suit your garden

Apr. 16, 2013 @ 03:21 PM

Fertilizers, both chemical and organic are usually chosen for N-P-K (nitrogen, phosphorous and potassium or potash) content. But is that all that’s needed to grow healthy and nutritious vegetables, herbs and fruits?

What does each nutrient do?

N — Nitrogen helps plant foliage to grow strongP — Phosphorous helps roots and flowers grow and developK — Potassium (Potash) is important for overall plant health

Is more better?

Higher N-P-K levels don’t necessarily mean healthier plants. For instance, high nitrogen fertilizers will result in quick growth, but quick growth can result in weaker plants that are more susceptible to attacks by diseases and pests.

Who came up with the idea that N-P-K was the answer to the fertilizer question?

German scientist Justus Von Liebig was responsible for the theory that nitrogen, phosphorous, and potassium levels are the basis for determining healthy plant growth. However, this theory, which dates to the 1800s, doesn’t take into account the dozens of other nutrients and elements that are essential to plant growth such as sulfur, hydrogen, oxygen, carbon, magnesium, etc. Nor does the theory include the importance of beneficial soil organisms that helps plants flourish and fight off pests and diseases. Additional nutrients such as carbon, hydrogen, oxygen, sulfur, magnesium, copper, cobalt, sodium, boron, molybdenum, and zinc are just as important to plant development as N-P-K. Unfortunately, Justus Von Liebig’s theory has developed into the dominant formula by which we nourish our edible and ornamental plants, lawns and trees.

What’s in your fertilizer?

When reading fertilizer labels, you’ll notice that the N-P-K numbers don’t add up to 100 percent. So, what is the rest of your fertilizer made up of?

Chemical fertilizers can have any number of additional ingredients including dirt, sand, and even materials that are potentially hazardous to your health and to the environment. Chemical fertilizer fillers are needed so that the nutrients aren’t so concentrated that they will “burn” your plants, your skin, and anything else they touch. Organic fertilizers don’t necessarily contain fillers, because they are made up of a variety of natural components that in one way or another benefit your plants.

Chemical fertilizers differ from organic fertilizers in the rate at which the nutrients become available to the plant. For example, the type of nitrogen typically found in chemical fertilizers dissolves very quickly in water. This means that excess nitrogen may find its way into groundwater and freshwater sources and contaminate the water. Additionally, many chemical fertilizers are now using phosphoric acid to create a high phosphorous content quickly and cheaply. According to research, this kind of phosphorous essentially neutralizes other important trace minerals from the soil that your plants need.

Although organic and natural fertilizers usually have a lower N-P-K number, they are considered soil amendments that work slowly over time to improve your soil and to help your plants grow strong. They avoid the fast growth and flowering provided by chemical fertilizers that can actually weaken plants. High N-P-K numbers don’t necessarily mean a better fertilizer.

Do You Need a Soil Amendment or a Fertilizer? You might not need much fertilizer in your garden. You may just need to liberate the nutrients already present in your soil with beneficial soil organisms, proper soil aeration, soil drainage, and re-mineralization. For the gardener, this means that organic fertilizers with low N-P-K labels are perfectly adequate for your garden as long as you’re properly caring for your soil. Without proper soil aeration, mineral nutrients, and other factors, your plants may not be able to absorb phosphorous and potassium anyway, so loading up your soil with high levels of phosphorous and potassium may not make much difference with the health of your plants.

Nitrogen is typically available to the soil without additional fertilizers. The trick is having healthy soil full of beneficial microorganisms that can make use of the nitrogen that is available in the air. Earthworm castings and properly prepared compost are teeming with these beneficial organisms. Also, companion and cover crop plants like beans and clover can fix nitrogen that boost soil’s health. Other excellent natural sources of nutrients are fish meal, kelp, alfalfa meal, and bat guano.

According to the National Academy of Sciences, even though farmers uses fertilizers more widely, homeowners uses 10 times more fertilizer per acre. You should only use the proper amount of any fertilizer, and not anything more. This will save you money, and it will also keep your yard and garden healthier at the same time. This applies to organics as well as chemical fertilizers!

At Windcrest Farm, our motto is: Feed the soil that feeds the plants that feed you!

 

Mary Roberts and Ray Tarlton are owners and managers of  Windcrest Farm, a USDA Certified Organic farm and greenhouse in Monroe, NC . Visit www.WindcrestOrganics.com for more information about plants, produce and classes.