For the Love of PlantsJune, 2020
By Emma Erler
When it comes to growing healthy productive plants, all New Hampshire gardeners must make a choice about which type of fertilizer to use. Let’s break it down.
Garden center shelves are stacked with myriad of choices, and it’s hard to know which product is right for your plants. One clear distinction is whether a fertilizer is organic or inorganic. There are pros and cons to each option, and understanding which to apply to your garden requires a little background on how fertilizers work and the nutrients that all plants need.
Organic fertilizers are especially popular among home gardeners because of the many potential benefits they provide to the soil. Here’s what you need to know if you’re thinking about switching to organic fertilizers this season.
The defining feature of natural organic fertilizers is that they come directly from plant or animal sources, such as manure, fish emulsion or alfalfa meal. Inorganic fertilizers, such as ammonium nitrate, are often called synthetic or conventional fertilizers because they go through a manufacturing process, though many come from natural mineral sources. Either type of fertilizer is safe for plants and the environment as long as they are used properly. The process of how plants absorb nutrients is the same whether they come from an organic or inorganic fertilizer, so, from a plant’s perspective, it doesn’t really matter which type of fertilizer you use.
How plants use fertilizer
Plants are only able to take up nutrients in certain ionic forms. For example, nitrogen is only absorbed by plants as nitrate (NO_3-) or ammonium (NH_4+) ions, and potassium is only taken up as the potassium ion (K+). Soluble inorganic fertilizers provide nutrients in these forms, so they are immediately available for plant uptake. They are fast acting and fairly inexpensive.
However, because all of the nutrients are available at once, plants can’t use them all, and many are quickly lost from the soil. This means that you may need to fertilize more than once throughout the growing season unless you are using a slow-release formula. Another downside is that it is easy to damage plants by applying too much.
In contrast, organic fertilizers have to be broken down by microorganisms in the soil for the nutrients to be released in the right form to be absorbed through plant roots. Due to this fact, organic fertilizers release nutrients over a fairly long period of time. Though they are more expensive and less concentrated pound for pound than inorganic fertilizers, organic fertilizers do provide benefits to the soil that inorganic fertilizers do not. Over time, they can improve soil structure, increase soil water holding capacity and promote the activity of important soil microorganisms, contributing to a healthier soil environment for plants.
The main downside to organic fertilizers is that the proportions of nutrients are usually different from what plants require for growth. If you try to apply enough of an organic fertilizer to meet a specific nutrient need, you may end up applying too much of another nutrient. In particular, many of these materials often contain far more phosphorus than plants need. If gardeners try to apply enough organic fertilizer to meet the nitrogen needs of their plants, they will usually add way more phosphorus than plants can absorb.
Over time, this can lead to high levels of soil phosphorus. New Hampshire soils tend to be high in phosphorus, and it is a rare garden that requires phosphorus fertilization. While having extra phosphorus in the soil doesn’t hurt plants, it can contribute to surface water pollution when it runs off the landscape. Excess phosphorus in water bodies can cause algae blooms that harm water quality and aquatic organisms. To use organic fertilizers in an effective and environmentally sound way, have your soil tested to learn its pH and nutrient status.
Selecting an organic fertilizer
All packaged organic fertilizer products will have a fertilizer analysis or grade that refers to the percentages of nitrogen (N), phosphate (P_2 O_5), and potassium (K_2 O) in the fertilizer. On a fertilizer label, these nutrients will be listed as three numbers in the order N-P-K. For example, a 6-15-0 fertilizer contains 6% nitrogen, 15 percent phosphate and 0% potassium. One hundred pounds of this fertilizer would hold 6 pounds of nitrogen, 15 pounds of phosphate and zero pounds of potash. If a soil test indicates that the levels of some nutrients are high, try to choose a product that has low concentrations of those nutrients.
A few common organic nitrogen sources for gardens are alfalfa meal, blood meal, cottonseed meal, fish emulsion and soybean meal. These organic materials break down at different rates and some must be applied more than once to address plant needs. Fish emulsion and blood meal break down rapidly in comparison with other organic fertilizers, so they are usually used at planting and again as a side-dressing mid-season.
If phosphorus is needed in the garden, bone meal, rock phosphate, fish meal and poultry manure are good sources, and potassium can be applied with alfalfa meal or Sul-Po-Mag; the latter if magnesium is required.
Composted manure is a complete fertilizer, which means it contains nitrogen, phosphate and potassium, albeit in low concentrations. Manures vary in their nutrient content by animal source, but in general have a fertilization ratio of 1-1-1. They are an excellent fertilizer but should not be used as the sole source of nutrients over the long-term due to concerns about soil phosphorus buildup.
Fresh manure should never be used in vegetable gardens because it poses a high risk of microbial contamination. Animal manures can harbor pathogens that are harmful to people, such as E. coli and Salmonella. If you intend to use manure around edible crops, make sure it has been composted and aged for six months before application. Another benefit of composting is that most weed seeds will be killed.
Ultimately, organic fertilizers can provide a lot of benefits to garden soil and plants, but they must be used judiciously. Though they are often viewed as safer and the best choice for the environment, that is only true if the right organic fertilizer is chosen based on soil nutrient availability and crop needs. Like in all other aspects of gardening, you’ll have better results the more thought and care you put into choosing the right nutrient source.
Emma Erler is the UNH Cooperative Extension Education Center Program Coordinator