A Question of the Week


A key part of maintaining a lush, healthy lawn is proper fertilization. Garden center shelves are filled with multiple choices, and it can be hard to know which product is right for your lawn. One clear distinction is whether a fertilizer is organic or conventional. There are pros and cons to each option, and understanding which to apply to your lawn 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.

How plants use fertilizer

Organic and conventional fertilizers are safe for lawns 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 the grasses’ perspective, it doesn’t really matter which type of fertilizer you use.

Plants are only able to take up nutrients in certain ionic forms. For example, nitrogen is only absorbed by plants as nitrate (NO3-) or ammonium (NH4+) 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, perennial grasses 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 that contains greater than 50 percent water-insoluble nitrogen.

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 real 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.

Environmental concerns

The main downside to organic fertilizers is that the proportions of nutrients are usually different from what grass requires for growth. In particular, many of these materials often contain far more phosphorus than lawns need. If gardeners try to apply enough organic fertilizer to meet the nitrogen needs of their lawn, they will usually add way more phosphorus than the grass can absorb.

Over time, this can lead to very high levels of soil phosphorus. New Hampshire soils tend to be very high in phosphorus, and it is a rare lawn that requires phosphorus fertilization. While having extra phosphorus in the soil doesn’t hurt the lawn, 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. In order 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 lawn fertilizer

All packaged organic fertilizer products will have a fertilizer analysis or grade that refers to the percentages of nitrogen (N), phosphate (P2O5) and potassium (K2O) in the fertilizer. On a fertilizer label, these nutrients will be listed as three numbers in the order N-P-K. For example, a 10-2-2 fertilizer contains 10 percent nitrogen, 2 percent phosphate and 2 percent potassium. One hundred pounds of this fertilizer would hold 10 pounds of nitrogen, 2 pounds of phosphate and 2 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.

There is a turf fertilizer law in New Hampshire that dictates that no retail fertilizer product should exceed 0.9 pounds per 1,000 square feet of total nitrogen can be put down per fertilizer application, when applied according to the product label. That means that with each fertilizer application you should be putting down no more than 0.9 pounds of actual nitrogen per 1,000 square feet. You can figure out how much fertilizer to apply by dong a few simple calculations. Additionally, no more than 3.25 pounds per 1000 square feet of total nitrogen should be applied in a year, as directed by the instructions.

Exceeding this rate of nitrogen fertilization can not only cause environmental harm, but it can make turf grasses more susceptible to pest issues. The most important times to fertilize the lawn are in mid to late spring and early fall. If you only want to fertilize your lawn once, choose to fertilize in early fall when the grass can make the most use of the nutrients. 

Organic fertilizers can be a good a good choice for New Hampshire lawns, but they must be used appropriately. Though they are often viewed as safer for children and pets, and the best choice for the environment, that is only true if the right organic fertilizer is chosen based on soil nutrient availability. Ultimately, using a fertilizer properly according to the label instructions or based on soil test results is most important. Whether the nutrients come from organic or synthetic sources does not matter to the lawn.


Got questions? The Ask UNH Extension Infoline offers practical help finding answers for your home, yard, and garden questions. Call toll free at 1-877-398-4769, Monday to Friday, 9 a.m. to 2 p.m., or e-mail us at answers@unh.edu.



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Got questions? The Ask UNH Extension Infoline offers practical help finding answers for your home, yard, and garden questions.
Call toll free at 1-877-398-4769, Monday to Friday, 9 a.m. to 2 p.m., or e-mail us at answers@unh.edu.