Characteristics of natural and manufactured fertilizers for lawns
The environmental movement has stimulated interest in the use of natural organic fertilizers for lawn care. This has spurred debates on the qualities of natural versus manufactured materials. Unfortunately, misconceptions confuse the issues for people selecting fertilizers. The goal of this fact sheet is to discuss characteristics of common fertilizers in an objective manner, and to allow consumers to choose for their own needs.
Why do we fertilize?
In discussing fertilizers, it is useful to know why they are important in lawn care. Nutrients are needed by grass plants because of the many stresses we apply to them. These nutrients are needed by the grass to repair damage done by normal wear and mowing. The nutrients are also consumed by the other plants in the landscape, such as trees, weeds, etc. Nutrients are also lost when clippings are removed while mowing. Fertilizers supplement the soil with these needed nutrients.
What makes up fertilizer?
The three main nutrients in fertilizer and needed by the plants are nitrogen (N), phosphorus (P) and potassium (K). Many other nutrients are also needed, but in very small amounts, hence the term micronutrients. For a home lawn, the most important nutrient to add on a regular basis is nitrogen, and is the one addressed most frequently in this fact sheet. Phosphorous is present in high levels in many midwestern soils, therefore, more in the form of fertilizer is usually not needed. A soil test can be used to determine if phosphorous should be added. Because phosphorous does not easily move in the soil, additional amounts are often beneficial during establishment of new turf. However, established lawns show little response to added phosphorous and it should only be applied if a soil test indicates a need. Potassium may need to be supplemented. Coarse-textured (sandy) soils are often deficient, so regular additions on these soils are needed. Again, a soil test is recommended to determine when potassium should be added.
The analysis (concentration of nutrients) of the fertilizer is printed prominently on the bag. Each number represents a percentage of the total weight of the product that is actual nutrient. The first number indicates the amount of nitrogen expressed as N, the second is the amount of phosphate expressed as P2O5, and the third, potash expressed as K2O. For more information on calculating fertilizer amounts, refer to FO-3338, Fertilizing Lawns.
Commonly, high analysis is interpreted as excessive fertilizer. This is definitely not true. Simply less of a high analysis fertilizer is applied. This is similar to differences between a concentrated laundry detergent and a more dilute one. Natural organic fertilizers usually have a low analysis as compared to the manufactured forms. As a result, more of the organic fertilizer is needed to supply the same amount of nutrients. This is an important consideration when figuring cost, since more bags of the low analysis fertilizer will be needed to supply the same amount of nutrients.
Types of fertilizer
There are many kinds of fertilizers available for the homeowner. For comparison purposes, the products will be grouped according to their source into the groups listed in Table 1.
|Table 1. Examples of the two fertilizer groups.|
|Natural||Milorganite, Ringer, Sustane, manure,
grass clippings, phosphate rock,
potash (source of potassium)
|Manufactured||ammonium nitrate, urea, IBDU,
sulfur-coated urea, urea formaldehyde,
Compost is not included in Table 1 because ordinarily it has little nutrient value. However composts can be very beneficial as a soil amendment, adding organic matter, and improving many soil properties.
Natural organic fertilizers are commonly made from waste products of various sources ranging from chicken feathers and manures, to treated sewage sludge from city waste systems. Rock phosphate and potash are mined from the earth. The manufactured fertilizers normally are made from petroleum and natural gas, except for superphosphate and triple superphosphate, which are rock phosphate that has been concentrated using acid reactions.
Whether natural or manufactured, the nutrients in fertilizer are generally not in a form the plant can use directly. Only two forms of nitrogen are readily available to the plant: ammonium (NH4) and nitrate (NO3). All nitrogen fertilizers must be broken down to these forms regardless of source. Some chemical changes and decomposition must occur before the nutrients are taken up by the plant.
The speed at which nutrients in the fertilizer become available to the plant varies depending on the product. Natural organic fertilizers are typically slow release (delivering nutrient over a period of time) because decomposition of the organic matter occurs slowly. Some of the manufactured fertilizers are also slow release. This group includes sulfur coated ureas, urea formaldehyde and IBDU. Microbial activity in the soil gradually breaks down the fertilizer. Other forms, termed fast release, are rapidly converted to available nutrients. Examples include ammonium nitrate and urea. Fast release fertilizers are applied more frequently, at low rates.
Common questions concerning natural and manufactured fertilizers:
Are manufactured fertilizers going to burn the lawn?
"Burning" occurs when fertilizer is so concentrated around the roots that it essentially pulls water out of the plant. Burning most commonly occurs when a lawn is fertilized in hot weather and/or without being watered. This is particularly a problem with the fast-release fertilizers. Slow-release products, both natural or manufactured, also have the potential of burning, but the nutrients usually do not build up to such high concentrations in the soil. To prevent burning of the lawn, always water after applying any fertilizer, unless the label recommends otherwise.
Are manufactured fertilizers more likely to cause pollution?
As mentioned, all fertilizers must be broken down to the same nutrient form. Therefore, all types of fertilizer can contribute to pollution, if used incorrectly. The leaching of nitrates may be of concern in areas of sandy soils when over-watering and over-fertilization occur, particularly with the use of fast-release forms. Excess phosphorous has also been involved in lake and stream pollution. However, if the lawn is dense and vigorous, it contributes very little, if any, to runoff. The grass roots bind the soil tightly, promoting infiltration of water and nutrients. Care should be exercised so as not to leave fertilizer and organic matter (lawn clippings, leaves) on sidewalks, driveways and gutters. These contribute substantially to nutrient runoff into lakes and streams. Refer to Preventing Pollution Problems from Lawn and Garden Fertilizer for more information.
Does fertilizer adversely affect the soil and its inhabitants?
Some nitrogen fertilizers tend to reduce the pH of the soil and thatch slightly. pH is a measure of acidity with 7 being neutral, below 7 acidic, and above 7 alkaline. Most commonly grown turfgrasses do best at a pH in the 6.0 to 6.5 range. In areas of already low pH (less than 5), acidifying fertilizers may be of concern. This is partly because low pH has been shown to reduce the population of earthworms and certain microbes. In Minnesota, and many other parts of the Midwest, pH tends to be rather high (above 7) along with high buffering capacity1. In those soils, the use of acidifying fertilizers may actually be beneficial.
There are many fertilizers available for use by the consumer, coming from many different sources. However, as discussed, all fertilizer types essentially do the same thing: add nutrients to the soil to assist plant growth. When properly applied, the use of fertilizers generally involves few risks to humans or other animals. Few special precautions need to be taken when using them. If used correctly, fertilizers will improve and maintain lawns to their best, and provide us with their many benefits such as controlling erosion, cooling of the environment, and the control of allergens, among many others.
1Buffering capacity refers to the soil's ability to resist pH change. In a soil with high buffering capacity, it is very difficult to reduce the pH by means of fertilizers and other soil amendments.
Reviewed by Bradley W. Pedersen, Associate Professor, Department of Horticulture.