The term “organic fertilizer” refers to fertilizer derived from a plant, animal or human source. Those sources might be biosolid (human waste), chicken litter/manure, blood and bone, or plant material like corn gluten meal. While synthetic fertilizers are sometimes referred to as inorganic because they are manufactured, many also come from natural sources, like mineral deposits. An example of this would be phosphorus, derived from phosphate rock. Like inorganic fertilizers, organic fertilizers can be manufactured and shaped into a prill so that they can be applied with a rotary or drop spreader. If the biosolid or animal or plant waste is mixed with a bulking agent like woodchip, it is referred to as compost and they are applied on a volume basis, rather than a rate per 1,000 sq. ft. A typical application rate of compost on an athletic field would be 0.25-inch depth spread evenly over the field in conjunction with soil cultivation and coring, while an organic fertilizer application rate might be 1 lb. nitrogen per 1,000 sq. ft.
Organic fertilizers usually have a lower nutrient analysis than inorganic fertilizers, since they come from a source that typically contains less than 10% nitrogen. This means that more organic fertilizer needs to be applied to the turf compared to an inorganic source. For example, if the desired application rate of fertilizer is 1 lb. nitrogen per 1,000 sq. ft., then 5 lbs. of product per 1,000 sq. ft. would be required with an inorganic fertilizer containing 20% nitrogen, compared to 17 lbs. of product per 1,000 sq. ft. with an organic fertilizer containing 6% nitrogen.
Bottom line is, more organic fertilizer is typically needed than inorganic fertilizer. Keep in mind also that the cost of organic is generally higher than inorganic. How often any kind of fertilizer in applied to the field is dependent upon the results of a chemical soil test, done through a reputable soil-testing lab. On cool-season turf, most applications are made in the fall, with little applied in the spring and summer months. A typical amount of nitrogen fertilizer would be 3 to 4 lbs. applied over the growing season.
Organic fertilizers are usually slow-release because the nutrients need to be converted into inorganic nitrogen by fungi and bacteria in the soil before the grass plant can take them up. The rate of release depends on soil temperature, so warm soils and ideal growing conditions are perfect times to apply it. In the Midwest, this would be between late spring and early fall. The more extreme the weather is, the slower the rate of release. Being slow-release, there is little chance of burning the turf, and they generally don’t need to be watered in. For this reason they are considered “safe” to turf, but avoid applying any kind of manure-derived compost or fertilizer in hot weather. Manure and biosolid composts have salt contents that could cause turf damage in hot weather. They can also be a source of weed seeds. Some organic fertilizers, particularly those derived from biosolid and manure, may also have an odor for a couple of weeks, so be prepared to answer questions from field users. If a turf response is needed quickly, due to field damage or to get seed established quickly, an inorganic, quick-release fertilizer might be a better option.
Organic fertilizer generally contains lots of different nutrients, like phosphorus, iron and a range of micronutrients. In states or regions where phosphorus applications are banned or heavily regulated this might create a challenge. Some organic fertilizers contain appreciable amounts of phosphorus, sometimes on a 1:1 or 2:1 ratio with the nitrogen source. If this is the case, carefully monitor soil phosphorus levels to make sure it is not being over-applied. This is especially important if compost is being applied as topdressing. Inorganic fertilizers usually contain nitrogen, phosphorus and potassium and sometimes micronutrients depending on the job it is designed for. Inorganic fertilizer can be tailor-made for different situations like “starter” fertilizers used during seed establishment. They can also be manufactured to be quick or slow-release and can be mixed with other products so that they play more than one role. An example would be starter fertilizer plus the herbicide mesotrione applied at seeding to aid seed establishment at the same time as preventing and controlling broadleaf and grassy weeds.
If soil improvement is a goal, using composts on the field is a good idea. They contain nutrients for turf growth and the bulking agent used in the compost (usually woodchip) helps improve soil structure. It’s very important to pick high-quality, mature compost that has a pH between 6 and 8.5, low soluble salt content, 30-65% organic matter and meets EPA standards for harmful pathogens. A reputable supplier, turf diagnostic lab, or University Extension Service can help pick a good local compost product.
One final advantage of using organic fertilizers and composts is social acceptance. Reduce, reuse and recycle are all programs the public understands and supports. However, as stated earlier, be very vigilant on how much phosphorus there is in a product. We should never over-apply products that contain large amounts of phosphorus, especially near water.