Bob Mugaas, UMN Extension Educator
Photo 1: Applying lawn fertilizer using a rotary spreader.
While it may only be March 1 and there is still plenty of snow covering the ground, in a few short weeks our attention will be turning to taking care of our lawns and gardens for another year. One of the spring activities that many homeowners focus on is getting their lawns off to a healthy, vigorous start; an activity that usually means applying some fertilizer to the lawn. Appealing to that desire is the rather large array of home lawn fertilizers available at local garden and home improvement centers. Frequently, that leaves the homeowner asking the question, "Which one should I buy?" or perhaps even asking the question, "Do I really need to fertilize my lawn at all?"
Over the next three months and beginning with this issue, the emphasis of this lawn care section will be on information to better understand the basics of lawn fertilizer packaging. In turn, that should help with the decision about which one to buy and some tips on getting the most from that fertilizer product.
Plant Nutrient Needs
Photo 2. The basics of photosynthesis.
Before launching into fertilizer product labeling, it's important to review why we would want to fertilize our lawn in the first place. Grass plants growing in our lawn are no different than any other green plant in that through the process of photosynthesis they manufacture all of their own needed food. See Picture 2. Biologically, we call them autotrophs. That is, we don't 'feed' out plants directly they make what they need for growth using elements or nutrients obtained from the soil or atmosphere. The three elements needed by the grass plant in the largest quantity are carbon (C), Hydrogen (H) and Oxygen (O); all of which are obtained from the air or water.
Beyond those three, the elements needed in the largest quantity to support healthy grass growth are nitrogen (N), phosphorus (P) and potassium (K); the majority of which are taken up from the soil by the root system. These three are known as primary macronutrients. In addition to these primary macronutrients, there are considered to be about 10 or 11 other essential nutrients to support healthy growth. These too, are all extracted by from the soil by plant roots. However, it is the primary macronutrients of N, P and K that are often insufficient in soils to sustain and support healthy grass growth. Hence, they often need to be supplemented in the form of fertilizers to provide an ongoing supply of these nutrients throughout the growing season.
Soil Testing to Know What's Needed
The only reliable way to determine what may be needed in the way of nutrition is to take a soil test from the lawn. Soil samples taken or sent to the University of Minnesota Soil Testing Lab for analysis will provide results regarding soil pH, organic matter content, an approximation of soil texture and the amount of available P and K present. They will also provide a basic interpretation of these results along with recommendations for what, if any, fertilizer might be needed for the lawn.
It is beyond the scope of this article to go into the details of taking a soil test or interpreting the results. However, additional information about taking a soil test can be found at the Soil Testing Lab's website . Additional information about interpreting soil test results can be found in the Extension publication Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants
Remember Minnesota's phosphorus law!
Taking a soil test is always a good first step to determine what one might be looking for in the way of a lawn fertilizer. It is especially important in Minnesota as we have a state law that restricts the application of any fertilizer containing phosphorus to lawns without a soil test that indicates a need for additional phosphorus or a new lawn is being established, either by seeding or sodding, when additional phosphorus can be added to aid in the early growth and establishment of that lawn. Thus, without either of those two conditions being met, it is against Minnesota law to apply any fertilizer containing phosphorus to an existing, established lawn.
So, what's in that lawn fertilizer bag anyway?
When looking at a lawn fertilizer bag, the most common container for lawn fertilizers, there will always be three numbers present that state, from left to right, the amount of nitrogen, phosphorus and potassium contained in the bag. This specific ratio of N-P-K is known as the fertilizer analysis. For example, a bag of lawn fertilizer with the analysis of 25-5-15 would be 25% N - 5% P - 15% K. This is also known as the guaranteed minimum analysis as the fertilizer bag cannot contain any less than the percentages stated on the bag for the respective nutrients. Because this is a guaranteed minimum analysis, by law the bag could contain more than that stated but cannot be less. This fertilizer analysis is a percentage by weight basis. That is, if a bag of fertilizer with the above analysis weighed 50 pounds, then it would contain 12.5# N (50 x 0.25), 2.5# P (50 x 0.05) and 7.5#K (50 x 0.15) for a total of 22.5# of nutrients in this 50# bag of fertilizer.
The difference between the amount of nutrients contained in the bag and the total weight of the bag is largely attributed to the weight of the carrier. That is, the material used to bind to the nutrient or 'carry' them such that it can be applied using various types of spreaders. Frequently, these materials are in the form of dry granules that help disperse the fertilizer uniformly across a lawn surface when applied through a spreader.
The fertilizer analysis gives the percentages of N, phosphate (P2O5) and potash (K2O) contained in the bag on weight basis. The percent nitrogen is considered to be the actual amount of nitrogen contained in the bag while phosphorus and potassium are given in their oxide forms (i.e., they are combined with oxygen). For example, the element phosphorus only makes up 44% of the molecular mass of P2O5. Thus, in order to determine the amount of actual phosphorus contained in the bag we must multiply the percent phosphate in the fertilizer bag by 0.44. From our example above, the 5% phosphate equates to 2.2% or 1.1 pounds of actual elemental phosphorus in our 50# bag. Likewise, the element potassium makes up 83% of the molecular mass of the K2O molecule. Thus, in our example above, we would multiply the 15% by 0.83 to determine the amount of actual K present in the bag. Carrying out that multiplication we get 12.45% actual K or 6.22 pounds of actual K in our 50 pound bag.
Unless there is a specific need to know the amount of actual phosphorus or potassium present in the bag we usually use the percent phosphate and potash for making fertilizer recommendations. Most soil test recommendations also base fertilizer needs on these same phosphate and potash percentages. Hence, when applying the amount of phosphate or potash suggested in the soil test recommendations, one will be applying the needed amount of phosphorus and potassium to the lawn. Picture 3.
In next month's article, we will be taking a close look at the nutrient nitrogen as it is the nutrient required in the largest amount next to C, H and O. It is also the nutrient for which nearly all rates of lawn fertilizer applications are based. And, it may just be the reason why you would select one fertilizer over another. Check in next month for a thorough overview of this very important fertilizer nutrient.