WHAT IS A STARTER FERTILIZER?
, 4 min reading time
If you have been researching the correct way to seed a new turf area, you have probably been told that you need a starter fertilizer. That is sound advice, but what exactly is a starter fertilizer and how does it differ from other fertilizers used in turf areas?
Let’s begin with a few facts about commercial fertilizers. The term fertilizer can refer to many different materials that provide essential elements for the growth of plants. There are currently 17 elements considered essential for all plants. They include the following elements: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), magnesium (Mg), and calcium (Ca) are macronutrients, whereas molybdenum (Mo), copper (Cu), zinc (Zn), manganese (Mn), boron (B), iron (Fe), chlorine (Cl), and nickel (Ni) are termed micronutrients. The carbon, hydrogen, and oxygen are obtained from the air and water, but the others are generally taken up by the roots from the soil.
The most important elements supplied by a commercial fertilizer are generally N (nitrogen), P (phosphorus), and K (potassium), although you may find some types that include iron, magnesium, or some of the other minor elements. All fertilizers sold in the United States use a standard system for listing the elements and the amount of each element found in the container. This is done by listing three numbers on the label. For example, a 50 lb. bag of fertilizer may have the following numbers listed in a prominent location on the front of the bag:
10-10-10
These numbers represent the percentage by weight of three distinct materials. They are N, P2O5, and K2O. Notice that this is not N-P-K. In the example above, the fertilizer would contain 10% by weight N, 10% by weight P2O5, and 10% by weight K2O.
To determine how much of the elements P and K are in the fertilizer, the following conversion factors are needed:
P2O5 x 0.44 = P
K2O x 0.83 = K
In the above example, a 50 lb bag would contain
50 x .10 = 5 lbs. N,
and 50 x .10 = 5 lbs. K2O.
To convert the P2O5 to P, multiply the
5 lbs of. P2O5 by .44 = 2.2 lbs P,
and the
5 lbs K2O by .83 = 4.15 lbs. K.
The bag of fertilizer then contains 5 lbs of N, 2.2 lbs. P, and 4.15 lbs. K. This sounds unnecessarily complicated, but it is the way all fertilizers in the US are sold.
Now back to starter fertilizers. The most important element for the fertilization of mature grasses is almost always N. This is the element that is generally limits growth. The addition of N will usually increase tissue growth and it also will generally result in a dark green color. Things are different at the time of seeding, however. As a seed begins to grow, the first structure that appears is a primary root, this is followed by the shoot that appears above ground.
Phosphorus is quite immobile in the soil. It tends to stay put, whereas many of the other elements like N readily move in the soil solution. Root development needs a lot of P. At germination, the root is not able to reach out and obtain enough P from the surrounding soil and P does not move readily to the root. As a result, we generally add extra P at the time of seeding. Nitrogen is still important at this stage, but too much of it can burn the seedling and we generally use smaller amounts of N at establishment and larger amounts of P.
A starter fertilizer will be high in P. The middle number will be higher than the other two. Analysis like 10-20-10, 5-10-5, 18-26-0, and 13-25-10 among many others high in P are commonly marketed for this purpose. A commercial starter fertilizer will generally have the application rate on the bag. If not, apply enough fertilizer to provide 2 to 4 lbs P2O5 per 1000 sq. ft. of surface area. In the case of 13-25-10, the fertilizer is 25% by weight P2O5. Therefore, if you were to apply 8 lbs of fertilizer per 1000 sq. ft., it would provide:
8 lbs x .25 = 2 lbs P2O5
and
16 lbs x .25 = 4 lbs P2O5
Therefore, you would apply 8 to 16 lbs. of 13-25-10 per 1000 sq. ft.
Once the grass is established and root system has formed a fibrous mesh that extends into the surrounding soil, the plant can obtain enough P from most soils. At that time, N becomes the most important element for growth and fertilizers such as 20-2-10, 18-5-9, 25-5-15 that are high in N and low in P will be more appropriate. We will consider the fertilization of mature grasses in another posting.
Nick Christians, University Professor of Horticulture, Emeritus, Iowa State University
books: http://www.amazon.com/Nick-Edward-Christians/e/B001H6SB7A/ref=ntt_dp_epwbk_0
turf blog: http://www.iaturf.blogspot.com/
Fundamentals of Turfgrass Mgt. 5th ed http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119204631.html
Web site: https://www.hort.iastate.edu/directory/nick-christians/