David M. Kopec, Extension Turfgrass Specialist
Definitions and Terms
In the January issue, we talked about fertilizer analysis and grade, fertilizer ratio, and what constitutes a complete fertilizer. In this issue we will address starter fertilizers, the cost per unit of fertilizer and slow release fertilizers.
Starter fertilizers can represent a broad range of fertilizers for lawns and landscape plants. In general, they have higher analysis (or grade) values for phosphorous (20% or more) and a quick release source of nitrogen (usually the nitrate form).
The high phosphorous is included to enhance root growth of the new seedlings at establishment. Usually a fertilizer which has 20% or more phosphorous can be regarded as a starter fertilizer. Ammonium phosphate 16-20-0 also qualifies as a starter fertilizer because it is 20% phosphorous. Super phosphate 0-27-0 and triple superphosphate 0-45-0 also qualify as starter fertilizers even though they have no nitrogen or potassium.
Regular fertilizers with a 1-2-1 ratio or so can be used as starter fertilizers for lawns as well, as long as the fertilizer grade for phosphorous is 20%, or higher.
Slow release fertilizers:
Slow release fertilizers are those which do not release their nutrients all at once when watered or rained upon. Slow release can be from organic of synthetic sources.
Synthetic sources (granules usually) can have different ways they achieve the slow release of the fertilizer nutrients. Some products are coated with sulphur (like sulphur coated urea).The sulphur breaks down over several months in the soil. Thus, the process results in a slow release rate.
The size of the particles themselves also influence the breakdown or release rate of the slow release fertilizer. Smaller granules break down faster than larger granules. This provides some initial response. The medium size granules release next, followed by the largest granules which provide the long-lasting component. This is one reason why some fertilizers come in different sized particles in the fertilizer bag. This is true for sulphur coated urea as well.
Other products have a plastic coating on them which is slow to break down when watered. Thus a little fertilizer is released each time the lawn is irrigated. The plastic may vary in thickness and may contain cracks which also control the slow release of the fertilizer.
Still others need to go through a chemical change in the soil solution (the water that the roots use) to make the fertilizer available. This usually does not happen in cold soil temperatures such as in the winters in Arizona.
The slow release fertilizers often have a high analysis or grade for -N-. This is because so much of the total nitrogen amount is not readily available, but is in the slow release form. When applying these synthetic slow release forms, follow the label directions closely for the amount of fertilizer to apply per 1000 ft2 of lawn space. It is usually a large amount! Also note how long the product will last and when the next application should be. You can still apply a quick release from of regular fertilizer at the same time you apply a slow release type.
Note that on some fertilizer bags, ammonium (NH4) may be included as a slow release form. This is because the NH4 is converted in the soil to nitrate (NO3) nitrogen. The conversion is soil temperature dependent, so when the soil is cold the conversion is slow. Note that the ammonium (NH4) itself is highly water soluble.
Organic forms of slow release fertilizers include manure mulches and organic by-products from animal and plant product industries. These included turkey and chicken manure products, processed municipal waste sludge (safe to use), decomposed plant products and even mine tailing products that contain iron and other true elements.
The plant and animal organic by-product fertilizers are usually low in fertilizer grade analysis. Typical nitrogen in the organic products has to be converted from proteins to amines, to ammonium, and then to nitrate. This takes a long time and thus results in slow release. The real benefit of these products is the addition of organic matter to the soil. This allows for better root growth on heavy texture soils.
Cost per unit of fertilizer:
It is easy to get confused by these terms discussed (fertilizer analysis, fertilizer ratio, complete fertilizer, starter fertilizers, etc.) when it comes to laying your money down on the table at purchase time.
The cost per unit of fertilizer is calculated by performing two calculations. Usually, cost is commonly figured and the price of nitrogen. To determine the cost per unit of nitrogen, you need the following information. 1 = the fertilizer analysis or grade, 2 = amount of product weight, 3 = cost per unit of fertilizer product (cost of a bag, or truckload of fertilizer product itself).
Find out how much actual nitrogen you have in the purchase unit. Our example includes a 50 lb. bag of 10-6-4 at $12.50 per bag.
A 50 lb. bag of 10-6-4 has .10 x 50 lbs. = 5 lbs. -N-. Then divide the cost of the purchase unit by the actual weight of the fertilizer element in question.
$12.50/bag divided by 5 lbs. -N-/bag equals $2.50 per lb. of -N-.
Compare this to another fertilizer which costs $16.00 for a 25 lb. bag which has a fertilizer ratio of 16-5-9. For the cost per unit of nitrogen, multiply the 25 lbs. (weight of the bag) by 16% -N-. Therefore 16 .lbs x 25 lbs. = 4 lbs. of -N-. Finish by again dividing the cost per bag, by the amount of fertilizer. So $16.00 bag/4 lbs. -N-/bag results in a cost of $4.00 per pound of nitrogen. Therefore, the 10-6-4 fertilizer is actually cheaper ($2.50 lb. vs. $4.00 lb.), even though the 16-5-9 has a higher analysis content of -N-.
Now you know how to go fertilizer shopping!
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.