Express News Weekly Article
Saturday, January 29, 2005
By Calvin Finch, Conservation Director, SAWS, and Horticulturist

Soils, Why a 3-1-2 Ratio Fertilizer           

            Fertilizers are described as a percentage by weight of nitrogen-phosphorus-potassium in the bag.  In simplified terms, a fertilizer with a 19-5-9 formula is 19% nitrogen (N), 5% phosphorus (P) and 9% potassium (K).  If the bag is 40 lbs that would mean that is contains 7.6 lbs of N, 2.0 lbs of P and 3.6 lbs of K.  The rest of the bag is inert filler or other nutrients.  In the case of a 40 lb bag of 19-5-9, the balance would be 67% or 13.3 lbs of filler.

Why is a fertilizer with a 3-1-2 ratio recommended for the lawn in San Antonio.  The obvious answer is that that is the ratio that lawn grass uses nitrogen, phosphorus and potassium.  For every unit of phosphorus (middle number on a fertilizer bag) that most grasses use, it means 3 or 4 units of nitrogen (first number) and two units of potassium.

            That makes sense; we are applying fertilizer with a nutrient mix that replaces the nutrients used by the grass.  When you take into account, however, that our soils are loaded with phosphorus and potassium, a fertilizer with a 3-1-2 ratio does not make nearly so much sense.  In fact, the best fertilizer for lawns in San Antonio on our heavy soils would be one that only provides nitrogen.  We really do not need the phosphorus and potassium because our soils are loaded with the nutrients. 

            So why do we recommend fertilizers with a 3-1-2 ratio rather than a fertilizer like ammonium sulphate at 21-0-0?  There are several reasons.  The first reason is that fertilizer manufacturers cannot efficiently formulate a customized bagged fertilizer for every market’s conditions.  To achieve economies of scale, fertilizers have to be manufactured for large markets.  Since most of the South has sandy infertile soils, a 3-1-2 ratio fertilizer is ideal for a huge area.  The fertilizer is also good for raised beds in our area because they often are well-drained, low fertility mixes dominated by sand and organic material.

            Taking into account that most of our South Texas soils are loaded with phosphorus and potassium, why then would we recommend a 3-1-2 slow release lawn fertilizer rather than a fertilizer like 21-0-0 (ammonium sulphate), which is available on the market?  The key reason that 21-0-0 is not desirable in that it is a fast release source of nitrogen.  Nitrogen is a volatile nutrient in many forms, including the form in ammonium sulphate.  Apply it to your lawn and large portions of it volatize into the air and leach into the soil.  In addition to much of the nitrogen being wasted, it is easy to burn the lawn and is a threat to the groundwater and rivers.

            The slow release 3-1-2 ratio fertilizers are a reasonable compromise.  They are an efficient source of nitrogen, are relatively safe for the environment, do not burn the lawn, are very useful in raised beds and can meet the nutrient needs of landscapes over a large geographic area.

            We have discussed “hot” fertilizers like 21-0-0 and the more tame 3-1-2 ratio fertilizers, there are some other materials on the market as well.  At least one fertilizer company offers a slow release lawn fertilizer in a ratio of 27-3-3.  That would be a great formulation for lawns in the San Antonio area.  It has a large amount of nitrogen (27%) in comparison to phosphorous and potassium (8% each) and it is slow release.  The only hesitation would be the price.  Nitrogen is the most valuable nutrient.  If 27-3-3 fertilizer is twice as expensive as 19-5-9, you are spending lots of money for the extra 3% of nitrogen and a reduced load of phosphorus and potassium.

            Organic fertilizers are also very useful but the nitrogen is expensive.  Compost can have as little as 1% nitrogen and the relatively potent formulated organic fertilizers only have 6 or 7% of nitrogen.  All organic fertilizers are slow release nutrient sources and they contribute to soil structure improvement.  Over the long run, organic fertilizers can make soils more efficient in storing and releasing nutrients.  Remember, however, in our climate that organic materials do not usually accumulate, the materials added one year are usually decomposed by the next.  We will discuss organic materials in more detail in future articles in this series on soils.

            The summary of the discussion is, “that even though a 3-1-2 slow release fertilizer is not ideal for many fertilization situations in San Antonio, it is a good compromise.”