Primetime Newspapers
By Calvin Finch, PhD, SAWS Conservation Director, and Horticulturist
Week of January 31, 2005

Fertilizer Formulas for San Antonio

            Do you ever wonder what the numbers on a fertilizer bag mean or why most of us use a 3-1-2 ratio fertilizer in San Antonio?  Fertilizers are described by the ratio of a source of nitrogen-phosphorus-potassium as a percentage by weight of 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 50 lbs that would mean that is contains 9.5 lbs of N, 2.5 lbs of P and 4.5 lbs of K.  The rest of the bag is inert filler or other nutrients.  In the case of a 50 lb bag of 19-5-9, the balance would be 67% or 33.5 lbs of filler.

            Ammonium sulfate is 21-0-0-5, which translates to 21% nitrogen with no phosphorus or potassium.  The fourth number, if one is included, is sulfur.  Sulfur is usually so common that it is not listed, but in our alkaline soils, sulfur is important because it is the nutrient that reacts with hydrogen to acidify.  A fertilizer like ammonium sulfate with a relatively large quantity of sulfur can be useful in alkaline soils. 

            Many plants, especially our lawn grasses need nitrogen, phosphorus and potassium in the ratio of 3-1-2.  Grasses need three parts nitrogen (N) for every one part of phosphorus (P) and 2 parts of potassium (K) that are available.  If the nutrients are not available in those ratios, the plant cannot prosper and the excess of any of the nutrients are wasted.  You probably see now why a 3-1-2 ratio fertilizer is recommended for lawns.  It makes sense to provide a growing lawn a fertilizer that provides nutrients in the ratio that plants require. 

            What happens, however, if the soil is loaded with phosphorous and potassium?  That is exactly the case here is Central Texas.  Our ideal fertilizer would only include high levels of nitrogen because we have plenty (sometimes excessive) quantities of P and K.  Why then are we recommending a 3-1-2 ratio of fertilizer for Central Texas lawns?  The reason is that economic and environmental factors limit our choice of fertilizers.

            Fertilizer manufacturers cannot produce a separate blend of nutrients for every locale.  To be viable, a fertilizer manufacturer must produce a product that can be marketed and sold over a large geographic area.  A 3-1-2 fertilizer is ideal for the majority of the South because the sandy soils are generally devoid of N, P and K.  Lawns or infertile sands need a 3-1-2 ratio fertilizer.  A 3-1-2 is not ideal for Central Texas but it is acceptable.  The 3-1-2 ratio becomes even more acceptable for our area when it is in a slow release formulation.  The slow release formulation is important to sandy soils and to areas where gardeners are trying to protect an aquifer.

            Nitrogen is volatile (“hot”) and in many formulations, such as that in 21-0-0 fertilizer, it leaches into the soil or volatizes into the air before the grass can utilize it.  Adding to the problem of nitrogen being wasted, the nitrates (nitrogen form) can pollute groundwater and rivers or burn lawn grass.

            If you garden in a raised-bed, the conditions often approximate that of a sandy, infertile soil.  The best raised bed mixes are dominated by sand and organic material.  They drain well and are easy to work but usually are devoid of nutrients.  A 3-1-2 ratio fertilizer is perfect to meet the nutrient needs of heavy feeding vegetables and flowers.

            Organic material is often mentioned as a fertilizer.  Organic fertilizers are useful because in addition to the nutrients (very limited quantities), they improve soil structure.  Unless you are recycling your own landscape waste, organic fertilizers are very expensive sources of nutrients and are sometimes difficult to use.  I will discuss organic materials in more detail in future articles. 

            In summary, the numbers on the bag of fertilizer indicate the percentage of a source of nitrogen, phosphorous, potassium and sometime sulfur within the bag.  We recommend and most people use a 3-1-2 ratio slow release fertilizer in San Antonio because it provides the nutrients in the ratio that most plants use them and despite the fact that such a fertilizer provides more P and K than we need for most soil situations in our area, the nitrogen is provided in a relatively safe and economical form.