Soil Test and Fertilizer

You don’t have to do a soil test ever time you are going to fertilize, but you should done one at least once a year. For this post when I talk about soil test I am referring to Soil Nutrient test, which let you know what nutrients are in the ground. There are several other type of soil test/ analysis that can be done but usually are only done if there is a drainage/erosion problem or there is a possible soil born disease. Soil Nutrient test can be divide into two general categories, either Basic or Complete analysis.


Basic test usually only give you results for Nitrogen (N), Phosphorus (P) and Potassium (K); with some frequency you will also get soil pH. You sometime receive the results in number for ppm (parts per million) but usually on a basic test you are give the results at Inadequate/Low, Adequate/Good or High/Too High. You will get the results for each of the three nutrients/minerals.

N-P-K are the three numbers on all fertilizers. The numbers represent the percents of the nutrient by weight. The three numbers can never exceed 100, but usually are less than 50. Often there are other things in the fertilizer, like more minerals or product to slow the release of the nutrients into the soil. A common general fertilizer is 15-15-15; it is 15% of each of the nutrients. This would be a good fertilizer is you got inadequate results for all three nutrients. It you got a good result for a nutrient you might look for a low percentage for that nutrient. But if you got a high/too high result you want to look for a fertile without that particular nutrient/mineral.

pH is a good number to have (scale of 0 to 14, with 7 being natural), it lets you know the availability of some of the nutrients that are already in the soil. For example, Arizona’s soil usually has good/adequate results for Iron (Fe), but the soils pH is somewhere around 8 (Alkaline.) With that pH there iron that is in the soil is not available for most plants to get enough. You can try lowering the pH with something like sulfur or an acidic produce like pine much (I would recommend the second because of how easy it is to overdo the sulfur.) Or, you can use a fertilizer with a little iron (will remain available to most plants along as it is suspended the soil water.)

Complete test will give ppm or ppb (parts per billion) results for all (at least most of the) 13 essential minerals for plants. It almost always includes the pH of the soil, and sometimes includes even more information that could be helpful. This test obviously cost more, and usually needs a professional to help analyze the number, but it will help keep you landscape healthy for years to come.