By Randall Mallette, Parish Agent
This time of year a lot of my time is spent taking soil samples, interpreting sample results, and making soil recommendations. Having a basic understanding of soil amendments and plant nutrition, specifically pertaining to soil pH, nitrogen, phosphorus, and potassium, will make the soil sample process much less strenuous.
First, let’s talk about soil pH. The acidity/alkalinity of the soil is referred to as the pH. A pH value less than 7 is acidic, while a pH above 7 is alkaline or basic. These values are of extreme importance. Different types of plants have different pH tolerances. Blueberries and azaleas for example like a strongly acidic soil around a 5.5 while many turf grasses and vegetable crops thrive in a soil pH of ~6.5. The soil pH also effects nutrient availability. Soil nutrients are only available for uptake by the plant in certain pH ranges. If the pH of your soil needs adjusting, the soil sample will also tell us how to do that. To raise the pH we use lime, and to lower it we use sulfur. Pay careful attention to the magnesium levels of the soil. The magnesium level will determine what type of lime to use. Use caution when lowering a soil pH. I prefer to use ammonium sulfate instead of elemental sulfur. It is easy to mess up a soil’s chemistry with sulfur. The ammonium sulfate option not only lowers the pH, but also provides a nitrogen source.
The soil sample results will also give recommendations for nitrogen, phosphorus and potassium. These are the three most important nutrients for plant growth. Nitrogen is responsible for vigorous growth, dark green color, and causes a high protein content in the plant tissues, which is important for forage crops and crops for human consumption. Phosphorus is important to plant reproduction, photosynthesis, cold resistance, root growth, and assists in nitrogen uptake. Potassium encourages strong stems, disease resistance, and cold hardiness.
Nitrogen, phosphorus and potassium are the three nutrients found in many “complete” fertilizers. The three numbers on the fertilizer are the Nitrogen-phosphorus-potassium (N-P-K) contents of the fertilizer, expressed as a percentage of the bag’s contents (ex. 13-13-13). With that in mind, we have to do a little math to figure out how much we have to get out of the bag in order to get the correct amount of the desired nutrient. The math is simple, but I won’t get into that part here.
Soil sampling is one of the best things you can do to improve your landscape, garden, pasture, or crop. If you are looking at starting a new flower bed or garden, a soil sample should be your first move. It often holds the answer to poor performing gardens and beds at a low cost.
For more information contact Randall Mallette, County Agent, at the local LSU AgCenter Extension Office 318-357-2224.