Submitted by:
Leo Vieira-Filho, Postdoctoral Fellow
Liliane Silva, CU Assistant Professor and CUCES Forages Specialist
Soil Fertility Assessment = Soil Test!
Soil fertility is “the quality of a soil that enables it to provide nutrients in adequate amounts and in proper balance for the growth of specified plants when other growth factors, such as light, moisture, temperature, and the physical condition of the soil, are favorable” (USDA, 2024). The soil fertility level is assessed by a soil test. The first result that should be checked in a soil test report is soil pH. Most forages require soil pH between 5.5 and 6.5; however, it is always important to remember that certain forages (e.g., legumes) require higher soil pH. Acidic soils (pH < 5.5) can hold macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium and sulfur) and make them unavailable for plants (Figure 1).
On the other hand, an alkaline soil can hold micronutrients (e.g., zinc, iron, manganese, boron, copper) and have the same effect. When plants do not have access to proper nutrient levels, that limits their production, quality and even persistence over time (Figure 2).
It is important to ensure that the soil test will correctly represent the fertility of a certain field, therefore, a representative soil sampling must be done. In order to collect a representative composite soil sample, there are some considerations to make:
• Collect a composite sample per area: a composite sample consists of combining all the soil cores (ideally 15 to 20) taken from a certain area into one sample. By collecting a composite soil sample, it will be possible to have a sample that will represent the average pH and nutrient availability.
• Size of the area sampled: the area should not be larger than 10 acres. In case of larger areas, the variability in soil characteristics will likely be high. Therefore, only one composite sample will not be enough to correctly detect differences in pH and nutrient availability across the whole area.
• Sources of variability: topography, management history, fertilization history, land use history, etc. It may be necessary to collect separate composite samples for different fields depending on those factors previously mentioned.
• Proper soil depth: should be taken into consideration based on the forage species used when establishing a new field. Consider sampling a depth of 8 in when establishing a new forage (forage roots normally explore more than 4 in of soil). For well established fields, sampling to 6 in should be enough, unless you have a forage stand that requires deeper routine sampling (e.g., alfalfa stand).
• Be careful with contamination of soil from other fields or other sources: do not use dirt or rusted buckets, buckets that you transport fertilizer or minerals for animals, etc.
• Never dry soil samples in the oven: it will significantly change soil nutrient concentrations and induce wrong soil fertility diagnostic.
A soil test performed at Clemson Agricultural Service Laboratory is at a current price of US$ 6. This is a low investment considering the information you will receive and that adjusting soil pH can lead to an increase in forage production and nutrient availability in the soil. In addition, being able to apply the needed amount of a certain nutrient will also lead to an increase in forage production, prevent nutrient losses and proper allocation of resources.
References:
Kichler, J. 2021. Strategically fertilizing forages. UGA Extension. https://site.extension.uga.edu/forageteam/2021/03/strategically-fertilizing-forages/
Msimbira, L.A., & Smith, D.L. 2020. The roles of plant growth promoting microbes in enhancing plant tolerance to acidity and alkalinity stresses. Frontiers in Sustainable Food Systems, 4: 1-14. https://doi.org/10.3389/fsufs.2020.00106
USDA – United States Department of Agriculture. 2024. Agricultural Thesaurus. https://agclass.nal.usda.gov/vocabularies/nalt-core/concept?uri=https%3A//lod.nal.usda.gov/nalt/968
