Understanding the seasonal fluctuations of soil test measurements during the year can aid in understanding and interpreting soil test results that vary from year to year or samples taken within the same year. Many factors can cause soil results to differ from year to year, with seasonal fluctuations being one of them. Seasonal fluctuations are mainly controlled by the crop’s uptake and release patterns of nutrients and environmental conditions. Suppose soil samples are taken from a field the same way and at the same time each year with the help of a soil nutrient tester in the USA, and the results are higher or lower than expected. In that case, seasonal fluctuations can sometimes explain these results.
What Does the Hanging Season Mean for the Soil Nutrients?
A spring soil sampling gives an accurate picture of which nutrients will be available to the plant that year. However, a fall soil sample offers many advantages to the producer, such as sampling during good weather, allowing time for planning the coming crops, and time to react before spring planting. Understanding the seasonal fluctuations of soil tests will allow a producer to take fall soil samples that better represent the fertility in the soil and obtain a more efficient lime and fertilizer recommendation. Of the commonly reported soil test measurements, phosphorus (P), potassium (K), and pH are the most affected by seasonal fluctuations. These factors work together to lower the pH during the growing season.
Source: Freepik
Phosphorus (P):
Seasonal fluctuations of soil tests for P are more minor than for pH and K. The quantity of crop P uptake is much lower than K, with most of the P ending up in the harvest grain. The smaller amounts left in the residue are not easily leached from the plant and require microbial decomposition for release (a much slower process). The soil is also an excellent buffer for P. Fertilizer P is converted to chemical compounds in the soil that significantly reduce its solubility, and it also stabilizes the soil test P and prevents rapid fluctuations.
Potassium (K):
Seasonal fluctuations in soil test K are seen almost yearly and can be considerable. As the crop grows and matures, uptake lowers the soil-available K. Large amounts of K are taken up by the plant, about 160 lb/ac of K2 0 in a 150 bu/ac corn crop. For corn, wheat, and grain sorghum, most of the K is in the vegetation, with only about 25 to 30% in the grain. Once the grain is harvested, the K in the residue is washed back into the soil. The net effect is that soil test K drops until the crop reaches physiological maturity, increasing slowly as the K is leached from the residue. However, soybean grain contains about 60% of the crop’s total K, so removal is higher, and the soil test K does not rebound to the same extent as other grains. K removal is highest for crops where the entire plant is harvested, such as tobacco, hay, and silage, since everything taken up by the plant is removed from the field. In these cases, the K soil test can drop rapidly in the field with a slight recovery later in the season.
pH Levels:
Seasonal fluctuations in pH are not unusual. They are affected by fertilizer rate, time of fertilizer application, organic matter, root and bacterial activity, and soil moisture. The pH usually lowers during the summer and early fall and then increases as the soil moisture increases. The reduction in soil pH during this time is generally attributed to soil drying, root and bacteria activity, and nitrification of nitrogen fertilizers. Nitrogen fertilizers containing ammonium are changed to nitrate (nitrified), releasing acidity (hydrogen ions). The roots and bacteria in the soil produce carbon dioxide that temporarily lowers the pH during the height of their activity. The salts in the soil (natural and added as fertilizers) can concentrate near the soil surface as the soil dries and displace hydrogen ions (acidity) from the cation exchange complex into the soil solution, which also lowers.
In Conclusion:
Fall soil sampling is a widespread practice. It offers the advantages of sampling during good weather, allows time for planning for the coming crops, and gives lime time to react with the soil before spring planting. However, it has some disadvantages. Because of seasonal test results fluctuations, the pH and soil test K measurements tend to be at their lowest. The amount of rainfall and crop uptake will affect this. Soil test results should be compared to previous tests to help determine if seasonal fluctuations are significant enough in a year to make fertilizer and lime recommendations adjustments. When soil sampling in the fall, sampling beside the row may be a more accurate representation of real soil test K but would have little or no effect on the P and pH. At the very least, samples should be taken in equal numbers from both the row and row middle positions to help reduce the seasonal effects.
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