A question often asked is, if fertilizer in soil will have an effect on the measured concentrations of radionuclides. Fertilizer has elevated concentrations of radionuclides and adding fertilizer to soil could have an effect on the concentrations that are measured by a gamma-ray spectrometer. Whether an increased concentration of radionuclides can be measured from fertilizer added to soil, depends on the background concentration of radionuclides in soil, the concentration of radionuclides in the fertilizer and the amount of fertilizer added to farmland.
Background in soil
The concentration of Potassium (K) in soils depends on the provenance and composition of the soil particles and the type of weathering. Most clay-rich areas have higher concentrations K than areas that contain mostly quartz minerals. In typical northern hemisphere (non-tropical) soils, the concentration of Potassium (K) ranges between 0.5%-2% (which is 0.005-0.02 kg K/kg soil). However, tropical soils can be weathered to a state where the concentration of K is extremely low.
This potassium present in the soil is the total potassium within the crystal structure, only a small amount of this potassium (<5%) is available for uptake by pants.
Concentration in fertilizer
Different types of fertilizer exist, but the fertilizer with the highest concentration of potassium is Potash, containing 50% of K.
Amount of fertilizer added to farmland
the amount of fertilizer that is added to farmland depends on the fertility of the soil, but typical amounts are 100 kg fertilizer/ha. This fertilizer is added to the entire tillage layer of 30 cm. With the assumption that the soil has a density of 1500 kg/m3, a hectare of soil has a weight of 4.5 x106 kg. The 100 kg fertilizer/ha (with 50% K) results in ±1x 10-5 kg K/kg soil.
Can we measure a changes in K when fertilizer is added to the farmland?
The added fertilizer with the concentration of 1 x 10-5 kg K/kg, is much lower than background concentration from the crystal structure (500-2000 x 10-5 kg K/kg) and this difference in smaller than variations due to changes in clay content of soils. For typical northern hemisphere (non-tropical) soils, changes due to the addition of fertilizer can not be measured. Only in soils that are by nature devoid of K, the addition of fertilizer can be measured.
In a field experiment (Dierke, et al, 2013), plots were treated with different amounts of fertilizer and mapped with a gamma spectrometer. These measurements did not reveil changes in the measured 40K concentration.
Dierke, C., Werban, U., 2013. Relationships between gamma-ray data and soil properties at an agricultural test site. Geoderma 199, 90–98. doi:10.1016/j.geoderma.2012.10.017