Salt Index

POLY4 has a salt index of 76

Most fertilizers are salts containing macro and micro nutrients which, when added to soil, cause an increase in the osmotic pressure of the soil solution. Fertilizers vary with regards to their osmotic effects and their potential for crop injury. They are measured in comparison to a (w/w) standard reference material, sodium nitrate, giving a ratio referred to as the salt index [16].

The Jackson (1958) method [17] is the most recent employed and was used to assess a range of potash fertilizers. Results from seven independent laboratories show a lower salt index for POLY4 than MOP, SOP and SOP-M (Figure 7).

POLY4’s relatively lower index supports a plant’s ability to absorb water and the nutrients contained within the soil solution.

Figure 7: Salt Index (SI) +/- Standard Deviation for MOP, SOP, SOP-M and POLY4 using the Jackson (1958) method [18]

[16] The ratio of the increase in osmotic pressure produced by a fertilizer to that produced by the same weight of sodium nitrate multiplied by 100 to give whole numbers is called the “salt index” or SI (Rader, L.F., Jr., White, L.M., and Whittaker, C.W. (1943). The Salt Index – a measure of the effect of fertilizers on the concentration of the soil solution. Soil Science, V.55, No. 3, pp. 201–218. [17] Jackson W.L. (1958). Soil Chemical Analysis, Prentice Hall, Englewood Cliffs, NJ. [18] Salt index is average of results from in 2013 from Thornton Laboratories, Spectrum Analytic Inc., Southern jesting, Midwest Laboratories, University of Florida, University of São Paulo and Shandong Agricultural University.