Critical Relative Humidity

Uncoated POLY4 has a Crh of 70%

Critical relative humidity (CRH) is the value of the relative humidity of the surrounding air, above which a fertilizer will absorb moisture and below which it does not absorb moisture. Water absorption influences caking propensity which is undesirable as it leads to difficulties in handling and spreading.

A typical curve from which CRH is determined is shown in Figure 4 which compares uncoated POLY4 with coated MOP. CRH values of 70% for MAP, DAP, MOP and urea, and lower values — near 55% — for blends have been reported [7].

Uncoated POLY4 has CRH of 70%. This is similar to other substitute products such as MOP (72%), giving blenders and growers confidence in the product’s shelf life.

Figure 4: Plot of moisture content and relative humidity of uncoated POLY4 and coated MOP measured at 25˚C [8]

[7] Clayton, W.E. (1984). Humidity Factors Affecting Storage and Handling of Fertilizers. International Fertilizer Development Center. [8] University of Limerick. (2015)