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 .
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.