Summit Fertilizers introduced custom blending facilities to WA in 1992. Farmers can design fertilizers that are suitable to their individual situation or paddock requirement. Summit recommends that only products that are of similar sizing be blended to avoid separation in transport. All Summit's blending products are between 2 and 4mm.
A small number of products can not be mixed, due to chemical reaction or increased hygroscopic properties of the mixture.
In the highlighted example, both DAP and Urea absorb moisture at around 75% Relative Humidity. When mixed together they absorb moisture at 62% RH - making this mix potentially difficult to use.
This chart illustrates the way that two fertilizers, when blended, will absorb atmospheric moisture at a reduced humidity. Under these conditions these blends will need to be treated carefully by adding builders lime and covering in storage with black plastic. Generally, mixtures with a critical humidity of less than 65% are not suited to bulk handling unless used immediately after mixing.