Can you mix urea and DAP? Blending, storage and seed safety

Short answer: yes, it is a standard blend

• Incitec Pivot describes DAP as ideal for blending with other fertilisers, including urea.
• Its custom blend safety data sheet lists 'Urea + DAP + Muriate of Potash' as an example NPK blend, and 'Green Urea NV + DAP' as a blend made with protected urea.
• Urea and DAP are both in Incitec Pivot's list of most-used blend ingredients, alongside ammonium sulphate, MAP, single super and potash.

A blend lets you spread nitrogen (N) and phosphorus (P) in one pass instead of two. Incitec Pivot notes that blends vary a little around their stated analysis, more than compound (all-in-one granule) fertilisers do.

Spread it soon after mixing

• Incitec Pivot says DAP blends should be applied shortly after blending to avoid storage issues. DAP is also more hygroscopic (it takes up moisture from the air) than MAP.
• Some fertiliser pairs are not compatible at all. Incitec Pivot's example: urea with ammonium nitrate products such as Cal-Am turns to a watery slush in a dry blend, so they are not mixed.
• Granules of a similar size spread more evenly and separate less in handling (segregation), which is why suppliers match ingredient sizes.
• So plan to spread a blend soon after it is mixed rather than storing it (our advice, based on the above).

Do not put a urea and DAP blend in the seed row at high rates

Both release ammonia, which can burn germinating seed. Incitec Pivot ranks the risk as urea first, then DAP, then MAP, then ammonium sulphate, and warns that adding urea to a MAP blend at planting raises the risk too.

• Canola is the most sensitive winter crop; oats, barley and wheat are more tolerant than lupins and chickpeas.
• Sandy soils, dry or drying seedbeds and wide rows all raise the risk.
• The safest option is a separate band below and to the side of the seed, at least 50 mm away.
• If you must sow with the seed, calculate your Seed Bed Utilisation (SBU%) and stay within the manufacturer's rate tables. See how much DAP per hectare for the working.

Working out a blend

Our arithmetic for a 1:1 blend by weight of urea (46% N) and DAP (18% N, 20% P): each 100 kg contains 50 kg urea and 50 kg DAP, so about 32 kg N and 10 kg P. In practice ask your supplier for the analysis of the blend they make, because recipes and ingredient analyses vary. The NPK fertiliser guide explains how to read the label.

Rather than blend, some farmers use a compound starter with P at sowing and topdress urea later; see MAP vs DAP, urea vs ammonium sulphate and protected urea vs urea. Compare urea and DAP side by side, or browse nitrogen and phosphorus products such as Granular Urea, DAP and Green Urea NV. Always check with your agronomist before acting.