How much DAP per hectare? Working out the rate

Step 1: decide how much phosphorus you need

There is no single right DAP rate. The rate comes from a phosphorus (P) target, which depends on your soil test, the crop and its expected yield. See reading a soil test for Colwell P and PBI.

• Replacement: Soil Quality (WA) puts removal at about 3 kg P per tonne of wheat or barley grain, 5 kg for lupins and 7 kg for canola.
• Building up a low-P soil needs more than replacement. Soil Quality (NSW) notes only 5–30% of applied P is used by the crop in its first year; the rest builds soil reserves.
• Soils that lock up (fix) P need significantly more, and acidic soils below pH 5.0 fix much more P (Soil Quality NSW).

Step 2: convert kg P to kg DAP

Incitec Pivot's DAP is 18% N, 20% P and 1.6% S. So divide your P target by 0.20, which is the same as multiplying by 5. Our worked examples:

Check the bag: analyses vary slightly between brands. Summit DAP, Impact DAP and Agfert DAP are listed with their own figures. Count the nitrogen DAP supplies in your N budget; see how much urea per hectare.

Step 3: check the rate is safe with the seed

DAP releases ammonia as it dissolves, which can burn germinating seed. Incitec Pivot ranks the risk to seed as urea, then DAP, then MAP, then ammonium sulphate. Soil Quality (WA) also says nitrogen fertilisers such as DAP are more damaging than superphosphate.

• Crop: Incitec Pivot says oats, barley and wheat are more tolerant than lupins and chickpeas, and canola is the most sensitive winter crop.
• Soil: any rate on sandy soil carries more risk than on clay.
• Moisture: in seasons with good soil moisture at sowing, damage is generally not an issue; drying soil after sowing raises the risk.
• Row spacing: wider rows put more fertiliser in each metre of row. Incitec Pivot warns that widening rows while keeping the same rate can cut plant numbers. GRDC suggests spreading seed and fertiliser more widely in the row, cutting in-furrow rates, or separating the bands.
• Placement: Soil Quality (WA) says high P rates should go at least 2 cm below the seed; Incitec Pivot says placing fertiliser below and to the side with at least 50 mm separation is the most effective way to avoid contact.

To size the risk, Incitec Pivot uses Seed Bed Utilisation: SBU% = seed row width (mm) ÷ row spacing (mm) × 100. A higher SBU% means a more dilute band and a higher safe rate. Look up your crop, soil and moisture in the manufacturer's current rate tables, and run a no-fertiliser test strip at sowing to see if your usual rate is hurting establishment.

DAP or MAP?

Incitec Pivot describes MAP (10% N, 21.9% P) as safer near the seed because it releases less ammonia, and DAP as best on acidic soils or where it is not in contact with seed, such as blends and pre-plant or broadcast use. Compare MAP and DAP side by side, read MAP vs DAP, or browse all phosphorus fertilisers. Always check with your agronomist before acting.