Germinating seeds can be damaged by fertiliser placed too close or applied at too high rates. You can check how much fertiliser you can safely drill using the seed damage calculator. The amount of fertiliser that can be safely placed with seed depends on:
Overall canola and lentils are particularly sensitive, while wheat and barley are relatively tolerant. The order of sensitivity for crop species can vary based on fertiliser type. In general the order (from most to least sensitive) in major grain crops is:
canola > lentil > peas > oats > wheat > barley
Fertilisers can affect delicate germinating seeds in at least three ways:
Salt Index: Most fertilisers are salts, which can affect the ability of the seedling to absorb water. Too much fertiliser salt can ‘burn’ the seedling. Desiccation results from an osmotic effect. Most common N and K fertilisers have higher salt index than P fertilisers, and sulphate forms tend to have lower salt indexes.
Ammonia formation potential of fertiliser: Free ammonia can be toxic to seed. Placement of urea-containing fertilizers in-furrow is usually not advisable. In some cases UAN is applied successfully in-furrow, but there is a risk in this practice. Polymer coatings or urease inhibitors that slow the rate of ammonia production make these products less likely to cause crop damage.
Soil texture and conditions
The risk of fertiliser damage increases with lighter soil texture (sands) and in drier soil conditions. Environmental conditions that induce stress or slow germination (e.g., cold temperature) prolong fertilizer-seed contact and thus increase the likelihood of damage.
Placement and machinery configuration
The type of point used and the spacing between the drill rows affect the concentration of fertiliser near seed and so the likelihood of damage:
Row spacing: The safe rate of fertiliser per hectare increases as row space narrows, all else being equal. Closer row spacing has the effect of diluting fertilizer over the length of row.
Twin Chuting systems: Separate seed and fertilizer delivery chutes, either to different tynes or to different parts of the same tyne give separation of seed and fertilizer. Fertiliser is placed in bands to the side or below the seed bands – 3-5 cm of separation is usually adequate to provide crop safety.
Seed Bed Utilisation: The more scatter there is between seed and fertiliser in the seed band or row the more fertilizer can be safely applied. The concept of Seed Bed Utilization (SBU) is used to address this factor. SBU is the seed row width divided by the tyne spacing or row width – it’s the proportion of row width occupied by seed row. The wider the seed row for a specific row width the greater the SBU. As SBU increases so does the safe rate of in-furrow fertilisation. Tables for fertiliser/crop combination thresholds are available from the IPNI website.
What about fluid fertilisers? There are few guidelines for in-furrow fluid fertiliser application, although the seed damage calculator does include several fluid fertilisers. As a general rule apply the same rules for maximum N or P rates as for granulated (solid) products, based on nutrient concentration. So urea/ammonium nitrate can be treated as essentially the same as urea, and ammoniated phosphoric acid the same as MAP.
Rob Norton, IPNI, Graeme Sandral, NSW DPI.
Photo courtesy of the GRDC