Give seeds the best chance in a dry start – how to avoid fertiliser burn

Seeds can be damaged by fertiliser placed too close or at too high rates. Dry soils increase the risk of harm to germinating seeds from fertilisers. Take extra care with fertiliser and seed placement this year. The dry lead in to 2018 winter sowing means last year’s strategy might not be safe this year.

Use the seed damage calculator to check how much fertiliser to you can apply with seed through the same chute.

The calculator doesn’t address fertiliser placed below or to the side of seed. With separation of seed and fertiliser around 3–5 cm is usually enough distance to protect the seed.

Safe fertiliser rates with seeds depends on:

Soil texture and conditions

The risk of fertiliser damage increases with drier and sandier soils. Conditions that cause stress or slow germination prolongs fertiliser-seed contact. This increases the chance of damage.

Crop type

Canola and lentils are more sensitive. Wheat and barley are relatively tolerant. The order of sensitivity for crop species can vary for fertiliser type. In general the order from most to least sensitive in major grain crops is:

canola > lentil >  peas > oats > wheat > barley

Fertiliser type

Fertilisers can affect germinating seeds in at least two ways:

  • Salt Index

Most fertilisers are salts. Too much fertiliser salt can ‘burn’ the seedling or stop seedlings from absorbing water. Nitrogen and K fertilisers tend to have a higher salt index than P fertilisers. Sulphate forms tend to have lower salt indexes.

  • Ammonia formation potential

Free ammonia can be toxic to seed. Placing urea-containing fertilisers in-furrow is risky because they produce ammonia.  A fertiliser with polymer coatings or urease inhibitors may slow the rate of ammonia production enough to protect seed. These fertilisers are still considered risky to place near seeds.

Placement and machinery configuration

Row spacing

The safe rate of fertiliser per hectare increases as row space narrows – all else being equal. Closer row spacing ‘dilutes’ fertiliser over the length of row.

Twin chuting systems

Twin chuting systems separate seed and fertiliser.  Fertiliser is placed in bands to the side or below the seed bands. Usually separation of 3–5 cm is enough to protect seed.  

Seed Bed Utilization

The more scatter there is between seed and fertiliser, the more fertilizer can be safely applied. The concept of Seed Bed Utilization (SBU) addresses this factor. SBU is the proportion of row width occupied by seed row. It’s the seed row width divided by the tyne spacing or row width. 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?

The seed damage calculator includes several fluid fertilisers. As a general rule, use the same maximum N or P rates as for solid products, based on nutrient concentration. Treat urea/ammonium nitrate like urea. Treat ammoniated phosphoric acid the same as MAP.

Photo courtesy of the GRDC

This article is a revision of our 2016 post Give seeds the best chance by avoiding fertiliser damage.

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