Various products broadly called 'wetting agents' can improve crop establishment on water repellent sands and water repellent gravels.
Wetting agents are often applied away from the seed (for example, using nozzles mounted behind the press wheels).
Some products, such as SACOA's SE14®, are recommended for application with or near the seed to form a band of moisture or 'wick' that gives the seed the best chance of germinating and accessing soluble nutrients.
Wetting agents commonly have pesticides, trace elements and liquid fertilisers such as urea ammonium nitrate (UAN) mixed with them.
Given UAN is a salt and can cause localised increases in ammonium and ammonia, there may be concerns that putting UAN with the agent near the seed will inhibit germination.
UAN has a relatively high salt index (63 to 71), albeit lower than ammonium sulphate (88) - a common ingredient in many Western Australian compound fertilisers - and muriate of potash (116).
When placed too close together, seeds and fertiliser can compete for moisture, causing the seed to dehydrate or die.
Localised toxic levels of ammonia/ammonium are probably of greater concern than UAN's salt index, as these nutrients can damage the seminal root, stunt the seedling and delay emergence. This means less-resilient plants have to face the rest of the season with just nodal roots.
Will mixing cause problems?
One WA Department of Primary Industries and Regional Development (DPIRD)-GRDC trial undertaken on a loamy forest gravel, and anecdotal evidence, suggests that applying 50 litres per hectare or less of UAN with the wetting agent is unlikely to inhibit emergence. The moisture the agent attracts may actually help reduce toxicity by diluting the salts.
Problems are more likely to occur with:
- rates greater than 50 L/ha of UAN with the wetting agent;
- wider row spacings. Wider rows mean more product is concentrated in the furrow, increasing the risk of toxicity. The effect could be halved where paired-row seeding boots are used;
- using granular fertiliser as well as the UAN. Higher rates of granular fertiliser applied closer to the seed increases the risk of toxicity issues. But the potentially negative effect of granular fertiliser plus wetting agent/UAN applied near the seed is probably no worse than that from granular fertiliser placed near the seed alone; and
- deep sandy soils. Trials so far have mixed results on deep sandy soils and more work is needed to work out why.
Tips for mixing and application
Recent DPIRD-GRDC research suggests placing the UAN and wetting agent mix within 20 millimetres of the seed can lead to better results. If you are worried about potential toxicity:
- in situations where plant establishment is more important than adding additional nitrogen at seeding, keep the nitrogen away from the agent on the bar - or apply it in a separate application;
- increase seeding rate or add more water to the wetting agent/UAN mix to increase the total volume applied;
- separate compound fertiliser from the seed;
- start on a tougher crop. Some crops are more susceptible to fertiliser toxicity than others. Generally, in order of most to least susceptible are canola, lentils, wheat, peas, barley and oats; and
- do a germination test and adjust the seeding rate as required.
GRDC Research Code PLT1909-001SAX
NOTE: This article was produced as part of the GRDC 'Maintain the longevity of soil contstraints investments and increase grower adoption through extension western region' investment.
Useful Resources: Soil wetting agents for water repellent forest gravel soils 2015 trial report; New generation soil wetters perform in forest gravels; New opportunities for soil wetting agents on repellent soils