Too wet to fertilise – a good problem to have

06 October, 2010

The ongoing wet spring across Australia’s cotton growing regions and subsequent increase in crop area is causing headaches for growers trying to fertilise land for the upcoming crop.

According to CSIRO research scientist Dr Ian Rochester, there are several options available to growers to apply fertiliser close to planting or in-crop.

Speaking on CSD’s Web on Wednesday broadcast, Dr Rochester advised growers to develop a fertiliser plan for each field based on pre-plant soil tests.

“It’s always been a good idea to do some soil tests before you put any fertilisers in the soil to make sure you know how much nitrogen, phosphorus, potassium the soil requires to maximise yield.”

“Then you develop a fertiliser plan to work out how much, how often you’re going to put those fertilisers on and try to adhere to that.”

“To back that up you might want to do some leaf tissue testing just to make sure you’re on target with that fertiliser plan. Petiole tests are great for nitrogen. Leaf tests for all the other nutrients as well as nitrogen so I prefer people to do leaf tests rather than petiole tests these days,” he said.

Following several years of drought, many people will be planting into fields that have been fallowed, allowing them to naturally mineralise some soil nitrogen which according to Dr Rochester will reduce the urgency to apply fertilisers.

“If growers haven’t got their nitrogen on at this stage, I wouldn’t be panicking because most crops would be able to get through to about Christmas time without having any nitrogen applied to those fields that are already N-fertile,” he said.

For fields that grew cotton last season or other crops on a short fallow, Dr Rochester said more immediate attention would be required.

“If your soil tests tell you that you are in a fairly N-deficient situation, as may occur with back-to-back cotton, you’ll need to apply some N pre-plant and more in-crop by side-dressing etc. In more fertile fields, you may not need to apply N fertiliser until the crop’s demand for N increases through flowering and boll-fill. It is important to have all of your N applied before early-boll fill.”

“There is a great danger in over-use of N fertiliser – our surveys over the past 4 years have shown that most cotton fields are over–fertilised with N by 25 percent, causing excessive vegetative growth, reducing yield and gross margins,” he said.

For growers attempting to apply fertiliser close to planting, Dr Rochester warned to place it away from the seed to avoid burning the roots of the emerging plants.

“The ammonia becomes quite toxic to developing roots so you can kill seedlings by putting N fertilisers close to those developing seedlings so you need to place you ammonia deeply – 30cm is good, and away from the plant line.”

“Normally we’ll put our anhydrous ammonia or urea 20 to 30cm deep – as deep as possible and at least three weeks before you want to plant. – that avoids any danger,” he said.

Dr Rochester said there were many ways to apply fertiliser in-crop but it depended what equipment growers have available and what products they use.

“If they’re reliant on anhydrous ammonia there are side-dressing options with that. If they’re using urea, they can side-dress with that or water-run urea with the first couple of irrigations to supply the nitrogen requirements of those crops.”

According to Dr Rochester, potassium fertiliser can be applied any time early in the season and watered-in with an irrigation. However, he said this was not possible with phosphorus because is not mobile in the soil.

“If you’re in that deficient level of soil phosphate then you should have some phosphate on – ideally at planting or close to planting but not too close to the seed because of the ammonia burn from MAP.”

6 October 2010

Further Information:
Dr Ian Rochester, CSIRO 0267 991500
See the full interview