Agronomic Insights

Supporting better summer crop choices

Rain has been patchy in the northern cropping region so far, giving only a few growers the opportunity to consider planting early. While waiting for planting rains, Bede O’Mara, subtropical systems agronomist with Incitec Pivot Fertilisers, has put together the following guiding principles to help summer crop growers with crop choices.

“Moisture is still the biggest hurdle to overcome before summer crops can be planted, but growers can begin to plan by considering each paddock on its own merit and soil testing,” he said.

“Not all paddocks will be suitable for all crop types, whether that is due to subsoil constraints, disease or paddock and herbicide history.

“From a nutrition perspective, the best approach is to arrange segmented soil samples to check what nutrients are available and at what depth in the soil profile, as well as looking into whether problems like salinity, chloride and sodicity are lurking in the subsoil.”

Mr O’Mara also encouraged growers and their advisers to remember that long fallows can deplete arbuscular mycorrhizal fungi (AMF), which help plants better access immobile nutrients from the soil, such as phosphorus and zinc.

“Most summer crops have a high dependency on AMF, which require a host plant or root material to survive, so where paddocks have been clean for a long time, there’s likely to be lower spore survival rates,” he said.

Mr O’Mara said crops with very high dependency on AMF such as cotton and corn should not be ruled out after long fallows, but they would need extra support.

“Without AMF, crops will need higher phosphorus and zinc rates to ensure adequate availability to the developing seedling,” he said.

“Granulock^® Z is ideal for sowing with these crops, as it supplies good levels of phosphorus, other starter nutrients and plant available zinc in every granule.”

When it comes to salinity, Mr O’Mara said the summer crops most likely to be affected are mungbeans and corn, along with sudan grasses. More tolerant crops include cotton and grain sorghum, forage sorghum and millet.

Salinity can be measured in two electrical conductivity tests. The inherent soil salinity is measured using the 1:5 water test and the species sensitivity is calculated using the soil texture from the saturated extract (SE) test.

“Depending on the results of soil tests, salinity levels may rule out some sensitive crops known to have setbacks in germination or poor growth due to lower water availability when there are high concentrations of salts in the soil,” he said.

Mr O’Mara said forage sorghum or millet crops could produce high fodder yields in good growing conditions, but were very heavy users of nitrogen.

He said well-grown and managed forages rarely presented prussic acid problems, but graziers should take care not to introduce very hungry stock, or allow grazing of rank, overgrown, moisture-stressed or blue crops. Stock should be offered hay or stubble and sulphur lick blocks and be slowly introduced when grazing new forages.

After paddock or strip grazing forage sorghum or millet, he suggested soil testing the paddock prior to the next season to understand the redistribution of nutrients. Crops cut for hay or silage can be plant tested to assess nutrient removal.

“Bear in mind that potassium removal increases dramatically when produce is removed from the field,” he said.

Some summer crops are also sensitive to high chloride levels in soils and in irrigation water, for example, mungbeans.

“High levels of chloride can have similar affects to salinity in sensitive crops – burnt leaf tips, yellow and split leaves and poor growth,” he said.

“It may be a good idea to test irrigation water for chloride if you are planning on growing chloride sensitive crops.”

Soil structural constraints can also impede root development and plant uptake of nutrients and water.

Mr O’Mara encouraged growers and their advisers to check the sodicity of soils at the surface and in the subsoil by requesting an exchangeable sodium percentage (ESP) in their soil tests, particularly where corn or maize, sunflowers or mungbeans were planned.

“Sodicity at the surface can set back seedling emergence, while subsoil sodicity reduces the drainage capacity of soils, limiting nutrient availability to plants and aeration,” he explained.

Check before you sow

Crop	Crop development	AMF dependency	EC(se) sensitivity	Chloride sensitivity	ESP tolerance
Corn (grain)	Medium-slow Peak 30-70 DAS	Very High	Highly sensitive	Moderately tolerant	Sensitive
Corn (silage)	Medium Peak 30-70 DAS	Very High	Highly sensitive	Moderately tolerant	Sensitive
Cotton	Slow Peak 80-150 DAS	Very High	Tolerant	Tolerant	Semi-tolerant
Grain sorghum	Medium-slow Peak 30-80 DAS	High	Tolerant	Moderately tolerant	Semi-tolerant
Sunflowers	Quick Peak 20-60 DAS	High	Moderately tolerant	Tolerant	Sensitive
Mungbean	Very quick Peak 0-50 DAS	High	Very sensitive	Sensitive	Sensitive

He said other considerations such as the length of the growing season, availability of seed, nutrient uptake demands and current market prices should also be factored in.

“For example, sunflowers and mungbeans can be flowering within 60 to 80 days of sowing,” he said.

“They generally have a lower overall nutrient requirement than cotton or corn due to their lower yields, but quite rapid uptake requirements, so they need good upfront nutrition.

“The best approach, as with moisture, will vary paddock to paddock, so consider each paddock on its own merit.”

The Nutrient Advantage^® laboratory can provide soil test results before or after planting summer crops to help guide fertiliser decisions.

To discuss crop choice planning or nutrient planning for summer crops further, contact Bede O’Mara on 0417 896 377 or write to bede.omara@incitecpivot.com.au.