Switching to Summer Crop


Many winter crop growers in the north are in the unfortunate position of making an unplanned switch straight to summer cropping due to the ongoing dry conditions.

Once that decision has been made and the winter crop sowing window has closed, growers and their advisers can start to plan ahead and readjust their fertiliser programs.

The nutrition needs of the upcoming summer crop are likely to be very different to the needs of a winter crop in a range of areas.

Planning will need to consider current subsoil moisture levels and estimates of rainfall required. New soil tests may be needed, particularly in assessing nitrogen and sulphur levels and the position of these nutrients in the soil profile.

Summer crops may need higher rates of phosphorus and zinc to combat long fallow disorder and are likely to have a higher potassium removal than winter crops.

With some attractive commodity prices available in a range of summer crops, it is well worth the time to fine-tune nutrition to optimise the yield potential and quality of the coming summer cropping program.

Know your nitrogen

Even if soil nitrogen tests were taken prior to the intended winter crop, it may be worth testing again.

The soil nitrogen status changes over time naturally with mineralisation, but these changes are difficult to predict. Temperature, moisture and biological factors all play a role.

Where pre-winter nitrogen applications have already occurred, re-sampling those fields is advisable.

Segmented sampling from the prescribed profile depths is best for understanding both the quantity and position of nitrate nitrogen in the soil profile. This may reveal that while nitrogen is adequate, it is at a depth where crops are likely to suffer deficiencies before they reach it.

The best time to soil sample is as close as possible to planting time, when all potential soil mineralisation and nitrification from fertiliser applications has occurred, while still leaving time for any nitrogen fertiliser additions that may be necessary.

Bear in mind that dry conditions slow the decomposition of stubble and crop residues.

When the rain returns, the processes involved in the decomposition of those residues may immobilise some nitrogen. Consequently, results from soil samples taken during this time may show lower soil nitrogen levels than those taken during more stable environmental conditions.

I have been monitoring soil nitrogen at numerous sites on the Darling Downs this year and have seen some large variations in values between sample times.

In general, paddocks that have been fallowed for the longest seem to give the most stable soil nitrogen test values over time, while those that have been recently harvested and are quite dry can be subject to large variations over short timeframes.

This is likely to be due to active nitrification and mineralisation processes, soil and biological responses to intermittent rainfall and soil temperature variations.

While nitrogen is a flexible nutrient, it’s best not to rely too much on in-crop applications, especially where high rates will be needed.

Growers have been known to be caught short when the season turns wet, or when large proportions of the farm are sown to the same crop on similar dates and it all needs nitrogen at once.

Be prepared by having a plan for those blocks that are likely to need nitrogen, considering options such as ®BIG N, granular urea, Green Urea NV®, Gran-Am®and EASY N®.

With the majority of the expected nitrogen requirement available up front, growers are then in a strong position to top up nitrogen as required in season if growth is better than expected due to rainfall and good management.

Around 50% of nitrogen used by summer crops is removed in grain, although this can vary with season type, grain yield and protein outcomes.

In sorghum, around 18 to 20 kgN/t of grain is removed. Nitrogen removal from legumes is much higher. For example, nitrogen removal from mungbean crops can be more than 40 kgN/t of grain.

Plan on phosphorus

The results of soil tests taken in the past three to five years will suffice for phosphorus decisions.

If new soil testing is needed, consider segmented surface (0-10 cm) and subsoil (10-30 cm) sampling, testing for Colwell P, BSES P and Phosphorus Buffering Index (PBI).

An opportunity may exist to apply some deep phosphorus bands in test strips if you suspect crops will be responsive. The soil profile is dry, so ripping shouldn’t affect stored soil moisture.

As well as higher potential yields, applying phosphorus can promote more even establishment, as well as flowering synchronicity which can allow easier management of issues like sorghum midge and ergot.

Pop-up rates of 3 to 5 kg/ha of phosphorus can assist the early setting of grain and panicle development.

Phosphorus uptake for most summer crops is around 1.6 times that removed in grain or lint. This equates to a total of 4.5 to 5 kgP/t of intended grain yield for summer cereals. Irrigated cotton requires around 30 to 40 kgP/ha, depending on the targeted yield.

The following phosphorus removal values have been recorded from summer crops:

  • Corn – around 2.9 kgP/t of grain
  • Sorghum – up to 5.25 kgP/t of grain, with an average of 3.7 kgP/t grain
  • Mungbean – 4.2 kgP/t of grain
  • Soybean – 4.7 kgP/t of grain
  • Cotton – 2.5 kgP/bale.

High yielding crops or those which are removed fully as silage or hay will take off nutrients in larger quantities.

Add some potassium

Previous soil test results may be used to guide potassium requirements. Where no data is available, or it is more than five years old, new soil tests should be taken.

The ammonium acetate potassium test in conjunction with the cation exchange capacity (CEC) gives advisers a good guide to likely potassium responses.

Also check on other constraints, like magnesium and sodium. These disruptive exchangeable cations can impede potassium uptake. If the magnesium percentage is greater than 25 or the sodium percentage is greater than 5, adequate potassium will be particularly important, especially in crops like cotton and grain legumes which have usually high uptake during peak demand.

As with phosphorus, the drier soils may offer a good opportunity to apply test strips of surface or deep placed potassium.

Where soil potassium reserves need to be improved, a pre-plant banded application of potassium using the narrowest row spacing possible allows growers to treat a large volume of soil in one application.

Nitrogen can be applied at the same time, bearing in mind the need to avoid the intended planting row position.

Once soil test levels have improved, Muriate of Potash can be applied by spreading followed by incorporation or pre-plant banding.

In the longer term, broadcasting manures may be able to provide total potassium requirements, depending on the source. These need to be incorporated to allow adequate quantities of potassium to be released prior to the next crop planting.

Summer cereals require about 17 kgK/t of grain for crop growth and sorghum can remove up to 7.25 kgK/t of grain.

Pulses remove proportionally more potassium in grain than cereals. Mungbean crops can remove 24.3 kgK/t of grain and soybean 16 kgK/t of grain.

Cotton removes modest amounts of potassium, at just 3.2 kgK/bale, but if yields are high the total can be significant.

Around 20 to 30% of total potassium uptake in summer crops is removed from the paddock.

Soil test for sulphur

Soil sampling for sulphur should follow the same depth increments as used in nitrogen sampling, as sulphate sulphur is mobile in the soil.

This will allow you to see whether the surface and shallow soil supply of sulphur will be adequate to avoid potential yield losses during the early crop growth stages.

Many northern subsoils contain naturally occurring gypsum at depth. If this is the case, sulphur testing is only needed in the surface layers, as it will always be sufficient at depth.

The mono-calcium phosphate (MCP) test has shown more robust results than the potassium chloride (KCl40) test in cropping soils with lower organic carbon levels, but either can be used to monitor soil sulphur levels.

If soil sulphur is low and applications are necessary, a fertiliser with some sulphate sulphur is most effective in the season of application.

Elemental sulphur from fertilisers will contribute to soil supply, but it must be converted to sulphate sulphur in the soil before it can be taken up by plants.

Because sulphate sulphur is subject to leaching, it can often be needed following favourable or excessive rainfall. In fact, only one significant grain response to sulphur has occurred at Incitec Pivot Fertilisers’ long-term experiment at Colonsay on the Darling Downs, and that was in the 1998-99 grain sorghum crop, following the big wet of 1998.

For sulphur critical crops like sunflower, pay attention to sulphur nutrition and ensure an adequate, season-long sulphur supply. Severe oil yield penalties (17-30%) can occur if sulphur is deficient, but nitrogen is adequate.

In summer crops, the total sulphur uptake is four times that removed by grain or produce.

Removal is around 1 to 2 kgS/bale in cotton, 3.8 kgS/t of grain in corn, 2.8 kgS/t of grain in sorghum, 2 kgS/t of grain in mungbean and 3 kgS/t of grain in soybean.

Think zinc

Many summer crops will require zinc applications this year to achieve their potential.

Soil tests can be used to check on the need for zinc. These are more reliable when used in conjunction with plant tissue tests, as is the case for all micronutrients.

Remember that grain sorghum is one of the higher zinc removing crops. Removals of up to 70 gZn/t of grain have been recorded.

Grain sorghum is followed by corn, sunflower and sorghums, which can remove 25 gZn/t of grain and mungbean at 16 gZn/t of grain.

Another important consideration is arbuscular mycorrhizae fungi (AMF), previously known as VAM.

A good community of AMF in the soil improves the uptake of poorly soluble nutrients, like phosphorus and zinc.

AMF levels can be severely depleted by long periods of fallow, such as those experienced in droughts or after the growth of non-host crops.

Generally, summer crops have a high to very high AMF dependency, while winter crops have a lower dependency. Supplementation with a phosphorus and zinc fertiliser in low AMF situations could avoid unexpected crop setbacks and yield reductions.

Take stock of seasonal considerations

Drier conditions can also exacerbate subsoil constraints like chloride or salinity.

Soil samples segmented to at least the rooting depth of the intended crop can help identify and locate any subsoil problems.

A hostile subsoil may not allow full water and nutrient extraction, prompting a downward revision of yield estimates.

It may also help guide crop choice. In some of the paddocks I have been monitoring this year, subsoil chloride levels have increased to the point where planting a sensitive crop like mungbean would not be appropriate.

These subsoil constraints tend to increase in concentration when rainfall is sparse, but they can be diluted again once better seasons return. It’s better to know before you sow.

Of course, any growers applying fertilisers between now and summer crop planting should be mindful of placement.

Unreacted nitrogen bands placed too close to the intended planting row are a recipe for disaster.

The deeper into cotton soils, the better, as far as placement goes. Nitrogen bands should be at least 10 cm to the side of the plant row and at depths below the bottom of the irrigation furrows.

For dryland or bed crops, ensure nitrogen fertiliser is placed greater than 10 cm below and 10 cm to the side of the intended planting line.

If fertiliser is to be applied in contact with the seed at planting, growers and advisers should use a seed bed utilisation percentage (SBU) calculator to ensure the crop is not at risk of seedling damage from osmotic damage or ammonia burn. See the calculator (http://seed-damage-calculator.herokuapp.com/) or for more information, read Incitec Pivot Fertilisers’ guide on seed safety (https://www.incitecpivotfertilisers.com.au/~/media/Files/IPF/Documents/Agritopics/SBU.pdf).

Crops are at greater risk in dry soil conditions, on wide row spacings, where there is minimal soil disturbance from disc planters and where higher rates of fertiliser are used.

Incitec Pivot Fertilisers and the Nutrient Advantage® laboratory can help growers and advisers with soil testing and fertiliser requirements in the switch to summer cropping.

The laboratory can provide timely, accurate and accredited results to allow you to make the right decisions on a range of quality fertilisers, including Granulock® Z, Granulock Z Extra, Gran-Am and Granulock BIG Z for blending. Our custom blending service can match exacting nutrient requirements using quality ingredients.

We look forward to helping growers and advisers as they plan a successful summer crop ahead.

For more information and advice, feel free to give me a call on 0417 896 377 or contact me at bede.omara@incitecpivot.com.au.

Central Downs Grower Group (2017) Grain nutrient removal data. Unpublished.
Dowling, C. (2015) Pacific Seeds Corn Nutrition Guide. Pacific Seeds, Toowoomba.
Dowling, C. (2015) Pacific Seeds Grain Sorghum Nutrition Guide. Pacific Seeds, Toowoomba.
Lester, D. & Dowling, C. (2001) Grain sorghum – a yield response to N or S? In Science and Technology: Delivering results for agriculture? Proceedings of the 10th Agronomy Australia Conference, Hobart.
O’Mara, B. (2016) Nitrogen and phosphorus learnings from 14 summer crops in a sub-tropical continuous cropping system on a vertosol in Southern Queensland. Proceedings of the 2016 Australian Summer Grains Conference, Gold Coast.
O’Mara, B. (2016) Better dryland cotton yields with phosphorus. Incitec Pivot Fertilisers.
O’Mara, B. (2017) More money from mungbeans with nitrogen. Incitec Pivot Fertilisers.
Rochester, I. (ed) (2001) Cotton Nutri-pak. Australian Cotton Cooperative Research Centre, Narrabri.
Rochester, I. & Constable, G. (2006) Nutrients removed in high-yielding cotton crops. The Australian Cottongrower, June-July, page 26.
Walker, C. & O’Mara, B. (2015) Lessons learnt about nitrogen and phosphorus from a 30 years study in a sub-tropical continuous cropping system on a vertosol. In Building Productive Landscapes. Proceedings of the 17th Agronomy Australia Conference, Hobart.