How to account for stubble nutrients next season

Stubbles can be a source of nutrients for next season. Different crops grown under different conditions have varying amounts of nutrients in their stubbles. Nutrient budgets can consider the amount and type of stubble.

How much stubble?

Stubble loads can be estimated from grain yield and the amount of growth. For cereals, there is usually a ratio of grain to stubble of 1:1.5. So a 4 t/ha wheat yield means around 6 t/ha of stubble. This is a harvest index of 0.4. The Harvest index is the ratio of grain to total biomass (the stubble plus the grain). For canola and some legumes, the ratio is 1:2. A 3 t/ha canola yield comes with around 6 t/ha of stubble. That’s a harvest index of 0.3.
With good finishes, there’s a bit more grain relative to stubble, and the harvest index comes up a bit. Droughted or frosted crops tend to have less grain relative to stubble and lower harvest index scores.

Nutrients in stubble

Depending on the crop, stubbles contain different amounts of nutrients. The table below gives estimates of the nutrients present in crop stubbles. It comes from the Canola best practice management guide for south-eastern Australia.

Crop Nutrient (kg/ha per tonne of stubble)
Nitrogen (N) Phosphorus (P) Potassium (K) Sulfur (S)
Canola 10* 2 26 3.2
Wheat 8 0.7 21 1.5
Barley 7 0.7 18 1.5
Oats 7 0.6 18 1
Lupins 10 0.4 16 2.5

* The amount of N in canola stubble varies with time of windrowing. The value of 10 kg N/t is at the high end of the range. More mature crops might have 5 kg N/t.

When are stubble nutrients released?

Retaining stubbles for soil fertility is a long-term strategy. The rate stubbles break down is influenced by:

  • crop type – legumes are fastest, cereals slowest, canola in the middle
  • soil moisture – wetter is generally faster
  • Temperature – warmer is generally faster
  • degree of incorporation into soil – more mixing leads to faster breakdown
  • carbon (C): nitrogen (N) ratio – lower C:N breaks down faster
  • other nutrients present – sufficient nutrient speeds breakdown
  • the amount of stubble – more stubble takes longer.

What is the fate of these nutrients?

Soil organic matter includes charcoal, humus, microbes and decomposed organic residues less than 2mm. Microbes form organic matter by breaking down the stubble, which is mostly carbon. The microbes need adequate amounts of N, phosphorus (P) and Sulfur (S). Most stubbles have a low C:N ratio. All the nitrogen in the stubble is consumed by the microbes to break down only part of the stubble.  In rough terms, soil microbes need an additional 3 to 5 kg N from the soil for each tonne of stubble. The tie-up of soil N in the breakdown of stubble is called immobilisation. It’s likely that most of the N in cereal and canola stubbles will be tied up in the short term – months rather than weeks.

Most of the P, K and S in crop residues are present in soluble forms. They get washed out of the stubble into soil with rain, soon after crop maturity. So most of the nutrient release occurs in the first month. These nutrients may not be available in the short term (several months) if summer weeds take them up.

Positive Priming

Positive priming is when crop residues mobilise soil nutrients.  Stubbles can interact with soil organic matter to increase mineralisation and nutrient availability. For example crop residues with higher C:N ratio such as grain legumes can increase soil mineral N by 18 +/- 9 kg N/ha for each t/ha of grain harvested (Peoples et al 2017).

Studies on P and S have found that P bound to the soil may be mobilised by working crop residues into the soil. In South Australia, more P was detected in the crop and soil when stubble was incorporated into the soil. Canola released about 20% more sulfur (S) than wheat. Most of the nutrient release occurred in the first month after stubble incorporation. After four months, up to 30% more nutrients were released.

Stubbles and nutrients

Retaining stubbles is an important part of a crop management system, to protect soil and reduce erosion risk. Managing nutrients in these systems is part of a long-term strategy. Some nutrients – particularly N – get tied up in the short term, but become available later in the cropping cycle.

Burning stubbles results in nutrient losses. About 80% of N,  44% of P, 40% of K, 50% of S and 80% of carbon from stubbles is lost. Over the longer term, the losses add up, becoming a significant difference between burnt and retained stubble systems.

This article was updated on 29/11/17

More

ExtensionAUS

When do retained stubbles increase the need for nitrogen?

What happens to urea with high residue loads?

What happens to nutrients when stubbles burn?

 

GRDC

Canola best practice management guide for south-eastern Australia

Managing stubble

 

Was this article useful? Take our quick survey and let us know!

Share this:

Leave a comment