Straw Management in Dry Conditions

As harvest progresses under warm and dry conditions, straw management is becoming a critical consideration for growers. Shorter crops and reduced straw volumes are not the only challenges—nutrient dynamics, particularly potassium (K) behaviour, are shifting in response to the weather.

Elevated Potash Levels in Straw

In dry years, potassium is less likely to be remobilised and washed out of straw before harvest. This leads to elevated potash concentrations remaining in the straw, by as much as double the amount in dry years, which can have several implications:

• Nutrient Lock-Up: Without sufficient rainfall, potassium remains bound in the straw, delaying its return to the soil and potentially affecting nutrient availability for the following crop.
• Straw Value: High potash content reduces the combustion quality of straw, making it less suitable as a biomass fuel.
• Soil Fertility: When straw is removed from the field, the associated potash is also lost. Unless replaced, this can lead to long-term deficits in soil potassium, particularly in deeper layers where it is less accessible to crops.

Visual Indicators and Leaching Potential

Some straw has shown a pink tint, which some suggest is a result of potassium lock-up whilst others suggest it may be linked to anthocyanin accumulation—a plant pigment used by the plant in dealing with a stress response, triggered by drought and excess light (drought and excess light compared with the 5-year average in East Anglia is shown in the figure below). While research is limited, this could serve as a visual cue for elevated nutrient concentrations in straw.

Figure 1. Annual weather data for the East Anglia region in July, highlighting very low rainfall and very high sunshine hours compared with the 5-year rolling average. This weather dashboard can be accessed in the ‘interactive zone’ of your membership area.

Studies suggest that up to 50–60% of the potassium in straw can be leached back into the soil within 40 days, provided there is sufficient rainfall. However, in dry years, this natural leaching process is significantly reduced. Figure 2 below shows results from a scientific article investigating the effect of crop type of potassium leaching following rainfall events. The results provide further detail as to how much is ‘sufficient rain’, suggesting that potassium can leach back into the soil from <10mm rain, and continue leaching beyond 60mm of rain. Triticale providing the most amount of potassium leaching.

Figure 2. Considerations for Unharvested Plant Potassium, Springer Nature, 2020. 

Chopping straw: the pros and cons.

The benefits of chopping straw and leaving it on the ground include:

• Reducing soil erosion
• Shading soil, contributing to moisture retention which in dry years is very beneficial
• Nutrient retention
• Maintaining or improving soil organic levels
• Reducing the risk of compaction.

Some of the disadvantages include:

• Nitrogen tie-up which can lead to crop deficiencies in early growth stages
• Disease carryover
• Slug pressure
• Issues with establishment
• Potential for additional income from straw sales if baled.

An 11-year field experiment investigated straw retention versus no straw retention and its effects on soil health parameters. Straw retention increased macro-aggregates (> 0.25 mm), porosity, field water capacity (FWC), soil organic carbon (SOC) storage, total nitrogen storage, microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) by 17.3%, 3.2%, 13.0%, 5.5%, 3.2%, 15.5%, and 13.8%, respectively.

This was explained by the fact that straw forms a barrier of soil-straw-atmosphere which prevents direct wind and rainfall erosion. This reduces structural compaction. Additionally, the straw can slow down soil evaporation rates which increases soils’ water holding capacity. An example of these results are shown below in Figure 3. Porosity and field water capacity increased significantly more (shown by different letters) with retained straw than when straw was removed.

Figure 3. Example of some of the results from the 11-year field experiment investigating straw retention effects on soil health. TNS = no straw, TS = straw retained (the red legend entry relates to straw retention and reduced mineral fertiliser application). FWC = field water capacity. 

Improvements on physical properties were also attributed to an improvement in chemical and biological properties because the physically improved soil increased  aeration and water conditions for soil microbes to feed, reproduce and store carbon and produce plant available N.

Practical Recommendations

• Analyse Straw and Grain: Testing can help quantify nutrient offtake and inform fertiliser planning.
• Factor in Nutrient Value: When selling straw, consider the value of the nutrients being removed, especially potassium. The Potash Development Association has a phosphate and potash deficiency correction and nutrient offtake calculator available here. It takes into account crop, yield, soil tyle, desired nutrient index and provides an estimate on deficiencies.
• Adjust Cultivation Plans: Be mindful of nutrient dynamics when deciding whether to chop or bale straw, especially in dry years. Chopping returns nutrients to the soil, while baling removes them.