Impact of rotational sequences on nutrient balance

Key messages

• Crop species and their sequencing contributed differently to soil fertility and nutrient balance.
• Lupin contributed lower nitrogen balance than field pea due to higher grain nitrogen export from lupin.
• Canola was the only crop which resulted in significantly higher potassium balance compared to other treatments.
• Oaten hay contributed to lowest nitrogen and potassium balances than other crops.
• Large negative potassium balance occurred when oaten hay was grown after oaten hay.
• Green or brown mature consistently improved nitrogen, phosphorus and potassium balances compared to other crops. However, loss of income may deter adoption of green manure as break crop.

Background

• Soil type: Grey shallow sandy duplex (Chromosol)
• May-Oct rainfall: 350mm

Nutrient cycling through crop residues is an important component of nutrient supply to crops, but has not been studied extensively in grain production in Western Australia. This led us to investigate the effect of the choice of break crops, including pulses and canola, to grow in sequence with cereals with residue retained, on soil fertility aspects crucial to both agronomic and economic success of crop sequences. We monitored soil fertility over three years to understand how soil responds to crop sequences and whether soil fertility declined or improved.

Experimental treatments and method

A crop sequencing experiment was designed in a strip-plot matrix factorial of 10 crop treatments. The treatments were typical of cropping practice in the Great Southern region of WA:

• wheat
• wheat with fluquinconazole fungicide seed treatment, wheat+J and wheat-J)
• barley (B)
• oat for grain (O)
• oat for hay (oaten hay, OH)
• canola (C)
• narrow-leafed lupin (L)
• field pea (FP)
• green manure (oats plus vetch, GM)
• chemical fallow (F).

In the first year, the crop treatments were applied in strips of 10 metres and were arranged as a randomised complete block design with four replicates. In the second year the same 10 crops were seeded perpendicular to treatments applied in year one to generate 100 different crop sequences. In the third year, wheat treated with fluquinconazole fungicide was sown over all plots.

Partial nutrient balance for nitrogen, phosphorus and potassium was calculated for all treatments in all years from nutrient inputs as fertiliser and from recycled residues; and nutrient removal from grain or oaten hay. The nutrient removal in grain or hay was calculated based on measurements of weight and nutrient chemical analysis of the grain. The nitrogen fixation estimates from legume treatments and losses of nutrients due to leaching or chemical transformations and the nutrient input from rainfall were not taken into consideration.

In the first year, the crop treatments were applied in strips of 10 metres and were arranged as a randomised complete block design with four replicates. In the second year the same 10 crops were seeded perpendicular to treatments applied in year one to generate 100 different crop sequences. In the third year, wheat treated with fluquinconazole fungicide was sown over all plots.

Partial nutrient balance for nitrogen, phosphorus and potassium was calculated for all treatments in all years from nutrient inputs as fertiliser and from recycled residues; and nutrient removal from grain or oaten hay. The nutrient removal in grain or hay was calculated based on measurements of weight and nutrient chemical analysis of the grain. The nitrogen fixation estimates from legume treatments and losses of nutrients due to leaching or chemical transformations and the nutrient input from rainfall were not taken into consideration.

Conclusions

Crop species and their sequencing contributed differently to soil fertility and nutrient balance. For example, inclusion of field peas, lupin and green mature in crop sequences improved inorganic nitrogen content and balances whilst inclusion of canola improved potassium fertility and balances compared to cereal-only sequences, meaning that growing these crops in sequence with cereal cereals can reduce the amount of fertiliser needed in subsequent crops.

It was also evident that green or brown manures may provide feasible alternatives to the use of break crops in cereal systems in improving soil fertility, although the loss of immediate income can be prohibitive in adopting green or brown manure in sequences. Fallow in this study also resulted in increased inorganic nitrogen, but due to immediate loss of income, like with green manure, it may not be an attractive option to many growers unless soil moisture conservation, diseases and intractable weeds are serious issues are to deal with. Oaten hay is one of the most adopted sequence crop in the south-western agricultural region of Western Australia due to its agronomic and economic benefits. However, the negative potassium balance that usually occurs after oaten hay crops does lead to a depletion of soil potassium.

The study suggests that if the goal is to improve soil fertility and better manage weeds and diseases for long term sustainability, growers should consider using break crops in their crop sequences. Enhancing the capacity of soil to supply a significant portion of the nutrients required by following crops can lead to fertiliser savings and also reduce the risk of nutrients losses to the environment. It is anticipated that research findings from this study will provide farmers with crop choices that fit in their individual needs to meet their long-term strategy for sustainable cropping systems.