Being smart with nutrients

Bala Tikkisetty column for July

Being smart with nutrients

Bala Tikkisetty

We’re now well into winter, with early spring not that far away. Both are times when nutrients are most at risk of leaving farms due to high rainfall, reduced pasture growth, lots of stock urine being deposited, soil compaction and pugging.

That, as we know well, risks hitting farm production and damaging the health of waterways and groundwater.

Helping to handle those risks smartly, thereby boosting production while protecting fresh water, is what good nutrient management is all about.

A sound farm nutrient budget is one of the keys to this. It will show where fertilizer applications are inadequate, leading to a potential decline in the soil’s “nutrient status”. Conversely, it can indicate where too much input will result in a nutrient surplus and greater potential for losses to the environment.

So good nutrient management will keep nutrients cycling within the farm system and reduce losses to the environment to the bare minimum.

Some nutrients are more prone to loss than others, depending on the nature of the nutrient, soil type and climatic conditions.

Leaching through the soil – one of the biggest nutrient loss risks - can see the loss of the likes of nitrogen, potassium, calcium, magnesium and sulphur.

It occurs when water washes soluble nutrients through the root zone into deeper layers of the soil and they become inaccessible to plant roots. The leaching risk depends on various factors such as soil type, total rainfall, extreme weather events and the actual quantity of soluble nutrients present in the soil.

Don’t oversupply the soil with such types of soluble nutrients, especially not just before and during winter, as there is a very high risk of these getting washed out through the soil and lost from farm systems.

A good understanding of the processes and terminology involved with nutrient cycles is important for budgeting and management.

For nitrogen, one of the key nutrients that both grows grass and can harm waterways, there are two important processes - immobilisation and its opposite mineralisation.

Immobilization is the conversion of plant available nitrogen into organic forms.

Mineralization is the conversion of soil organic nitrogen into plant available forms such as nitrate and ammonium.

These processes are controlled by microbes, and the degree of their activity, in the soil.

Another point about nitrogen is that, generally, there will be an increase in nitrate leaching with increasing rate of nitrogenous fertiliser.

This highlights the environmental risk associated with too much nitrogen fertiliser use on farms. In one study, where 400 kilograms per hectare of nitrogen was used, the nitrate concentration in groundwater increased to an average value of almost twice the commonly-accepted recommended maximum for potable water (11.3 parts per million). In the same study, it was revealed that gaseous nitrogen losses, through de-nitrification and volatilisation, increased by approximately five times between zero nitrogen application and that 400 kg per hectare level.

Phosphorus loss, on the other hand, mainly occurs from erosion and run-off. Research has revealed that phosphorus losses will be high in soils with high Olsen-P levels and also on steep to rolling country. Managing these optimum levels and controlling soil erosion are keys to helping prevent this.

Overall, the ongoing challenge is to ensure our farming systems efficiently cycle nutrients. Smart nutrient management practices for all land uses and activities has the potential to bring about substantial improvements in the quality of our water resources and profits.

Waikato Regional Council is working with stakeholders to help farmers adopt good practices and supports the use of industry-developed codes of practices, particularly the Code of Practice for Nutrient Management, Fertmark and Spreadmark.

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