Bala Tikkisetty: Rains take nutrients from farming soils

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Winter and early spring are times when nutrients are most at risk of getting lost from farms due to high rainfall, reduced pasture growth, the huge amount of urine deposition, soil compaction and pugging.

Rivers, streams and wetlands are important natural ecosystems because they provide water for productive land use as well as help clean up the negative impacts of urban and rural activities, particularly pollutants that flow from the land.

A farm nutrient budget is a valuable indicator of the status of nutrients in a farm system. It indicates where fertiliser applications are inadequate and leading to a decline in the soil nutrient status. Conversely, it can indicate excessive inputs which result in a nutrient surplus and greater potential for losses to the environment.

The objective of nutrient management is to keep nutrients cycling within the farm system and to keep losses to bare a minimum. Most farmers know that some nutrients are more prone to loss than others, depending on the nature of the nutrient, soil type and climatic conditions.

Nutrients are getting lost from the farm system through various channels, such as produce (milk, meat, silage, hay, wool, vegetables and crops), atmospheric loss and leaching. Depending on the production levels, these figures can vary greatly between farms. Nutrient budget will provide all this information.

Nutrients, such as nitrogen, potassium, calcium, magnesium and sulphur can also be lost by a process known as leaching. Leaching occurs when water washes soluble nutrients through the root zone into deeper layers of the soil and become inaccessible to plant roots. The leaching risk depends on various factors such as soil type, total rainfall and extreme events, as well as the actual quantity of soluble nutrients present in the soil.

Due to the very high risk of soluble nutrients getting washed out through the soil and lost from the farm systems, it is recommended you do not oversupply the soil with soluble nutrients, especially before and during winter.

The high risk winter and early spring period requires careful planning and understanding of nutrient cycles to reduce the danger of nutrient inefficiencies.

A good understanding of the processes (and the terminology) in the nutrient cycles is important for nutrient budgeting and management. For example, in the nitrogen cycle there are two important processes, immobilisation and its opposite mineralisation. Soil biology plays an important role here, as these processes are microbially mediated and hence their speed is determined by the microbial activity in the soil. Plants cannot utilise organic nitrogen, so it must be first broken down to mineral nitrogen. Mineralisation occurs as the result of action by non-specific fungi and bacteria but the process of nitrification occurs as the result of two specific bacteria, nitrosomonas and nitrobacter.

Generally, there will be an increase in nitrate leaching with increasing rate of nitrogenous fertiliser.

This highlights the environmental risk associated with high (over and above agronomic requirement) nitrogen fertiliser use on farms.

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.

The challenge is to develop farming systems that efficiently cycle nutrients.
Adoption of good 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 industry-approved practices, particularly the Code of Practice for Sustainable Nutrient Management, Fertmark and Spreadmark.