Nutrient cycles (Nitrogen & Phosphorus)

Nitrogen and phosphorus are essential nutrients for plant growth and is cycled through agricultural and horticultural practices and natural processes. Here's an outline of these processes on farms:

Fertilizer Application: Farmers apply nitrogen and phosphorus-rich fertilizers to fields to supplement soil fertility and enhance plant growth.

Animal Manure: Livestock waste contains organic phosphorus and nitrogen. When applied to fields as manure, this becomes available to plants after decomposition.

Runoff and Erosion: Rainwater can wash away excess phosphorus and nitrates from fields, carrying it into nearby water bodies through runoff. Soil erosion can also transport phosphorus and nitrates from fields to waterways. In aquatic ecosystems, excessive phosphorus and nitrogen can promote the growth of algae blooms.

Algal Growth: Algae blooms deplete oxygen levels in waterways, potentially harming aquatic ecosystems in a process called eutrophication.

Plant Harvest and Consumption: When crops are harvested, nitrogen and phosphorus is removed from fields which requires replacement through application of fertilizers.

Crop Rotation and Cover Crops: Farmers may use crop rotation and cover crops to manage nitrogen and phosphorus levels in the soil, prevent nutrient depletion, and reduce the need for excessive fertilization.

Efficiency and Environmental Impact: Efficient fertiliser management is important to minimize nutrient loss, which can lead to air and water pollution, including the formation of greenhouse gases and eutrophication in water bodies.

Balancing nitrogen and phosphorus inputs, plant needs, and environmental impacts is crucial in sustainable farming practices. Proper management can enhance crop productivity while minimizing negative effects on surrounding ecosystems.

Simplified phosphorus cycle
Source: DairyNZ

Management of nutrients

Balanced nutrient management aims to optimize agricultural productivity while safeguarding the environment and maintaining long-term soil health. Customizing strategies to specific farm conditions and local ecosystems is crucial for sustainable nutrient management.

  • For best practice of N management, there are some variables to consider.
    • Moisture content of soil – varies the mineralisation on N.
    • Soil Temperature – correlates with soil moisture content.
    • Seasonal variations – Plants grown in winter will grow slower than the summer months, so less nitrogen is used. Fertiliser application is at a higher risk of leaching in winter months due to increased rainfall which can affect N absorption.
    • Plant species N absorption ability – Some species have an increased ability to absorb N which depletes the soil of it for future crops. Weeds, annual crops and some brassica species can affect livestock if nitrate levels are too high so timing of nitrogen-rich fertiliser should be considered.
    • Plant growth rates – Plants with faster growth rates need N more immediately then slower growing species.
  • For best practice of P management, remember the 4 Rs: Applying the right P fertiliser in the right places at the right time and rate can reduce losses. With the four Rs, recently applied P fertiliser is responsible for less than 10% of farm P losses. If this advice is not followed, losses of 30–80% of P applied have been recorded.
    • When and where to apply phosphorus (P) fertiliser
      • Apply annual P fertilisers when the probability of surface runoff or leaching is lowest.
      • Always avoid applying P-based fertiliser if significant (>10 mm) rain is forecast within the next 14 days.
      • Runoff can also occur in dry times of year when soils cannot absorb water. Avoid applying P-based fertiliser at these times (common from midsummer to autumn on north-facing slopes in the eastern regions of both islands, and in the Central Plateau) as surface crusting can cause increased runoff and fertiliser P loss.
      • Target fertiliser good management practices (the four Rs – the right fertiliser in the right place at the right time and rate) to critical source areas (such as land with a moderate to high slope, regular and frequent rainfall, or with soil that cannot retain P) to make them even more effective. In one case P losses were halved with minimal impacts on profitability.
    • Type of P fertiliser to apply
      • When runoff or leaching is unavoidable (for example, where rainfall is frequent or heavy enough to produce runoff or leaching events year-round) consider applying a partially water-soluble P fertiliser, such as reactive phosphate rock, Serpentine Super or products coated with a polymer to control the rate of P release.
      • Data shows that at a catchment scale, reactive phosphate rock can potentially decrease P in streams by up to 38% compared to superphosphate. Other products, such as Serpentine Super, can also decrease losses by about 20–30% compared to superphosphate.
      • A cautionary note: Data shows that products such as reactive phosphate rock only produce the same amount of pasture as superphosphate when soil pH is <6 and rainfall is >800 mm.
      • In ideal conditions, when transitioning to reactive phosphate rock from single or triple superphosphate, replace a third of the superphosphate with reactive phosphate rock each year: 33% in year one, 66% in year two, and 100% in year three.
  • This is a commonly used soil test used to measure the phosphorus in different types of soil. This test provides information about how much phosphorus is currently accessible to plants. From this test, farms can assess how much P is needed to maintain productivity.
  • Depending on land use, P requirements can change so using the Olsen P test is a more accurate way to decrease P leaching.
  • Good management practices
    • Farm environment plan (FEP)
    • Cultivation and soil structure
    • Ground cover
    • Sediment, phosphorus and bacteria
    • Nutrient management
    • Irrigation and water use
    • Animal feed
    • Farm effluent and wastewater management
    • Intensive grazing
  • Riparian strips planted near waterways to reduce leaching. Native species are ideal as they generally have a deep root system so filter runoff more efficiently.

Managing Nutrient Loss

Nutrients are naturally found in the soil. Some land management practices can lead to excessive loss of nutrients to waterways. Increased nutrients in waterways encourages algal growth, deteriorate water quality, and renders it unfit for drinking and recreational use. Land management practices have improved significantly over time, to reduce the loss of nutrients from the soil. Nutrients are vital for plant growth and have a fertiliser value. Holding onto them in the soil is the best outcome economically and environmentally.

Sites at risk of losing containments, typically bacteria, sediment, phosphorus, and nitrogen, are considered critical source areas (CSAs). Managing these sites can reduce the amount of topsoil loss and containments in waterways. DairyNZ's critical source areas page outlines the potential management options that could be implemented to manage CSAs.