Farm Advice: Important factors for soil nutrient retention

Soil Scientist Dr Gordon Rajendram offers advice around soil nutrient retention


There are many aspects to consider for healthy soil and pasture production. The appropriate soil testing can help you get it correct.

There are two key factors that drive pasture production. Firstly, there’s soil temperature measured at 10cm. At five or six degrees, pasture stops growing. The second is soil moisture. With soil moisture <25%, there can be a significant impact on pasture production.

Using a Field Calibrated Soil Test will help the farmer identify if they are achieving maximum pasture production, low productivity, or wasting money due to excessive use of a nutrient.

In addition, pasture species struggle on low pH soils due to increased aluminium in the soil solution, which slows root growth, resulting in less growth of herbage. New Zealand farmers are not applying lime particularly on hill country, so many soils are getting very acidic (<5.5pH), with soils getting down to as low as 5pH. However, with the right testing and planning, there’s room in every budget for both lime and fertiliser to reduce soil acidification.

Leaching is the loss of water-soluble plant nutrients from the soil, due to such factors as rainfall and irrigation. The amount of nutrients lost via subsurface flow is related to the amount of rainfall and drainage.


Plant essential nutrients exist as anions and cations, and the leaching of these anions and cations is conditional on various factors, including the amounts and form of nutrients applied in fertiliser, stocking rate, drainage, soil type, and extent of previous leaching.

Anion Storage Capacity (ASC) or Phosphate retention (P-retention) is a measure of the soil to remove P from the soil solution.

This is a key test, which is inherent of the soil type and should be included with every sampled area to determine its level.

This is exceptionally valuable information when bearing in mind the use of phosphorus and sulphur fertilisers. Research shows that 70% of sulphur applied in a soluble form is leached from New Zealand soils.

Phosphate is a key element for plant growth. Any phosphate applied needs to stay in the soil and not runoff as particulate P or leach into subsoils. Much of the loss is related to the soil type, climate, Anion storage capacity, and overuse of soluble forms of P.

Mitigating P losses

  • <40% ASC greater potential to lose P from soil
  • Apply a little bit often
  • Important to use the right product
  • Apply slow-release P & S products
  • Combo of water-soluble/slow-release P & S is best

It’s important to remember that fertiliser itself is only one aspect of a productive fertiliser management system.

If you don’t test and do it properly, you’re doing a disservice to your business. Ensuring that it’s applied at the right time, place, and rate is equally important.

Waikato-based soil scientist Gordon Rajendram PhD is considered one of New Zealand’s experts in soil fertility. He’s committed to helping New Zealand farmers get the most out of their soil so that their farm can work more proficiently, be sustainable while still increasing farm profitability. Dr Gordon Rajendram worked at AgResearch, Ruakura Research Centre, Hamilton. He has developed two field calibrated soil tests (N & S), which are used for agronomic advice in New Zealand, and his work on leaching has been included in the Overseer nutrient model.

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