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The 'soil fertility bank'

soil The concept of a ‘soil fertility bank’

  • It’s an easy concept to think of the soil as a bank.  If you remove nutrients in products sold off the farm, then you need to replace them to keep a state of ‘nutrient balance’ in the bank.
  • Clearly on a highly stocked dairy farm with milk going off the farm every day, more nutrients are being removed than on a beef and sheep farm with only lamb and wool sales removing nutrients a few times a year. 
  • There is also transfer of fertility within a farm from one area to another.
  • A key point is that there’s no point in having nutrients in the bank that are not being used. 
  • Your money invested in nutrients is locked up, and may not be retrievable when you want them, and the excess nutrients in the soil may leach out and pollute the environment.
  • But be warned – not everyone in the various factions concerned about the soil likes this concept of the ‘soil bank’.

Nutrient budgets

  • These have become an important requirement of fertiliser programmes.
  • A ‘nutrient budget’ is where you measure the nutrient Inputs and Outputs, and see how things balance out.
  • The nutrients concerned are N, P, K, Mg, Ca and Na from applied fertilisers, effluent from a dairy system, or anything else applied such as chicken shed litter, and any nutrients from feed supplements.
  • It asks the questions – what nutrients are in the soil bank at any one time, what are coming in each year, what are going out, and especially where have they gone. If things don’t balance or can’t be accounted for – then you need to get some help.
  • Nutrient budgets have become very important in recent years with the concern over the environment and the risk of polluting waterways from farm runoff and leaching.
  • Dairy farmers have led the concern over these issues, especially the few with effluent disposal systems that do not comply with regulations.

Doing the budget arithmetic

You’ll need help to work all this out, and consultants use a computer programme called ‘Overseer’. From the outputs of N and P, Overseer can also predict the amount of N leached below the rooting zone in the soil and the P runoff loss risk.

The current version of Overseer contains a nutrient budget for acidity and a maintenance lime requirement greenhouse gas emission, an energy use inventory report, recommendations for application of effluent to land and includes mitigating technologies. 

So you can see why you need a consultant! However, you can get a copy of Overseer free of charge from MAF Policy, Ruakura, Hamilton, or from the AgResearch website <>