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Soils 5: pH and Nutrients

Course: Sustainable Soils | Last Updates: 26th November 2015

Alice Midmer
Project coordinator
Biography >>

Grass and crop yields can be reduced by up to 20% where soil pH is below target. If the pH is wrong, nutrients applied to the crop may not be available to the crop, instead being lost through the soil, incurring costs and run-off issues.
The pH determines the relative acidity or alkalinity of a soil and is important to understand, and assess, in order to maximise crop growth. pH is measured on a scale of 1 to 14 (but less than 4 and more than 9 is uncommon), with 7 being neutral. Below 7 is acidic and above 7 alkaline.
Nutrients in the soil need to be managed so that the supply to the crop is matched by demand. N, P and K are all key nutrients which should be tested regularly, every three to five years. The availability of these nutrients is affected by soil pH in different ways and it is important to be aware of these impacts.

How to detect deficiency

Deficiency in nutrients and acidity cannot be detected visually in the soil. Poor crop performance is often the first sign that the soil is lacking nutrients. Regularly testing soils and adjusting lime and nutrient applications accordingly can ensure good growing conditions are maintained and sufficient nutrients provided.
For continuous arable cropping, experts advise that the maximum availability of nutrients from the soil is achieved at pH6.5. In order to maintain an appropriate pH, soils should be tested every 3-5 years and treat acidic soils with a liming material. On soils where acidity is known to occur, more frequent testing may be needed.

How can I measure pH and nutrients?

  • Twist a sampling auger/soil corer down to 7.5cm in grassland fields or 15cm in arable fields.
  • In a bucket, collect 25 cores of soil while walking the field in a ‘W’, avoiding gateways/feeding areas.
  • Transfer a sample to a plastic bag and label.
  • Send to soil laboratory. Where there are different soil types it’s best to send multiple samples from one field. Sample every three to five years and (ideally) not within 6 months of manure, fertiliser or lime application.

What influence does soil type have?

Some soil types and conditions allow land to remain neutral or alkaline without the need for lime additions. Other soil types require regular applications of lime to remain within optimum parameters. In addition, heavier land requires more lime per acre than lighter land to raise the pH the same amount.
The rock underneath the soil can play a part here also. For example, soil over limestone retains certain level of alkalinity inherently.
Similarly, soil type can have an influence over the natural fertility of the land. For example, sand is naturally less fertile whilst peat and silt retain more nutrients. Despite this, some form of additions are utilised in all conventional farming systems to maintain optimum conditions.

Improve soil pH and nutrient status?

Despite variation in soil type, there are a number of measures that will improve chemical conditions for plants on all soil types. Adjusting soil management practices appropriately can have a large influence on the soil quality and influence the long-term sustainability of your soil.

  • Awareness of the conditions A comprehensive soil history of the P, K and pH levels throughout the farm and from individual fields is important to achieve before remedial action can take place. Similarly any further field history can help gain a fuller understanding of conditions.
  • Lime Applying appropriate amounts of lime will raise the pH effectively. Quantity of lime required will depend on your soil type, the pH and cropping. Seek specialist advice or see RB209 for details.
  • Soil mapping Soil mapping is a technique that is becoming more popular throughout the UK. Detailed soil maps give a fuller picture of soil health instead of relying on field averages.
  • GPS steering This allow more accurate targeting of inputs when and where required, particularly for N, P and K. Whilst this can reduce your overall fertiliser use, it will primarily ensure that nutrients are applied where they are needed most. This can help uniform the yield across a field, increasing overall production.
  • Organic fertilisers Substituting some synthetic fertiliser requirements with organic replacements has the added benefit of aiding soil structure.
  • Livestock Grazing livestock will add valuable FYM to the soil and in some situations farmers on sandy soils have added pigs into the rotation to increase soil fertility.
  • Clovers enrich the soil quality as they are leguminous and fix nitrogen from the air. Including them within your grassland, provides a valuable addition to the grazing quality and will enhance the soils nutritional composition by returning nitrogen to the soil.
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