How to improve soil fertility

The production of nutritious pasture depends on fertile soil. Not only can healthy soil bring economic gains – but also environmental benefits, giving you the best of both. This guide will help you improve soil fertility by covering key aspects, including organic matter, pH, soil types, and more.

Contents

1. What is soil fertility?
2. Visual soil assessment
   
3. Soil types
4. Soil test
5. The impact of soil pH on nutrients
   
6. Applying lime

What is soil fertility?

Soil fertility is the ability of soil to supply nutrients and sustain plant growth. It is the combined effect of three major interacting components: the chemical, physical, and biological characteristics of soil.

Biological

This refers to living organisms in the soil, including bacteria, fungi, nematodes, earthworms, and more. These organisms break down litter and cycle nutrients through the soil. Earthworms help create a healthy soil structure by eating decomposing plant material and creating channels through which air and water can permeate.

Physical

Referring to the physical structure of soil, this includes particle size and compaction, ability to store water and allow drainage. When soil is compacted, water struggles to flow through, potentially leading to waterlogging. Root growth and air movement can be stunted by soil compaction.

Good physical structure will enable rain infiltration, reducing runoff and soil erosion while encouraging soil aeration. It will also benefit the root development of plants.

Chemical

Chemical relates to the pH and nutrients in the soil. Organic matter is a store of nutrients, including nitrogen (N) and phosphorous (P), which are available to the soil through mineralisation. Each 1% of soil organic matter represents approximately 1,000kg of organic nitrogen and other nutrients.

Visual soil assessment

Visually assessing different sites across your farm will help you identify the soil types you’re working with. These characteristics are found in good soils:

1. Soil structure: It should be friable – easy to crumble when moist.
2. Porosity: A balance of large and fine aggregates.
3. Colour: Dark soil shows good aeration, drainage, and soil organic matter (SOM).
4. Root depth: Look for healthy white roots of 0.5-1.0m in depth.
5. Low in mottles: Numerous and coarse mottles indicate poor soil structure.
6. Earthworms: More earthworms equal better soil structure.
7. Pugging: Livestock can trample topsoil and damage the structure beneath.

Soil types

You can now confirm your soil type by handling it to identify the feel:

1. Sandy: Feels gritty and cannot form a ball.
2. Loamy sand: Almost moulds into a ball but will disintegrate when pressed.
3. Sandy loam: Moulds into a ball but note the fissure when pressed.
4. Silt loam: Similar to sandy loam but smoother and without a gritty feel.
5. Clay loam: Smooth and slightly sticky, forming a ball that will deform without creating fissures.
6. Clay: Similar to clay loam but feels much stickier.

Soil test

Knowing the pH and nutrient levels of your soil will let you know exactly what it needs to be productive. This will ensure you only spend on the inputs you need.

When soil testing, you will work with a specialist company that can advise on sampling instructions. Here are four soil testing tips to help out:

1. Select testing zones across your farm: Make sure these are major areas and they are clearly identified. This is so you can return two years later to get fresh samples to compare progress.
2. Equipment selection: Pasture soils can be sampled using the full depth of a 75mm probe.
3. Timing: Sample in spring or autumn when conditions aren’t extreme. Urine and dung can impact results, so avoid sampling in recently grazed paddocks, along fence lines, or around water troughs. You also need to wait until three months after applying lime or fertiliser, as they can inflate results.
4. Samples: Take 15 to 20 samples and mark the details requested by your soil specialist.

The impact of soil pH on nutrients

The pH scale goes from 0 and extremely acidic to 14 and extremely alkaline, with New Zealand soils often ranging from 5.0 to 7.0. In pastoral farming, 5.8 to 6.2 is the ideal soil pH because it will ensure the best availability of nutrients.

Ultimately, pasture needs 16 essential nutrients to grow. Nitrogen, phosphorus, potassium, sulphur, and magnesium are five that can be added as fertilisers.

When soils fall outside the ideal pH range, those essential nutrients are low in supply and pasture can struggle to grow. Yield will even struggle when one essential nutrient is low.

Nitrogen

Clover can be an excellent nitrogen (N) source for itself and companion grasses, but you will likely need applied N to aid establishment or strategically during the year. After this, you can ease off because applied N can make clover lazy and it will stop fixing atmospheric N in soil.

In New Zealand, most soils are deficient in phosphorus (P). This is a problem because P is needed to support germination, root growth, and tillering of grass and clover leaves.

Potassium

Soils that are sandy, podzol, or volcanic are most likely to need potassium (K). QT K tests will only support an application if they average under 4 over a two-year period. A K application is recommended following hay or silage harvesting as it can be easily absorbed in excess by the plants, resulting in the nutrient being lost.

Sulphur is an essential nutrient

Clover and other legumes need sulphur (S) to produce proteins and fix atmospheric nitrogen in soil. You can often spot a sulphur deficiency in paddocks where clover is struggling.

Sulphate sulphur is readily available to pasture but can be washed away with heavy rainfall. This is why it makes sense to also use elemental sulphur as a slower-release option. There are three soil tests that can determine the availability of sulphate S, organic S, and total S.

Magnesium

Magnesium (Mg) can benefit soil in several ways, starting with its ability to balance pH and enhance structure. It also aids photosynthesis in pasture species, supports nutrient uptake, and activates enzymes.

Low molybdenum soils

Molybdenum (Mo) is an essential micronutrient that helps legumes create protein and fix atmospheric nitrogen in soil. You will find that Mo decreases when soil pH is below 6.0. This can be corrected by adding lime to bring soil pH back up to 6.0.

There are no soil tests for MO, but you can spot issues by looking for clovers with pale green or yellow leaves. This indicates that the plant is struggling to fix nitrogen.

Applying lime to soil

We know that New Zealand soil tends to be acidic. Before pasture renewal, if pH is below your target of 5.8-6.2, you can apply lime as a neutraliser. These are the benefits of lime on soil pH:

• Lime helps release nutrients and aids soil fertility by correcting acidic pH. Fertilisers and effluent cannot be fully effective if pH is low.
• Improve soil moisture.
• Increase phosphorus availability.
• Decrease aluminium and manganese toxicity in soil.
• Create a better habitat for microorganisms.
• Lime makes a tremendous difference to the productive potential of pasture. Targeting the optimum pH will result in a more profitable and sustainable farm.

Your initial lime application should be determined by your soil test analysis. Please consult a specialist if you need advice on exact lime or fertiliser applications.

Ready to grow quality pasture?

Check out our highly efficient Aber High Sugar Grasses and range of persistent clovers.