How to increase soil fertility

The production of nutritious homegrown grass and forage is simply not possible without healthy, fertile soil. Not only can fertile soil bring economic gains – but also environmental benefits. 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. The impact of soil pH on nutrients
3. Checklist: How to increase soil fertility
4. Managing soil types
5. Correcting soil pH
6. Maintaining P and K using soil index
7. Slurry testing
8. Ask Germinal

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.

1. 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.

2. Physical

Referring to the physical structure of soil, this includes particle size and compaction, the 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 supporting good soil aeration. It will also benefit the root development of plants.

3. 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.

This can be useful in explaining why grassland farms that are converted from long-term tillage often yield less than expected in the early years. Tillage farms, although generally acceptable in terms of P and potassium (K) content, are generally low in soil organic matter. The longer the land was under continuous tillage, in general, the greater the problem.

The impact of soil pH on nutrients

Soil analysis is the first step to maximise production, with Teagasc finding that only 16% of grassland soils have optimum fertility (pH > 6.2; P&K index 3 or 4). Ultimately, the greater the deficiency, the worse that grass and forage will perform.

Fertile soil will contain major nutrients, secondary nutrients, and trace elements to support plant nutrition.

Nitrogen exists in different forms and can be introduced to soil via nitrogen fixing legumes, artificial fertiliser, animal manure, or the breaking down of crop residue and soil organic matter.

Managing soil types

Peaty soils

On peaty soils, apply P and K where required in the growing season because peat is poor at retaining fertiliser.

Sandy soils

P shouldn’t be applied in late autumn or winter as it is readily leached from sandy soils. Similar to nitrogen, P must be applied by 15th September. The best response to P will be seen in spring and early summer. A little and often approach is generally best.

Potassium is best applied in small amounts in spring and summer. Where larger quantities are required, apply in autumn. There is no closed period for the application of K.

Sulphur is essential

Sulphur (S) is a fundamental nutrient needed for plant establishment and there is a close relationship between nitrogen and S in plant nutrition. Up to 30% of soils require sulphur for optimum yield, but there is no soil test for it. The optimum timing for S application is late spring to early/mid-summer.

Soils that need sulphur:

• Grazed ground requires about 20kg S/ha per year.
• Silage ground requires approximately 20kg S/ha per cut.

High molybdenum soils

Molybdenum (Mo) is an essential trace element but, when levels in the soil are high, it can cause copper deficiency in grazing stock. On high Mo soils, avoid increasing pH above 6.

Speak to a vet about copper supplementation to animals and beware of copper toxicity as too little or too much will negatively affect animal health.

Correcting soil pH

This is a critical aspect of soil health, with low pH resulting in poor yields and reduced efficiency of applied fertilisers.

Achieving the optimum soil pH will:

• Increase grass production
• Release up to 80kg N/ha per year from the soil alone
• Increase availability of P, K and other nutrients
• Increase the efficiency of applied fertiliser

Adding lime to soil

Lime is used to correct acidic soil and provide the right conditions for plants to develop. It has a major influence on:

• Plant development
• Efficient use of fertilisers
• Soil structure
• Bacterial activity in soil

Knowing the relationship between pH and lime means you can successfully manage your soil and grassland. Target the optimum pH of 6.3-6.5 for grassland to get the most from your soil.

Lime is generally applied as ground limestone. The finer fractions <0.15mm will start to correct soil pH immediately, with the remainder working over a 12 to 24-month period.

Lime can also be applied in granular form with 100% of particles less than 0.1mm fineness, meaning it starts to correct soil pH immediately.

Benefits of lime on soil PH

• Lime helps release nutrients and aids soil fertility by correcting ph. Fertilisers and manure cannot be fully effective if soil pH is low.
• Lime makes a tremendous difference to the productive potential of grassland. Targeting the optimum soil pH will result in a more profitable and sustainable farm.

Soil test

Base your lime application on soil test sample results. Soil sampling is the best way to evaluate pH levels and determine the need for lime.

• On mineral soils, the optimum soil pH is 6.3 - 6.5.
• On peaty soils, the optimum soil pH is 5.5.
• High Molybdenum areas (Mo) pH < 6.2.

When to apply lime

• After silage cutting is an ideal time to lime.
• In spring: After the end of the first grazing rotation, as there is a large area with low grass covers.
• At reseeding time: Always use lime when reseeding – it is especially important with minimum cultivation to break down the old sward as it decays.

Leave three months between liming and protected urea/slurry to reduce nitrogen loss through volatilisation. Overcome this by applying protected urea/slurry and then apply lime seven to 10 days later.

Applying lime to soil

Plan lime application as part of a fertiliser programme. It is a good idea to have the lime on-farm in time for a spreading opportunity.

• Spreading should be even and accurate.
• 125kg/ha of granular lime is equal to 370kg of ground limestone.
• Ideally, spread 20% of the farm each year.

Please note: It is not recommended to exceed 7t/ha (3t/acre) of lime in a single application.

Maintaining P and K using soil index

Soil index 3 is considered optimal for grass and forage production. To maintain soils at index 3, the P and K application should replace the P and K removed – for example, replace offtakes in crops. The rate applied will depend on your stocking rate and production system.

If your soil P or K index is 1 or 2, additional nutrients are needed to build up reserves. But if your soil P or K index is above 3, you can draw on soil reserves and do not need to apply chemical P or K fertiliser.

Slurry testing

Knowing the nutrient content of slurry on-farm will help you:

• Save money
• Reduce your chemical fertiliser use
• Improve soil health
• Protect the environment

If silage fields are low in P or K, then the silage will also be low in P or K, meaning the slurry will also be lower in P or K nutrients. Variability in the slurry dry matter will affect N, P, and K content of the slurry.

Test slurry with a hydrometer as it can estimate slurry DM content on-farm. This can be a useful tool to estimate the nutrient content of slurry.

Use data to determine supplementary chemical fertiliser needs. By managing the nutrient content of the manures and slurry on-farm, you can benefit from cost savings due to reduced inputs of chemical fertilisers. Generally, 3,000 gallons/acre at 7% DM slurry will supply 20kg P and 142kg K.

Grass and forage that increase soil fertility

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