Reduce fertiliser requirements by improving soil health and nutrient availability

Date published: 09 March 2022

A healthy soil will have the ability to store and supply nutrients and this function relies on a number of soil properties.

Earthworms are an ecosystem engineer looking after the physical structure of the soil and nutrient cycling.

Ruth Ruddell a Technologist with the College of Agriculture, Food and Rural Enterprise (CAFRE) commented: “With costs increasing it is more important than ever that we assess soil health and nutrient status, so that any limiting factors can be addressed before the growing season.”  A healthy soil is recognised by the ability of the physical, chemical and biological aspects of the soil to fulfil their functions. Issues in any of these three areas will limit plant nutrient uptake and yields.  In order to improve soil health you must first assess each of these aspects to establish what would reduce your soil function.

Completing a visual examination of soil structure (VESS) will allow you to assess the physical status of your soil. Use a spade to dig soil pits (20cm x 20cm x 20cm), then manually breaking down a sample of soil, by hand, to visually assess the structure of the soil particles, rooting depth and colour/smell of the soil. A well-structured soil has round crumbly particles that can be easily broken up between your fingers, allowing plant roots to penetrate down through the soil. The soil should smell earthy or have little odour; if there is a pungent smell this indicates poor drainage or lack of oxygen in the soil. Digging a soil pit will also allow you to determine if compaction and/or poor drainage is an issue. Compaction can lead to poor root growth and penetration with reduced soil air spaces. Reducing soil air spaces causes anaerobic (the absence of air) conditions in the soil, this has a negative impact because the soil organisms and plant roots live and function in the pore spaces. Compaction also reduces the capacity for water to infiltrate and drain through the soil.

Ruth said: “Management practices that can avoid damage include; restricting animal and machinery traffic when soils are vulnerable or wet, working with lighter axle loads and lower ground pressure tyres, or if applicable using remediation measures like sub soiling or aerators.”

Soil organic matter is essential for many of the processes that control nutrient supply and storage in soils. Organic matter varies depending on the soil texture, rate of decomposition, and environmental factors such as aeration and soil temperate. There will be higher levels of organic matter under grassland compared to continuous arable rotation. Organic matter levels are highest near the surface and decreases with depth, typically ranging between 1 and 6%. Low levels of organic matter will impede the soil’s ability to provide soluble forms of nutrients.

The level of soil organic matter can be increased by incorporation of crop residues, while optimising grazing management will help incorporate grass residues and animal manures into the soil.  Applying farm yard manure and slurry to crop requirement, combined with the application of lime to correct soil pH, will improve plant growth and also improve soil structure by making clay more porous and sand more water retentive. Choosing plant species with greater rooting depths, for inclusion in your sward, will increase and distribute the organic matter to greater depths within the soil profile. Also protect existing organic matter levels by avoiding poaching of the soil, this will limit loss of soil by erosion by wind and rain.  Minimum cultivation methods should also be considered as they reduce the rate of organic matter decomposition compared to conventional cultivation. The benefit of optimal organic matter levels will increase the resilience of crops/swards to adverse weather, improve nutrient holding and availability, increase carbon storage and improve water infiltration and traffic-ability of the soil.

Soil chemistry also plays a vital role in plant nutrient availability and having the correct pH is critical. Alastair Cameron, CAFRE technologist notes that “Improving the pH of your soil is one of the easiest and most cost effective improvements that you can make for soil health and nutrient availability.” Nitrogen, phosphorus and potassium are the key nutrients required for grass and crop growth and can be provided by fertilisers such as slurry and other organic manures or inorganic (chemical) fertilisers.  There is a potential cost to the environment and a financial cost to your farm business as a result of inefficient use of fertilisers, if nutrient uptake is not maximised. This cost is significant at current high fertiliser prices. Availability and uptake of nutrients is heavily influenced by the pH of the soil. At pH of 5.0 to 5.5 it is estimated that 32% and at pH 5.5-6.0 approximately 21% of nutrients applied are wasted as they are not available for uptake by the crop. In practice applying 150kg/ha of nitrogen fertiliser where pH is below 6, could result in a financial loss of £70 - 107/ha, based on a 27% nitrogen fertiliser price of £600/tonne.

You can determine the pH of your soil by carrying out a soil analysis. A soil sample, taken once every 4 years is adequate for 4 hectares (10 acres) in one or more fields, provided they have been under similar management. Soil sampling kits and augers can be obtained by emailing or telephoning your local DAERA Direct office on 0300 200 7840. Low pH soils can be corrected by applying lime. Your soil analysis results will indicate the amount of lime required. The quantities of plant available phosphorus and potassium in soil analysis results are expressed as an Index. Always aim for the optimum index to suit your production system.  Whilst in general the optimum index for phosphorus (P) is 2+, recommendations for less intensive grazing specify that a P index of 2- is adequate. The optimum index for potassium (K) is 2-.  As each soil index relates to the available nutrients in a sample it can be used to determine the need for any additional nutrients required for crop growth.  The higher the soil index the lower the need for additional nutrients from slurry/manure and fertilisers. To make best use of slurry and manure, protect the environment and save money on fertiliser target fields with low nutrient indexes and top up to crop requirement with inorganic fertiliser if required.  The CAFRE Crop Nutrient Calculator will help you determine nutrient requirements, enable you to maximise the potential of slurry and manures and provide a fertilisation plan for your farm. 

Soil is one of the most biodiverse habitats on earth, with organisms ranging from microscopic bacteria, fungi, protozoa and nematodes, up to the largest such as earthworms and ants. Soil biology performs key functions in recycling nutrients, making nutrients available to plants and maintaining soil structure. A key indicator of soil health are earthworms, as they are sensitive to changes within the soil environment such as pH, waterlogging and compaction. Counting the number of earthworms in a soil pit provides an indication of soil health.  A (20cm x 20cm x 20cm) soil pit on grassland, should contain around 20-30 earthworms and a soil pit on arable soil 7-10 earthworms. Aim to maintain good soil structure by avoiding compaction and waterlogging, this ensures good air pores and aerobic (the presence of air) conditions which benefit soil biology. Crop or sward diversity and the addition of organic material to soil can also help increase soil biology.

Taking care of the physical, biological and chemical properties of your soil will improve sustainability, performance and reduce input costs by decreasing losses and improving the efficient uptake of fertiliser.

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