Managing nutrient fertility of different soil types

Grass and crop production and nutrient cycling are two of the key functions that intensively farmed soils must perform. Farmers regularly manage the fertility of the soils on their farms by applying fertilisers and organic manures to build up or maintain the supply of nutrients required by the grass or crop types they produce.

However, experienced farmers will know that not all soils (or fields) have the same production potential (or suitability for certain crop types) or respond in terms of their soil fertility status to the nutrients that are applied. There may be a number of factors at play here, but one major denominator is soil type and most farms will have at least two different types dispersed within the farm boundary.

Given the nature of the Irish landscape and its origins (glaciation), there is often large variability in soils over relatively small areas, ie within towns-lands. This poses a challenge for individual farmers and their advisers when planning nutrient and fertiliser management strategies for their farms. A blanket fertiliser application approach, where all fields receive and are perceived to respond to similar nutrient application rates, may not be effective for attaining target yields or the most efficient in terms of financial return on investment.

This is because different soil types possess different characteristics and qualities. Some of the main characteristics related to soil fertility and nutrient cycling are the parent material (rock type, glacial till or mineral deposit) that soil is derived from and its nutrient composition, soil texture (ie proportions of sand, silt and clay present), soil organic matter level, water holding capacity and drainage class (ie free-draining versus poorly draining) etc.

Across all the 11 soil great groups identified by the soil information system in Ireland, the soil nutrient levels will have been influenced by agricultural production to some degree. However, four of these great groups are usually found in either upland or mountainous areas with shorter growing seasons (podzols and lithosols), are shallow or have much rock outcrop (rendzinas) or are too wet (ombrotrophic peat soils consisting of raised and blanket bogs) to yield a return on investment in soil fertility. These soils are therefore more likely to be used for extensive rough grazing by drystock.

The more intensively farmed and productive soils such as brown earths, brown podzolics, luvisols, alluvial soils, surface-water gleys, ground- water gleys and minerotrophic peats are more likely to receive active management by farmers to influence their soil fertility status. However, the weathering and soil formation processes under which these have been created and their distinct soil properties have a major influence on how they respond to fertiliser and manure additions and also how they maintain their long-term soil fertility status. Of the 14 essential plant nutrients, the primary nutrients – nitrogen (N), phosphorus (P), potassium (K) – and secondary nutrients – sulphur (S) magnesium (Mg) and calcium (Ca) – are required in largest quantities by grassland and crops.

Soil pH and acidity

The pH status of the soil is a key factor which regulates nutrient availability in soils and is optimum between the pH range of 6.3 and 7.0. (target pH of 6.3 for grassland and 6.5 for cereal crops). Soils formed from limestone parent material such as calcareous brown earths and luvisols have naturally high pH status (> 7.0) and therefore do not require lime applications. In contrast, brown podzolics naturally have inherently low nutrient and pH status (pH < 5.5) due to the podzolisation processes (stripping of nutrients, iron, aluminium and organic matter from the topsoil layer) and acid brown earths may be depleted in Ca and Mg which has been leached out. However, suboptimal soil pH is easy and cost-effective to correct with regular applications of lime. On more poorly drained gley soils, once corrected, the pH status will likely be maintained for longer as leaching of Ca and Mg from the profile is limited. The target soil pH for grass production on peat soils is lower (5.5), as these soils have low mineral content and therefore low exchangeable acidity (acidity that originates from aluminium-rich clays). However, applications of Ca, Mg and micronutrients are usually required when peat soils are drained and brought into agricultural use, due to their naturally low status.

Nitrogen

N is the main driver of plant growth and is the cornerstone of all plant protein production. Organic matter (OM) is the main store of N in soils – it can be in excess of 5,000 kg/ha. Therefore, soils with higher OM (eg peats and gley soils) have the potential to release more N to the growing crop. The rate at which N is released is dependent upon the ratio of C:N (10-12:1 is optimum) and soil conditions such as temperature, moisture and oxygen.

The highest N release from the soils generally occurs in late spring and early autumn when soil temperature and moisture levels are favourable. However, N is a volatile nutrient, and moves freely through soils, plant, water and air.

It exists in many different compound forms, some of which are available for uptake by plants (eg ammonium and nitrate), some of which are a potent greenhouse gas (eg nitrous oxide) and some of which can be lost in drainage water (eg nitrate).

Grassland and crops grown on lighter and well-drained soils such as brown earths and brown podzolics are likely to be very responsive to N fertiliser inputs. However, care must be taken when applying N to these soils, especially during wetter months, as there is a higher risk of the N being leached with draining water. In contrast, Gley soils are more poorly drained due to either impermeable clay rich layer which stops water and nutrients from draining through (surface-water gley) or from the up-welling of water from below (groundwater gley).

When conditions are favourable, such as when they are drying out or beginning to wet-up, these gley soils can release N from their organic matter reserves.

During prolonged wetter periods when Gleys and Alluvium soils (which are prone to flooding) become waterlogged, there is a risk of denitrification where N will be lost to the atmosphere as nitrous oxide gas. Many gley soils have artificial drainage systems which helps these soils to dry out quicker in spring, increasing the length of the grass-growing season and reducing the risk of N loss.

Phosphorus

Phosphorus plays a central role in the energy regulation of all organisms and its availability is likely to be most limited early in the season when soils are cold and wet. However, the behaviour of P is completely opposite to that of N, as it is easily bound to the soil or in soil organic matter.

When mining soil P for plant growth, we need to keep an eye on the total phosphorus reserves that remain in the soils. There are dramatic variations in the total P reserves between soils and this can simply be the result of the geology of the soil (derived from limestone versus acidic rock) which affects its storage and of past nutrient management.

Plant-available P in the soil is a much smaller proportion of these total P reserves (usually less than 1%) and is measured using the Morgan soil test. Soil type can influence how soil test P levels respond to P fertiliser applications and more heavy textured soils (such as acid brown earths with higher clay content) and have the potential to lock up fertiliser P. Therefore, farmers need to adopt different fertiliser management strategies to overcome this issue such as applying the P allowance for crop using a little and often approach rather than all at once.

Care should also be taken when applying P fertilisers and slurries on more poorly drained Gley soils as they pose a higher risk for P loss after heavy rainfall if water flows off the surface, representing a loss on investment.

Potassium

The behaviour of K lies somewhere in between that of nitrogen and phosphorus – it can be bound, but also move quite freely through soil, water and plants. Brown earth and brown podzolic soils may have high K reserves and supply for crop production, derived from their parent material, especially if formed from shale and mud stone geology. However, significant quantities of K can be removed from lighter textured and sandy examples of these soils. In contrast, Luvisols are the most prominent K fixing soils and this is more likely to occur when OM levels are low (and when these soils are used for tillage). Therefore, more frequent application of K fertilisers or manures are warranted on these soils to maintain soil K supply.

On peat soils both K and Mg are easily released and they are less likely to strongly bind K fertiliser applications.

Sulphur

Sulphur (S) is required by plants to convert N into proteins and a N:S ratio of 10:1 needs to be maintained. In Ireland, sulphur levels in the soil tend to be lower than in many parts of Europe, because we lack heavy industry and the associated S depositions from the atmosphere (typically 1-2 kg/ha). In lighter textured and well-drained soils (brown podzolics, brown earths and luvisols), crops can be very responsive to S applications as S can be leached with drainage water from the topsoil. Sulphur deficiencies may be more apparent on these soil types when high levels of N fertilisers are being used, or where high-yielding tillage and silage crops are removing large quantities of S. Therefore, applications of N+S fertilisers are required to meet crop demand.

More poorly drained, heavier textured and high-OM soils (such as gleys, Peats, or organic-rich brown earths) have greater potential to release S from OM reserves, making it available to meet the demands of crop uptake.

Nutrient Management

As not all soils are made equal, it is appropriate that nutrient applications need to be tailored to the particular soil type in order to achieve the correct balance for the growing crop. To this end, soil fertility research is being conducted at Teagasc, Johnstown Castle, utilising Irish soils information to develop more soil-specific nutrient advice.

For example, studies on soil N supply show that some soils have the potential to supply up 200kg N/ha whereas others just 50kg N /ha. To implement soil fertility advice for individual soil types at farm level, knowledge of the soils present is required.

The Irish Soil Information System has mapped our soils at a regional scale which is a major step towards identifying the typical soil types a farmer could expect to find on his farm which is located within a region on the map. The future challenge is to achieve improved nutrient management and increased profitability and sustainability of farming systems by linking soil fertility knowledge to these different soil types and to train and support professional advisors and farmers to identify and manage soil appropriately.