Soil

Soil Origins

Soil is the top layer of the earth’s surface. It consists of mineral and rock particles, organic matter, water and air. Agriculture could not exist without soil. Soil provides a medium in which crops can be grown, delivering nutrients and water to the crop and also provides physical support for plant roots.

Farmers choose to grow different crops depending on the type of soil they have on their land. Poor yields of crops can result from growing a crop in an unsuitable soil. Farmers also have to consider sustainable methods of farming to ensure the ability to grow crops on an ongoing basis.

Did you know that 4,455,800 hectares are dedicated to agriculture in Ireland (CSO, 2016).

Soil Microorganisms

The soil microbiome is a term used to describe the community of living microorganisms within our soils. These include fungi, bacteria and archaea. Each of these microorganisms have specific functions that keep our soils healthy. These are also called soil ecosystem services. Maintaining a large variety (or diversity) of microorganisms ensures the functioning of our soils. Some of the ecosystem services that the soil microbiome provides include nitrogen fixation and increased nutrient and water absorption.

Plants require nitrogen for photosynthesis but before it can be absorbed, nitrogen fixing bacteria must convert atmospheric nitrogen (N20) into the plant available forms - ammonia (NH3) and nitrate (NO3). Nitrogen fixation microorganisms can be free living in the soil or can be present through symbiotic relationships with certain plants. Azotobacter bacteria are a group of free-living soil microbes that fix nitrogen from the air. Rhizobium are a group of bacteria that form a symbiotic relationship with leguminous plants (eg clover) to form nodules at the roots. Symbiotic relationships are important for the survivability of each of these organisms. The rhizobium bacteria extract food from the plant and the plant extracts nitrogen from the soil after nitrogen fixation.

Another important grouping of microbes include fungi. Mycorrhizal fungi colonise the root system of plants and increase their nutrient and water absorption capabilities. Therefore, fungi play a critical role in food production. The mycelium of the fungi absorbs and retains nutrients and water and due to the symbiotic relationship, these important factors can then be more readily absorbed by plants.

Soil Types

Sand, silt and clay particles all have different properties, e.g. effect on drainage, aeration and fertility. Therefore the proportions of these mineral particles present in the soil will largely influence the water and nutrients available to crops, the aeration of the soil, drainage of the soil and how easy it is to cultivate. The properties of sand, silt and clay in a soil determine the soil texture. An ideal soil for cultivation contains 40% sand, 40% silt and 20% clay. A loam soil contains equal amounts of all three soil particles.

Sandy soils are well drained, well aerated, low fertility, easily tilled, warm up quickly in spring but are prone to drought and do not get waterlogged.

Clay soils have poor drainage, poor aeration, fertile and retain nutrients, they are not suited to tillage due to plasticity, they do not warm up due to lack of aeration and cold soils. They retain water during drought but are prone to waterlogging.

Loam soils have good drainage, good aeration, good fertilityand are easily tilled. They will warm up in spring and will retain water but will not become waterlogged or be prone to drought.

Peat Soils are a huge part of Ireland. They are characterised by their absence of horizons except for the O horizon which is dark brown/black in appearance. Peats are widely used for providing fuel in the form of turf. Peat soils are characterised by their poor drainage and acidic pH.

7.5% of all Irish bogs are owned by Bord na Móna.

3% of all Irish bogs are actively harvested by Bord na Móna.

24% is the percentage of raised bogs owned by Bord na Móna.

Peat operations are in 9 of the 26 counties in the Republic of Ireland.

Solar energy is used to dry the peat down to 45-55% moisture content for harvesting.

Soil Composition

O Horizon

This is not always present, due to the absence of vegetation. It consists of organic material.

A Horizon

This is commonly known as topsoil. It contains minerals and may have organic matter mixed through it. However, it can experience the effects of leaching and so it may be lacking in minerals in certain conditions.

B Horizon

Is known as the subsoil. It is normally a lighter colour than topsoil, except where minerals have been leached and have accumulated in this horizon.

C Horizon

Contains parent material and is rocky in nature.

R Horizon

Is the bedrock and is solid.

Chemical Properties

Soils have essential elements, macronutrients and micronutrients. Plants need these essential elements in order to be able to grow. The most important nutrients are the macronutrients that are found in large quantities such as nitrogen, phosphorus, potassium, calcium, magnesium and sulfur.

Particularly in farming Nitrogen (N), Phosphorus (P) and Potassium (K) play an extremely important role in soil management.

Nitrogen (N) is the most importa of the macronutrients. Without it, plant and animal life would not exist. Nitrogen has many essential functions in both plants and animals such as it is a component of chlorophyll which is needed for photosynthesis, it is a component of amino acids which is needed to create protein. It is also a component of DNA, which is responsible for growth and reproduction in plants. Sufficient levels of nitrogen provides rapid plant growth, creates dark green vegetation and high protein content in seeds.

Phosphorus (P) is the second most important macronutrient. Phosphorus like many mineral elements is found in ionic compounds in the soil. It is soluble in water and it is taken up by plants in this soluble form. Like nitrogen, phosphorus has many essential roles in the plant, it is required for optimum growth and reproduction, involved in energy transfer in the plant, it is important for the production and development of new cells as well as the transfer of DNA to new cells and for seed formation and development. When plants have sufficient phosphorus they are early maturing with vigorous growth, it has increased resistance to plant disease, it improves flower formation and increases stalk/stem strength.

Potassium (K) carries out a number of important functions in the plant such as protein synthesis, translocation of carbohydrates, regulation of plant stomata and water use, promotion of disease resistance and activation of plant enzymes. Sufficient levels of potassium increases crop yield and root growth.

Deep Dive Content

Soil Samples

Soil testing allows farmers to determine the nutrients available on their land and to determine how suitable an area is for crop growth. Soils can be tested for a variety of nutrients, lime requirement and for pH. Once carried out, a soil test is valid for 5 years. However, farmers engaged in intense crop production may test their soils more often to optimise crop production.

Guidelines for taking a soil sample

Divide the area to be sampled into regions 2-4 ha in size. Then take samples from a wide range of areas, accounting for differences including different soil types, previous cropping history and slopes. Avoid taking samples from areas that are not typical of the area, e.g. entrances and exits to fields, around drinking troughs, beside ditches and on marshland. It is important not to sample for P and K for at least 3 months after the last P and K application. Samples should be taken using a soil auger and at least 20 samples, 10 cm in depth should be taken in a W shape across the field. Samples of the same field should be stored together and sent to Teagasc for analysis.

The soil index system

A soil test is best carried out every 2 to 3 years to determine the nutrient requirements of an area of land. This is particularly important if the land is farmed intensively. When the soil test has been completed, it is possible to determine which fertiliser should be applied to the land and what quantity of the fertiliser should be used. Teagasc categories soils in a soil index system. This ranks a soil by its fertility level and its likely response to fertiliser application. The levels of nutrients required to grow a particular crop and the pH of the soil are also taken into account. If the pH is low, liming may be recommended. When a soil test is completed, the soil sample will be given an index value.

Fertiliser is an inorganic, manufactured material that may contain one or more of the essential elements required for crop growth.

Manure is an organic material that consists of the wastes of plants and animals.

Fertilisers and manures are any naturally or artificially produced materials that can be added to soil to provide one or more of the elements that are essential for plant growth. They exist in solid, liquid and gaseous forms. The quantity of an essential element present in a fertiliser can vary enormously and it is important for a farmer to know how much of a particular element is present when spreading the fertiliser or manure on their crop.

Fertilisers

Artificial Fertilisers

The most commonly used straight nitrogen fertilisers in Ireland are Urea and Calcium Ammonium Nitrate (CAN).

CAN contains nitrogen in two forms, nitrate and ammonium ions. The ammonium ions are acidic and can lower the pH of the soil. A lower soil pH would normally have a negative effect on crop growth, since some elements become unavailable to plants. However, since CAN contains calcium, which is alkaline, this acts as a buffer against the acidic ammonium ions and prevents pH levels from becoming too acidic. One of the main advantages of CAN is that it is a fast-acting fertiliser. The nitrate in CAN is immediately available for uptake by crops. The ammonium ions are slower acting as they must first be converted to the nitrate form. The combination of fast and slow acting fertilisers means that nitrogen is available on a long-term basis when CAN is spread on the land.

Urea is not as popular as CAN in terms of fertiliser usage however it does contain a higher concentration of nitrogen which means less fertiliser is needed for spreading on the land. This can rescue cost and labour. Urea is a slower acting fertiliser than CAN. it must be converted from urea to ammonium form and then into nitrate before it is available to crops.

Compound fertilisers such as 10-10-20 is one of the most commonly used fertilisers in Ireland for grassland, potatoes and cereal crops. Compound fertilisers can be produced by combining straight fertilisers and by mixing N, P and K compounds in their raw state. The main advantage of these fertilisers is that only one fertiliser may be needed for application to a crop or grassland.

Natural Fertilisers are in the form of manures. Manure is a cheap, alternative source of nutrients and it is a cost-effective way of recycling waste on a farm, particularly animal waste. It adds organic matter/humus to the soil which improves soil fertility and structure.

Farmyard manure (FYM) consists of animal dung, animal urine and straw from winter bedding. The manure rots over time and can be applied to the land. As it decomposes it releases nutrients. Farmyard manure is particularly useful on organic farms as a source of nutrients where artificial fertilisers are not used. Farmyard manure can be applied to the land with a muck spreader.

Slurry is a liquid manure that contains animal dung and urine. It is collected in a tank underneath the floor in slatted housing. The level of nutrients in animal slurry is similar to that in farmyard manure but it contains less organic matter due to the absence of straw from winter bedding. Therefore, it does not have as great a benefit to soil structure. It can be spread on the land using a slurry spreader and it is absorbed quickly because of its liquid composition. As a result, it gives a faster growth response than farmyard manure. The spread of pests and diseases is a factor to be considered when spreading slurry and as such it should be confined to grassland applications.

Slurry is full of valuable nutrients but can damage our environment if not managed correctly. For example, Phosphorus within the slurry can be washed into water bodies causing eutrophication or nitrogen can be lost through gaseous emissions.

Rules/laws concerning use of fertilisers

There are restrictions on the spreading of fertilisers enforced by the Nitrates Regulations. To minimise run-off, waste and water pollution fertilisers should not be spread in certain conditions such as if the land is waterlogged, flooded or is likely to flood, if the land is covered in snow or frozen, if rain is forecast within 48 hours or if the ground is steeply sloped and combined with other factors that may pose risks of water pollution.

There is a slurry spreading calendar each year. Slurry cannot be spread on land from 15th October to January 12th / 15th / 31st depending on which part of the country a farm is located. Closed periods are a feature of the Directive in all Member States. The closed periods in Ireland were decided following a period of extensive public consultation and were agreed with farming bodies and the European Commission.

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