Post category: Soils
A general fertiliser provides the three major nutrients – nitrogen, phosphorus and potash – in correct balance and it is suitable for general garden use to improve basic soil fertility. The formulae vary between the various brands: 7:6:17, 5:5:10, 10:10:20, 7:7:7. General fertilisers include Vegyflor, Growmore, John Innes Base Fertiliser, Potato and Vegetable Fertiliser and Potato Fertiliser.
Special purpose fertilisers
The special purpose fertilisers also provide all the major nutrients, but some of the minor ones too, and the balance of nutrients is set to suit the specific needs of certain plants, or groups of plants.
Purpose-made lawn fertilisers, such as Goulding’s Lawn Feed Special, Lawnsman Spring Feed, Lawnsman Winterizer, Special Lawn Fertilizer, Lawn Food, Toplawn and Autumn Toplawn are specially balanced for grass growth. The spring lawn feeds suit the spring and summer needs of grass. The autumn feeds suit the autumn and winter needs.
Rose fertilisers, such as Goulding’s Rose Fertiliser, Toprose, Rose Food, Rose ‘Plus’, and Special Rose Fertiliser have matching amounts of nitrogen and potash – nitrogen for growth, potash for flowers. Tomato and fruit fertilisers – Liquid Tomorite, Tomato and Fruit Fertiliser – have a balance towards potash.
The straight fertilisers contain just one major nutrient. Sulphate of potash contains just potash. Sulphate of ammonia, urea and C.A.N. contain only nitrogen, and superphosphate contains just phosphorus. These can be used on their own where plants need only one of the major nutrients.
For example, lawns may not need phosphorus or potash, just nitrogen, so sulphate of ammonia (or urea or C.A.N. on large areas) would do. Fruit trees may not need further boosting by nitrogen, being already too vigorous. An application of sulphate of potash would balance vigorous growth, and encourage fruiting.
The straight fertilisers can be mixed together to provide general fertiliser, or any specific formula as required needs are detected.
Organic manures have the dual effect of improving soil structure, by adding humus, and improving soil fertility by adding both major and minor plant nutrients. Farmyard manure contains the three major nutrients in small but significant amounts, and a full range of minor nutrients.
Garden compost contains a range of nutrients similar to farmyard manure, but in lesser quantity. Mushroom compost is at least as rich as farmyard manure, but contains a lot of lime. Meat-and-bonemeal contains nitrogen and phosphorus, but no potash. Leafmould had some nitrogen and potash.
Wood ash has some potash. Seaweed has considerable quantities of minor nutrients. Peat has very little fertiliser value, and is used solely for its fibrous organic matter. These manures do not have enough fertiliser value to improve soil fertility quickly, unless they are used at high rates.
For instance, farmyard manure or mushroom compost might have to be applied at rates of up to 25 kilograms per square metre to bring up soil fertility to an adequate level for vegetable growing. An annual application of 4 – 5 kilograms of farmyard manure per square metre is the equivalent of the recommended application of general fertiliser.
Artificial fertiliser can be used in conjunction with manures, but the two should not be mixed, unless the manure is very well rotted. Scorching of the plant roots and lower leaves can result as ammonia is released.
Plants get their food from the soil. Rock material and decaying organic matter contain tiny quantities of the nutrients essential for plant growth. As they are released gradually, they become bonded to clay and humus in the soil. From this reservoir, nutrients are slowly released back into the soil solution from which plant roots can absorb them easily.
Leafy rhubarb needs high nitrogen levels
Nitrogen (denoted by the letter N) is the scarcest of plant nutrients but among the most important, being essential for protein production. Dead plant and animal material, and animal waste, are sources of nitrogen as they break down. Bacteria in nodules on the roots of certain plants – particularly the pea family – are capable of extracting nitrogen from the air which is 80% nitrogen.
Lightning is another important source; the powerful electrical discharge ionises unreactive elemental nitrogen in the air, changing it to oxides of nitrogen that bond with water and fall in the rain of a thunderstorm.
Nitrogen is a vital part of green chlorophyll, which is essential for plant growth because it traps the energy of the sun. Without enough chlorophyll, plants turn pale, yellow-green all over and produce weak spindly growth. This often occurs on soils that are sandy and dry, low in organic material, or constantly wet.
Too much nitrogen causes leafy, rich green growth, at the expense of flowers and fruit. Nitrogen is especially important for grass and leafy vegetables.
Phosphorus (denoted by the letter P) is involved in root growth, and the development of buds and growing points. Plants short of phosphorus show stunted growth and a purple tinge to stems and leaves. Deficiency occurs most often on acid soils – especially in high rainfall areas, and in peat composts.
Potash (denoted by the letter K) is essential for flower and fruit production, and for balancing the vigorous leafy growth caused by nitrogen. Plants short of potash tend to be leafy, and often have pale, or brown, margins to the leaves. Deficiency occurs most commonly on light sandy soils, and strongly limy soils.
The minor nutrients are not needed in as great quantity, nor are they as scarce as the major nutrients, N, P and K.
Calcium (Ca) is important in its own right as a nutrient, apart from its neutralizing effects on soil acidity and its availability to bond with other nutrients, making them unavailable in high-calcium soils.
Calcium is involved in the building of plant structures. Shortage can cause browning within the tissue of plants such as tomatoes, apples and brussels sprouts. Deficiency occurs in peaty soil, acid soil and sandy soil. Apply lime as a remedy.
Magnesium (Mg) forms part of chlorophyll, making it essential for good growth. A shortage of magnesium causes yellow ‘netting’ of the leaves. Deficiency occurs on acid soils, or where too much potash has been given, because potash bonds with magnesium and makes it unavailable. Apply a spray of Epsom salts.
Iron (Fe), too, is part of chlorophyll. Shortage causes yellow leaves at the growing points, because iron is not mobile in plants. Deficiency is common on limy soils and it is the main reason why lime-hating plants suffer in such soils. Apply Sequestrene or sulphate of iron.
Boron (B) is closely associated with the actions of calcium in building the structure of plants. Similar symptoms of shortage appear – internal browning of brussels sprouts, swedes, cauliflowers and apples. Shortage occurs on limy soils where boron is bound up by calcium. Apply a spray of borax diluted at 30 grams in 10 litres of water to 10 square metres.
Other minor nutrients
Other nutrients that are important for plant growth include manganese, copper, sulphur and molybdenum, but plant deficiency symptoms rarely appear because most Irish soils contain adequate amounts.
Plants on very peaty soil can suffer copper deficiency and show shoot tip dieback. Lawns on very acid soil can show pale colour due to sulphur deficiency, because sulphur is part of green chlorophyll. Sulphate of iron applied to a lawn for moss control also greens up the grass by adding sulphur.
The application of organic material, especially farmyard manure, supplies additional quantities of most of the minor nutrients required. Organic material also provides plants with other useful food. As it is broken down, a vast range of organic chemical compounds is formed – protein derivatives, amino acids, sugars, oils, enmes and growth-promoting hormonal substances.
Plants can make these compounds, starting from the basic plant nutrients, but they obviously benefit from an energy saving if they can take in these derivatives, part-formed.
Soil types are described according to their main constituents. Thus, soil with a lot of sand is a sandy soil; soil with a lot of clay is a clay soil; and soil with a lot of organic material is an organic soil. Because this is usually peat, it is generally called peaty soil.
If soil has a good balance of all the important constituents – sand, silt, clay and organic material, but no preponderance of any one of them, it is called a ‘loam’. It might tend a little towards sand or clay and this would define it as a ‘sandy loam’ or ‘clay loam’.
Sandy loam might also be dscribed as a light loam; clay loam might be called heavy loam. The terms are interchangeable. Some soils contain a lot of stones or shale and can be called stony, or shaly loams.
Distribution of soil types
Potato pushes through a ridge of soil
There are peaty soils and other very poor soils associated with them in parts of many counties in the midlands, west and north particularly in mountainous areas, but most Irish soils are loams.
There are well-drained, sandy or shaly loams in part of Kerry, East Cork, Waterford, Kilkenny, Carlow, Wexford, Wicklow, Dublin, Louth, Armagh, Down, Derry and Antrim. These are easy soils to work, but need plenty of manure and fertiliser.
Parts of these counties and Tipperary, East Limerick, Kildare, East Galway, Laois, Offaly, East Donegal, Longford, Westmeath, Meath, Sligo and Roscommon, have medium-heavy loam soils. These are generally good, fertile soils, not quite as easy to work, but easier to keep fertile.
Counties Westmeath, Meath, Dublin, Roscommon, Longford, North Kildare, West Limerick, Clare, Cavan, Sligo, Monaghan, Tyrone, Fermanagh and Armagh, have clay loam soils that are heavy in varying degrees. Most of these soils are derived from glacial boulder clay deposits and can be stony. They are fairly fertile soils, but often difficult to work and need high potash fertilisers.
In counties Sligo, Leitrim, Cavan, Monaghan, Clare, Fermanagh, Tyrone and parts of other counties, impeded drainage adds to the heavy nature of the soil, making cultivation difficult and limiting growth.
Soil is composed of a complex variety of materials, some of which come from the actions of weather on rock, and some from living sources.
Good fertile soil is necessary for quick growth
The material that comes from rock includes stones, sand, silt and clay. Stones and sand are fairly obvious and easily understood components; silt and clay less so. Silt is made of very fine particles and compares in texture to the finest powder.
Clay is composed of very fine particles too, even finer that silt, and it is the result of the chemical decay of rocks as much as their physical breakdown. Clay is chemically active, which means that it can bind strongly to itself, and to other soil constituents. It bonds sand and silt together into tiny soil particles. The presence of a lot of clay causes too much binding, and the soil becomes a hard, lumpy mass.
The material from the decay of once-living organisms – both plant and animal – arises when plants and animals die and the remains are food for bacteria, fungi and algae in the soil. These, in turn, die and become food for other tiny organisms.
The cycle continues until the original dead plant tissue is broken down into tough, insoluble materials such as waxes, resins and gums. Mixed closely together, those materials, dark-brown in colour, are collectively called ‘humus’.
Humus, like clay, is chemically active and bonds itself to other soil constituents almost like an adhesive. Apart from stones, sand, silt, clay, dead plant and animal material, living organisms and humus, soil also contains air and water, and plant nutrients released by the decay of rock material and organic material.
The bonding action of humus and clay on the other soil constituents causes soil ‘crumbs’ to form. These are constantly forming, breaking up, and reforming because the bond, though strong, is not perfect.
Well-structured soil is easy to dig
The crumbs loosely bond to neighbouring crumbs but because they do not fit tightly together, spaces exist in between. These spaces, or pores, vary in size – the smaller ones fill with water, which sticks to the crumbs and provides a reservoir for plant roots.
The larger pores initially fill with water too, but this drains away and is replaced by air.
Good structure exists when there is extensive crumb formation, and the crumbs are arranged so that there is plenty of pore space for water and air. Poor structure exists in a soil where the soil constituents have not formed into crumbs, or have formed into very small, tight crumbs.
In poorly structured soil, there is little or no pore space and, though there is probably plenty of water, there will be very little air. The result is a heavy, wet, hard and lumpy soil.
Improving soil structure
Practically any soil type can have good or bad structure, depending on the presence or absence of good crumb formation. Certain soils however can have good structure even without crumbs.
In very sandy soils, the sand particles are large and about the same size as soil crumbs. They fill the function of crumbs, creating plenty of pore space. However, these are nearly all large pores, which drain quickly and, because of the absence of small pores, there is no reservoir of water for plants.
These soils tend to dry out very quickly. The addition of humus-forming organic material will create small pore space and improve their moisture-holding capacity.
Very peaty soils are composed almost purely of organic material. The fibrous nature of the peat creates both large pore space and small pore space, resulting in soil that is well structured and retains adequate moisture.
Heavy soils with poor structure can be improved by adding sand or organic material. Sand is not nearly as efficient as organic material because it works only by taking the place of soil crumbs. This also means that large quantities of sand must be added to achieve the same structural effect.
Organic material – farmyard manure, garden compost, straw, peat – breaking down into humus, is about ten times as efficient a soil improver as sand, on a weight for weight basis. However, sand has a permanent improving effect whereas organic material eventually decays.
Good soil structure develops naturally in well-drained soil by the action of roots, and the build-up of natural humus. Improving the drainage of wet soil is an important step towards better soil structure.
Removing plant material without adding organic matter back leads to progressive deterioration of soil structure. Walking on wet soil, and cultivating it when wet, quickly destroys the soil crumbs.
The acidity of a soil depends on the parent material – the rock from which it was formed. Soils over limestone, or those on boulder clay derived from limestone rock, in the Midland basin and North Munster, are limy.
Deciduous azaleas need acid soil
Limestone soil in Ireland is usually high in clay content with the result that these soils of often tend to be heavy and fertile, unlike the thin, chalk soils of other countries. Most plants thrive on these fertile limy soils, except the lime-haters such as pieris, rhododendron, most heathers, camellia, enkianthus and witch hazel.
These limy soils can have a layer of neutral or even acidified soil at the surface if old grassland has not been disturbed, or limed, for decades. They can be acidified by the addition of large quantities – 10 kilograms per square metre – of organic material, or by applying sulphate of iron at 100 grams per square metre and repeat this as required.
Over red sandstone, in South Munster, shale, in South-east Leinster, and granite, in Wicklow, Carlow, Kerry, Connemara and Ulster, soils tend to be acid, as are peat soils. Acid soils can be fertile if they can be classed as loams with plenty of organic material present, but they can also be low in nutrients, especially at higher altitude and in areas of high rainfall along the west coast.
Acid soils can be neutralised by applying lime at rates of about 500 grams per square metre. Simple kits are available to test soil acidity. Hydrangeas are good indicator plants; on acid soil, the flowers are blue; on limy soil, the flowers are red or pink.