Lesson 2 - Fertilisers
Fertilisers are materials added to the soil to supply essential nutrients for plant growth. Regardless of the fertilizer, these nutrient elements become soluble in the soil and are then chemically taken up plant roots. Two main types of fertilisers tend to be caterorised organic, and synthetic fertilisers. Organic fertilisers are derived from plants, animals and their byproducts, the most common being natural materials, such as manures and composts, and processed materials, such as blood and bone, fish wastes and seaweed extracts. Synthetic fertilisers are inorganic compounds made through an industrial process. Common examples include superphosphate and ammonium nitrate. There is another group of fertilisers those derived from natural minerals, including mined products such as gypsum, limestone or rock phosphate, or byproducts from processing, such as wood ash or coal ash.
Organic fertilizers tend to be slower in releasing nutrients to the soil compared to synthetic fertilisers. This happens through their solubility or the way they interact with other elements. This ‘slow release’ of nutrients is generally positive in that it matches the nutrient needs of growing plants, Not all organic fertilisers are slow release. Some, such as poultry manures, referred to as ‘hot’ organic fertilisers, are highly soluble and release their nutrients quickly. Some fertilisers, such as rock phosphate, release nutrients very slowly, and in some soils can the nutrients can be locked up even further. Good additions of compost and maintaining a neutral soil pH can help problems of nutrient release and solubility.
Most synthetic fertilisers are highly soluble with their nutrients immediate available for plant use. They have known amounts of elements and are highly concentrated making them more efficient and affordable to use, particularly to rapidly address any nutrient deficiency or imbalance. Synthetic fertilisers are used in hydroponic food production. However, because of this ‘quick release’ of nutrients there is a risk that any excess can be leached from the soil profile. This leaching of nutrients, through groundwater and surface runoff, can exacerbate problems of water pollution, particularly through the addition of soluble nitrogen and phosphorus.
Fertilisers can also be described to be simple or straight, containing one main nutrient element, or a complete fertiliser, incorporating a blend of different nutrients. Many organic fertilisers have most of the essential plant nutrients present, however in some cases important nutrients are lacking. Blood and bone for example lacks potassium, which can be corrected by additions of potassium sulphate (sulphate of potash). Most synthetic fertilisers are simple fertilisers, having one primary nutrient, such as nitrogen, phosphorus or potassium present. They are often used to address specific nutrient deficiencies in soils or applications where only one nutrient is needed. They are much cheaper when compared to organic and complete fertilisers.
Controlled-release fertiliser are used mainly in container and specialised growing situations (e.g. green roofs), and are designed to slowly release nutrients over a defined period. The release of nutrients is controlled by a semi-permeable resin or polymer that coats a small fertiliser pellet, slowly diffusing elements for plant use. The rate of nutrient release is largely determined by moisture and temperature and ranges from two to 24 months.
Liquid fertilisers are a nutrient solution, a concentrated form of soluble fertiliser. Mainly synthetic fertilisers formulated around a single major nutrient or a ratio of NPK, these need to be diluted significantly before use. Used mainly in container production, they are expensive but very effective in addressing short-term or specific nutrient deficiencies. Some synthetic liquid fertilisers are also as foliar sprays, rather than applied to the rootzones of plants. While these can be an effective treatment to address specific nutrient problem in the short-term, they can’t replace the need for soil-applied fertilisers as most nutrients are taken up by plant roots – that’s their job after all! There are also several organic liquid fertilisers made from animal manures, fish waste and seaweed. Some of the more popular are fish emulsion fertilisers, a byproduct from the fishing industry and a concentrated source of nutrients. They generally have high levels of NPK and other nutrients and need to be diluted well before use. Some have the added benefit of being produced from the harvest of invasive fish species like European Carp. Seaweed extracts are also used as a liquid fertiliser although their levels of nutrients, particularly NPK, are quite low. They are perhaps better described as a biostimulant due to the presence of different plant growth regulators and other bioactive compounds, they need to be used regularly to be effective and are expensive compared to other sources of nutrients.
The elements found in fertilisers include the major or macro nutrients – nitrogen (N), phosphorus (P), and potassium (K) (referred to as NPK), calcium (Ca), magnesium (Mg), and sulphur (S). Many also contain micronutrients or trace elements such as iron (Fe), copper (Cu), zinc (Zn), boron (B), molybdenum (Mo) and manganese (Mn).
Plants need a balance of nutrients in the soil with each of them playing a specific role in plant growth and development. However, most plants have greater demand for macronutrients, particularly NPK, which is why these are provided in larger proportions in most fertilizers. Micronutrients are still essential for many plant functions but are needed in much smaller amounts and are present in most soils in adequate quantities.
Comparing fertilisers starts with the label. All fertilisers must have a label with a complete nutrient analysis, containing the percentage of major and micronutrients in the product. The label allows you to know what’s exactly in the fertiliser and to compare different products. Always read the label carefully, especially the NPK. While most organic fertilisers tend to have a complete range of nutrients, they have varying concentrations of macro nutrients present. However, some composts and manures, even bagged products, will not always have composition information available. Knowing the indicators of quality for these products is perhaps the best way to judge their quality.
When using fertilisers, it’s a good idea to distinguish between preparing new growing spaces and applications where you are maintaining plants. In preparing new spaces, and as we have discussed previously, your goal is to have a well-structured, loamy soil with plenty of organic matter. If this is the case and you are growing vegetables, then addition of good quality compost and perhaps some aged manure will provide most of the nutrients you need to get started. If your garden soils are less than ideal, then consider getting your soil analysed to work out what is needed. There are laboratories in every capital city that provide this type of service with costs starting around $300. This includes testing for soil pH, salinity (electrical conductivity, total soluble salt), organic carbon, organic matter, nutrient balance and available NPK. Only use laboratories that include recommendations on what fertilisers should be used based on the test results and your own growing needs.
Vegetables need ample nutrition to produce but vary in their needs based on the types of plants you are growing and their stage of growth. Always add some fertiliser at the bottom of the furrow, covering this with soil before planting or seeding. Once established, a top or side dressing of fertiliser, applied once or twice during their growth cycle should be sufficient. A slow-release organic fertiliser, such as a pelleted poultry manure product or blood and bone is best, although synthetic fertilisers can also be used. Always follow the manufacturer directions for application rates but if you have added organic matter or compost to your soil then the rates of fertiliser required will be significantly less.
Fruit trees should always be planted in soils that have been well-prepared, including deep cultivation and the incorporation of compost, in spaces of up to one square metre in area. At planting there is no benefit gained from placing compost directly in the planting hole, but a tablespoon of controlled-release fertiliser (12 months release) mixed in with the planting soil will help the tree establish. As fruit trees develop, they benefit from regular additions of fertiliser, more being applied as the trees develop. Mature fruit trees can need up to 3 kg of fertiliser per year, half added at harvest and the other half during spring as the tree is actively growing. Always mulch your fruit trees with leaf litter or woody mulch of some kind. In addition to weed control, the organic matter breaks down and helps improve the soil over time.
In most growing situations, organic fertilisers are recommended. They release their nutrients slowly, matched to plant growth needs, and add useful amounts of organic matter and humus. They are also much more environmentally friendly in their manufacture. Most are waste products and have significantly less processing and energy use than synthetic fertilisers do. There are applications where synthetic fertilisers will be needed such as container production and in targeting specific nutrient deficiencies. Some organic fertilisers can also be improved by small additions of a synthetic fertiliser. Some pelleted poultry manures are low in potassium, so adding an inorganic fertiliser that supplies potassium, such as potassium sulphate, can provide a better-balanced organic fertiliser.
Fertile soils are those that have sufficient nutrients and a good structure. And while plants vary enormously in their nutritional needs, most food plants require relatively higher amounts of nutrients, so creating and sustaining fertile soils should be a goal when growing food. We have already discussed the importance of organic matter and composting, but another way to improve soil fertility is through cover crops.
Cover crops, or green manure crops as they are also referred to, are annual plants that are grown to be turned back into the soil, providing organic matter and improving soil structure. They have been used for centuries in farming systems to enhance soil fertility, control weeds, build levels of humus and ultimately maintain soil health. The most common examples of cover crops include legumes, like clover, lucerne, lupins and cowpeas; cereals such as oats, rye grass, sorghum, and millet; and oil seeds including mustard and sunflowers. Plants need to be selected for the right season of use, generally cool seasons (e.g. mustard or clover) or warm seasons (e.g. lucerne, sunflowers). Legumes are particularly beneficial as they fix atmospheric nitrogen in nodules on their roots, this nitrogen then becoming available in the soil as the plants decompose. Many gardeners like to mix up 2-3 different species for a cover crop, combining a legume with a cereal for example. Cover crops are grown from seed and sown directly into soils where a crop has recently been harvested. The seed can be broadcast or placed in rows but needs to be sown thickly enough to provide good germination and coverage, although this will vary with each species. The plants are left to grow to the point of almost flowering, somewhere between 6-10 weeks. They are then cut back, with larger plants slashed into pieces, and the plant matter left in situ. The plants are then dug into the soil,, making sure they are well-mixed. The site is then left for the plant matter to decompose before any further planting or sowing takes place, somewhere between 2 to 4 weeks.
While not as complex as a green manure crop, another simple way to help with soil fertility is through fallowing. Fallowing is simply leaving a garden space or bed unplanted, perhaps for a season or even up to a year, often allowing annual weeds to grow freely on the bed. This helps to disrupt any pest or disease cycles that may be present. It can also improve soil organic matter if these weeds are then turned into the soil, as with cover crops. Make sure you stop any weeds from seeding by cutting them or turning them into the bed before they flower. Fallowing often forms part of crop rotation system in a food garden.
Crop rotation involves planting a sequence of different crops in the same space across different seasons or years. It reduces problems with pests and diseases, especially soil-borne pathogens, helps with managing the nutrient needs of different crops, reduces the reliance on pesticides and improves overall soil health. Most rotation methods first categorize crops based on either their family (e.g. Solanaceae, Brassicaceae) or their habit (i.e. root crops, legumes). The objective is to plan and implement a rotation in which crops from each category are ‘cycled’ through the same bed over a three-to-four-year period. The objective is to not repeat the same category of plant in the same space over this period. Any crop rotation does require good record keeping and needs to be matched to the available space that you have for growing. A simple 4-season crop rotation system would be to start with a legume crop (e.g. peas or beans, to build soil nitrogen); followed by a ‘green’ of some kind (e.g. lettuce or bok choy, a big nitrogen user); then a Solanaceae crop (e.g. tomato or eggplant, more balanced NPK needs), followed by a root crop (e.g. carrots, onions – more PK than N). This rotation could be interspersed with a green manure crop or a period of fallow but would certainly be needed after four seasons or years of cropping in the same bed.