The term ‘fertiliser’ in an agricultural/horticultural context means a material that contains plant nutrients. There are 14 known nutrients, 6 major and 8 minor or ‘trace elements’. No single fertiliser supplies all the nutrients in exactly the correct proportions required by every plant, so in many instances a mixture of fertilisers is used to provide those nutrients most in demand by the plant. Of the 6 major elements (macro nutrients), three are more heavily utilised by plants, and so most fertilisers supply one or more of these. They are; nitrogen (N), phosphorus (P), and potassium (K). Synthetically produced fertilisers and mixtures are labelled with the proportions of these nutrients present in the fertiliser, for example N:P:K 7:7:7 (Growmore). Those fertilisers containing just one nutrient are termed ‘straight’ fertilisers, whilst those containing two or more are termed ‘compound’ fertilisers.
Synthetically produced fertilisers, sometimes referred to as ‘artificials’ or ‘inorganics’ are capable of being produced with a known and consistent N:P:K ratio, enabling the grower to adjust application rates depending on the plant’s stage of growth and nutrient requirements. They are also usually presented in a form most easily assimilated by the plant and are therefore ‘faster acting’.
Naturally occurring fertilisers, usually derived from dead organic matter may have varying proportions of nutrients dependent on their origin, and so are not usually labelled with an N:P:K value. They are referred to as ‘organics’ or ‘naturals’ and can also be ‘straight’ or ‘compound’, though they are usually compound. Frequently the nutrients are ‘locked’ inside large complex organic molecules, which need to be broken down by the action of bacteria and, or fungi and weather before the nutrient becomes available for assimilation by the crop.. Before they become available, the nutrients are said to be ‘immobilised’, and incapable of absorption. When they have been released from the complex compounds, they are said to be ‘mineralised’, and available for absorption by the plant. The rate at which ‘mineralisation’ occurs is dependent on the amount of biological activity within the soil and the weather, as moisture and warmth are necessary. For this reason ‘organic’ fertilisers are generally much ‘slower acting’ and can be very variable in the rate at which the nutrients become available. Many organic fertilisers were at one time called ‘manures’, e.g. ‘Hop Manure’, but the term ‘manure’ is now usually reserved for those materials consisting of animal (and human!) dung, or derivatives from them (more on manures and composts, later).
Plants can only take in their nutrients in an aqueous solution (carbon dioxide for photosynthesis, and oxygen for respiration are not normally classed as nutrients). The solubility of the nutrients in the soil water is influenced by how acid or alkaline the soil water is. This is referred to as the ‘soil pH’, and most nutrients are at their most soluble, and therefore greatest availability for plants, when the soil water is slightly acidic or almost ‘neutral’. The type of soil, its texture (sandy, silty or clay) and its structure (blocky or crumbly, ‘claggy’ or friable) also plays a part in the availability of the nutrients. Clay soils might be rich in nutrients, but the clay particles attract them and make it more difficult for plant roots to grow through and absorb them, whilst in sandy, free draining soils the nutrients are quickly leached down by rainfall below the root zone. Sandy soils quickly dry out and warm up in the spring more readily, enabling earlier sowing, but the loss of nutrients by leaching has led to them being called ‘hungry’ soils as they needs regular feeding to replenish the nutrients lost by the need for greater irrigation. In the case of very clay or very sandy soil, there is a need to ‘condition’ the soil by the use of ‘soil conditioners’ or soil ‘ameliorants’. Lime is a good example, as it reduces the acidity and, in the case of clay, flocculates the soil particles and gives it a more crumbly structure, increasing the solubility and availability of nutrients to the plants. Sandy soils can quickly become very acid with constant leaching and lime counteracts this.
Another ameliorant is decayed organic matter, manure, or compost, which when broken down to humus, again clumps the clay or sand particles together to form crumbs. This makes the clay more freely draining, but at the same time acts like a gel, binding sand particles together and enabling sideways movement of the soil water instead of vertically downwards. It also ‘holds’ the nutrients, lessening the effect of leaching by rainfall or irrigation. With silt soils humus helps prevent the formation of a hard ‘crust’ or ‘capping’ of the surface, preventing run-off of rainfall and providing less of a barrier to germinating crop seedlings. It is therefore important to get the soil structure and soil pH ‘right’ to ensure the best availability of plant nutrients for the crop. Otherwise, the addition of fertilisers would be a waste of time and expense as they would either be locked in the clay particles or washed out of sandy soil.