Various fertilizers are used in agriculture to improve soil stricture and fertility, many of them are industrially produced fertilizers, which are produced first by extracting minerals and then processing them. For example, for the production of phosphorus fertilizers, more than 130 million t of phosphorus-containing minerals are extracted and further treated with acids or heat treatments. In order to reduce the consumption of resources and energy, as well as the amount of existing waste, it is also possible to use secondary raw materials in agriculture. Many organic fertilizers not only contain plant nutrients but also improve soil structure.
What can be used as secondary raw material in agriculture?
Plant residues, such as plant leaves, straw, can be used from field-grown plants. Animal excrements are widely used in agriculture – litter manure, litterless manure, slurry, bird droppings, guano sediments (partially decomposed bird, bat, seal excrement), as well as digestates and various composts (to make the use of different materials more efficient or to reduce unwanted materials, mixtures of different organic and mineral substances are composted). From human-made waste sewage sludge, biological waste, as well as various other industrial waste and by-products can be used – such as fruit and vegetable processing by-products, alcohol production wastewater (are high in nutrients), sugar, starch, meat and fish processing waste, pharmaceutical industry wastewater, textile industry, leather processing by-products and waste, etc. Garbage that may be contaminated with infectious agents, helminths, pests and diseases must be composted, but materials with high levels of heavy metal contamination cannot be used.
Various plant residues as secondary material source
For increasing soil fertility whole plants for incorporation in soil, which are usually fast-growing crops that hinder nutrient leach between harvest and sowing, are used (also called green manuring). When these plants are incorporated, also N, P, K (basic elements that are very necessary for the plant) are incorporated in the soil. Also, the leaves of garden crops can be successfully used as green manure, especially beet leaves (for example, from 10 t of fodder beet their leaves contain 32 kg N, 8 kg P2O5, 64 kg K2O). Straw as a crop residue is a widely used fertilizer, which needs to be shredded to decompose faster. The effect of straw is long-lasting, the content depends on the kind of crop which was cultivated. 1 t of straw contains on average 4-5 kg N, 2 kg P2O5, 9-16 kg K2O, from which about 0.5 t of humus is formed. From wood processing waste sawdust (used for barn bedding), shavings, sawmill waste, tree bark can be used to improve the soil. Their disadvantage is the changing chemical composition and the content of persistent substances (cellulose, lignin, resins), therefore they have to be composted for a long time period. Red algae and kelp leached during storms are called sea manure, which contains 2.8% N, 0.3% P2O5, 3.5% K2O, as well as I, Br. It can be used as fresh sea manure or compost for fertilizing various plants, especially in gardens.
Secondary resource – wood ash
Ashes can be used for liming and fertilization, for example, wood ash which contains about 29-35% Ca, 4% P2O5, 9-13% K2O, therefore it is also a good source of potassium fertilizer. Deciduous ash is considered to be more valuable. Wood chips, bark and forestry waste are used in various major Latvian furnaces, as a result, large amounts of wood ash are produced. For example, in Riga, 700 tons of wood ash is produced annually from four furnaces of “Rīgas siltums” (Daugavgrīva, Vecmīlgrāvis, Zaslauks, Ziepniekkalns), but in Liepāja “Liepājas enerģija” produces 1600 t of wood ash. Wood ash is used in agriculture, forestry, and also in backyard gardens, it can be used as a soil acidifier, fertilizer and plant protection agent. In the Nordic countries, the use of wood ash is widespread due to the large forest areas and wood resources, as well as its use for heating. As ash might contain heavy metals, it is preferable to use it in agriculture to fertilize non-food crops, such as short-rotation crop plantations.
Secondary resource – liming materials
Several industrial by and waste products can be used for liming – dolomite screenings (containing about 21% Ca, 13% Mg), which is a by-product of dolomite chip production; filter lime (13% Ca, also slightly N, P, K), which is a waste product of sugar industry; electro filter dust (32% Ca, 1.6% Mg), which is, for example, a waste product of the cement industry, which may also contain metal contaminants. Wood ash can also be used for soil liming.
Secondary resource – meat and fish processing waste
For soil fertilization from meat and fish processing following waste products can be used: fishmeal (containing 9% N, 7% P2O5, some Ca); blood meal (slaughterhouse waste, dried blood, containing more than 12% N); a horn-nail meal which contains a similar amount of N; meat-and-bone meal (7.5-8.5% N, 12-14% P2O5); bovine skin meal (10.5-12.5% N; 0.2% P2O5). This processing waste is dried and shredded. It contains quite a lot of nitrogen, and some contain also phosphorus.
Secondary resource – human-made biological waste
Half of the municipal waste consists of bio-waste (one person produces up to 0.8 kg of bio-waste per day). They can be composted and are a suitable fertilizer for landscaping such areas as roadsides, as bio-waste can contain various types of contaminants, such as heavy metals. Bio-waste contains about 1-1.5% N; 0.5-1.0% P2O5; 0.5% K2O. In Latvia, their use could become more and more popular, as from 2023 all municipalities must ensure the sorting of biological waste.
Secondary resource – sewage sludge
Sewage sludge contains a lot of organic matter (65%), nitrogen (3.9%) and phosphorus (5.1% P2O5). 40,000 tons of naturally moist sewage sludge is annually produced in the biological sewage treatment plant “Daugavgrīva” in Riga. According to the amount of heavy metals, sewage sludge is divided into five classes, however, according to microbiological indicators, they are not divided, and the pharmacological contamination of sewage sludge is also a burning problem. Restrictions of sewage sludge use in agriculture depend on the amount of heavy metals, the kind of crop cultivated, the soil characteristics like pH and the characteristics of the field, for example, terrain. As a result, farmers rather avoid using sewage sludge, therefore it is more usable for short-rotation crop plantations, recultivation, landscape greening.