Nutrient Conservation and Cycling

Soil, nutrient, and water conservation

Nutrients can be conserved by reducing erosion, nutrient leaching, nutrient run-off, and burning, and investing in compensating the nutrients lost with the harvested product, nutrient recycling, and soil fertility.

Mandal (2022) described erosion and soil and water conservation more in an entire chapter at Wikifarmer. Even a summary can still be helpful. Nutrients are concentrated in the topsoil – particularly those that are immobile and not washed (leached) down. It can be identified where:

• the soil can be seen sorted and deposited by wind or water in fine and coarse fractions,
• soils have been washed away from roots and the sides of stones.

More cover near the surface, with live, dead, or other materials, can reduce the risk or level of erosion during the season. They can also help build and stabilize soil organic matter and biological activity. Tall trees can prevent-decrease wind erosion, but if the zone near the soil surface is open and the soil bare, then the erosive wind or bigger, joined water drops can get more speed and erosive energy per square meter. Likewise, slowing wind or water is better and safer than completely blocking them because the flow can bypass with concentrated force and accelerated speed. Also, when erosion starts, it causes more erosion, so it is better to prevent it before fine sand particles knock more of them loose or water runs faster by making rills or deeper gullies. More suitable plant cover across the direction of the slope, also when fields are bare, is important. That can require improved, low-cost establishment methods, multipurpose plants, early benefits like water conservation, and minimal competition with crops. Some innovative solutions by Mandal (2022) are mentioned with references in his Wikifarmer Library articles on Nitrogen and legume and on soil and water conservation with agroforestry. Co-benefits can include fodder, fuel, fruits, green manure, weed suppression, saved weeding efforts, and reduced direct damage from strong and drying winds or water flow. More benefits can be habitats for natural enemies of seasonal crop pests, the plants may catch nutrients that could be leached, etc.

Recycling of nutrients in general

This is important, even if it may not be enough, and must be complemented by improving the amount of total or biologically available that can be recycled. Recycling can be imperfect, for example, due to the sale of crops or losses that cannot be completely avoided. Some nutrients may be available in higher or lower amounts than needed. Biological nitrogen fixation is not recycling for the local but the global environment and is treated in the article on Nitrogen. Recycling sulphur and ammonia N from the air is possible in polluted areas.

Recycling of nutrient ions (salts) directly from the soil

Nutrients from eroded topsoil may be conserved at a terrace edge or field boundary with permanent vegetation or in flat or hollow parts of the field. It can occur in the form of nutrient minerals, salts, and organic matter. Less manure or fertilizer may be applied to these enriched areas. This is a principle in traditional manual farming and modern, automated, data-driven precision farming (using drone images, analysis of colours, GPS location, and many soil analyses). Also, nutrients can be taken up from enriched parts by preferably deep-rooted, fast-growing plants and moved for fodder, biomass fuel, fruit, or so. Some plants and mature plant parts are low in nutrients, including phosphorous (P). On the other hand, the young part may contain higher amounts. Also, nitrogen-fixing multipurpose shrubs or trees can be mixed with shrubs or other plants with higher P-concentration. The topic has been studied in western Kenya.

Shrubs, trees, and deep-rooted lasting grasses or other suitable plants can be useful for catching, and recycling nutrients leached deep into the soil. Also, surface cover crops, inter-crops (mixed with others), and relay intercrops (partly overlapping in time) can help recycle nutrients before they are leached and lost. In Denmark, it is usually illegal to leave agricultural fields bare (without any vegetation cover). Even young plants seeded after the summer harvest are suitable as a nitrate “catch-crop”. In conservation farming/ agriculture, these crops are terminated by ploughing or by, undercutting or pressing crops down so other crops can grow above them.

Recycling of organic matter and biomass (as compost)

Organic matter (dead material) and biomass (live material) are terms that are often mixed. They contain all nutrients but often in too low amounts or concentrations, and the availability of the nutrients from them can be a problem too. Limitations and benefits are also discussed in other articles of mine in this chapter and the soil conservation series of articles. Practical solutions to key challenges are in focus here. Crop residues like straw and stalks have more than 30 times more carbon than Nitrogen, so soil nitrogen (N) is used by microorganisms to get a balanced diet. Such a high C:N ratio is a problem in low-nitrogen soil. Composting will be slow without sources of Nitrogen, and much CO₂ will be lost to the air. The temperature rise will also be too low to stop the spread of weeds and other harmful organisms, which unsorted compost may include. Including enough nitrogen-fixing legumes can help, but enough phosphate and water are needed to increase their efficiency. A stick deep in the compost pile can help check moisture, temperature, looks, and smell.

Wet, poorly aerated compost can become acid, from organic acids remaining, unlike aerated, where CO₂ can escape rather than form carbonic acid. Furthermore, harmful levels of, e.g., ammonia (NH₃) or hydrogen sulphide (H₂S) can cause damage from fresh manure applied directly near tender roots or seeds. The risk is when it is still fresh or smells like faeces or urine rather than like fertile forest or garden soil. Livestock manure and urine can also emit ammonia, methane, and, smelling like rotting eggs, toxic hydrogen sulphide (H₂S).  The strong bad smell of H₂S usually prevents toxicity problems for people. However, the gasses inside big tanks with slurry or livestock urine are often acutely toxic and lethal to work in, even if they are open to the sky. In Denmark, for example, it is mandatory to have enough storage capacity, an airtight cover (that may form naturally), and incorporate the manure into the soil right after its application and at proper amounts and times when nutrients can be absorbed and used by the crop.

Dry compost mixtures (or surface mulch cover) will be inactive but may lose more ammonia to the air than other compost. Dry organic matter may also catch fire or harbour animals, including rats, snakes, or crop pests like the big C-shaped dung- or chaffer-beetle-larvae that eat roots and residues. A Canadian extension guideline for compost is freely available. [2]

However, organisms like dung beetles, worms, and others can also benefit from organic recycling by borrowing organic matter. Throwing some topsoil and decomposing residues on top of compost heaps can help biological diversity and activity, as well as using a good moist mixture of what is available combined with shade, drainage, and/or, at dry sandy sites, a pit. Biological activity and organic matter have many benefits. Roots and microbes can make some nutrients in the soil more available e.g., by releasing acids or enzymes and by taking nutrients up, so the concentration in the soil solution is reduced.  In areas where fertilizer has been used much or soils are relatively young (renewed by, e.g., ice ages or volcanoes), more nutrient-rich minerals are available for release by biological activity. Nevertheless, the soil’s biologically available (within a season) and total (extracted with extreme methods) nutrient content are much lower in many low-income, tropical countries than in temperate areas with completely different soil types and land-use history. Minerals that can release nutrients (weatherable minerals) are washed out of the topsoil or all the root zone in some tropical soil.  Iron- and Aluminium-Hydroxides can be almost all that remain in these old soil types, plus exchangeable ions and some organic matter. The soil types have a  low level of exchangeable ions at the surface and also of the stronger chemically bound phosphate. Likewise, sandy soils can have low total nutrient content and can be mainly Silicon Dioxide (SiO₂). Today, peer-reviewed soil- and agronomy-science and evidence usually recommend considering both the use of fertilizer and other ways to improve or maintain soil fertility. In practice, the focus of attention may be narrow.

Selection and breeding for tolerance to low levels of water-soluble nutrients can help to recycle nutrients. Some plant release acids or other substances from their roots, helping nutrient uptake by dissolving minerals or organic matter.

Root hair length from the root (not density) has been found important for P-uptake by Dr T S Gahoonia, and it could easily be used more in plant breeding and selection (pers. comm., Copenhagen, 1996). Recently, root hairs have been documented to take up, partly digest microorganisms, and recycle some of them (root’s eating: rhizophagy[3]). Similar uptake has been known for rhizobia bacteria before they fix Nitrogen.

Surface mulch and contour trash lines can benefit against erosion and water run-off, but a spread of fire, weed seeds, and snakes should be considered when placing them. Materials with many fibers, and without green parts, when crushed, can be used without risking too much Nitrogen being used from the soil. Likewise, if chewed leaves give a very sticky feeling in the mouth, they have much tannin binding and protecting proteins from releasing Nitrogen – at least if they cover the proteins completely. While mulch and shade keep the soil surface moister by shading and cooling, the inefficient part of the rainfall from light rains will increase. However, moist topsoil where roots are important for roots uptake of phosphorous and organic matter and biological activity helps both root growth and water infiltration. A free guide on mulch is available from India HYPERLINK “bookmark://_ftn4”[4].

Recycling human and urban waste 

Sustainability of phosphate supplies and nutrient levels in land-use systems without fertilizer are among essential reasons to consider recycling residues, faeces, and the nutrient-rich urine that contains high amounts of Nitrogen and other nutrients like phosphorous. However, cultural, hygienic, and cost factors should be taken seriously. For example, in rural Africa, cultural objections have been handled successfully with suitable methods like ecological sanitation (Look for “EcoSan toilets Uganda”) approaches and inclusion. Deep pit latrines are widespread today in developing countries. However, they rarely function as planned to trap flies in (maintained) mosquito nets on ventilation pipes as the only opening when not in use (look for “Ventilated Improved Pit latrines”). The content can be sucked up by a pipe and placed in trenched in the field and covered immediately or used for biogas first. It can also remain covered with soil for 6 months or more while using other pit latrines not deeper than it can be excavated locally if using a moveable structure. Care may still be needed because resistant life stages of some parasites and bacteria often can last longer, particularly without oxygen (anaerobic), and survive regular cooking. However, it will not be in contact with the eaten parts of most crops, and most dirt will be cleaned away or cooked. Medicine residues and heavy metals, e.g., Cadmium (Cd) or lead (Pb) from batteries – or Mercury (Hg) can also be a challenge with recycling waste – particularly from urban or industrialized areas without good sorting of waste at the source. Cadmium can be taken most up by plants in risky amounts, and prohibiting and sorting batteries with Cd is feasible. See also the ash section below. In Denmark, wastewater is cleaned mechanically, biologically, and chemically so N and P can be recycled. Mechanically settled sludge is used for biogas, and the remaining sludge is used for fields and forests with heavy metal content and hygiene regulations. Most waste is sorted, for example, as food waste for biogas and sludge with remains of phosphorus-rich bone.

References:

[1] Diagnosis of Plant Nutritional Disorders — Department of Plant Science (psu.edu). Penn State College of Agricultural Science, seen 2023 The Pennsylvania State University

[2] Livestock Engineering Unit & Environmental Practices Unit Technical Services Division Alberta Agriculture, Food and Rural Development (2005): Manure Composting Manual (gov.ab.ca). 27 pp.

[3] MDPI: Microorganisms | Free Full-Text | Rhizophagy Cycle: An Oxidative Process in Plants for Nutrient Extraction from Symbiotic Microbes | HTML (mdpi.com). Seen 2023.

[4] Mulches: Types, Effects and Factor | Conservation | Soil Management (soilmanagementindia.com). Seen 2023.

Sustainable Plant Nutrient Management (SPNM): An overview

Sustainable Nutrient management: Introduction to concept, strategies, and principles

Nutrient conservation and cycling

Mineral fertilizers (including ash) and sustainability

Nitrogen (N): Essential for plant growth and yield but can have high costs to farmers, the environment, and health

Biological Nitrogen Fixation and seeding Legumes for Soil Fertility

The importance and management of Phosphorus (P) and Potassium (K) in plant production

Ion charges and secondary (=meso) nutrients: Calcium, Magnesium and Sulphur

How important are the Micronutrients for plants

Soil and plant analysis and field observations

When are approaches to Plant Nutrient Management actually Sustainable?