Ivydene Gardens Soil:
What are Soil Nutrients?
 

What are soil nutrients?

Nutrients can be defined as any substance used by an organism as food. Macronutrients are found in concentrations in excess of 1000mg/Kg and micronutrients have concentrations below 1000mg/Kg. Carbon, hydrogen and oxygen are obtained predominantly from water and atmospheric sources

.

Sedum acre

Cations are atoms that have lost an electron to become positively charged.

Anions are atoms or groups of atoms that have gained electrons.

In agriculture, leaching refers to the loss of water-soluble plant nutrients from the soil, due to rain and irrigation.

How is the availability of nitrogen, potassium and phosphorus affected by the soil colloids?

Colloidal particles (clays and humus) are the seat of most chemical, physical, and biological properties of soils. They are extremely small in size, being mostly less than 2 micrometers in diameter, a size below the range of the ordinary light microscope.  Their small size, coupled with plate-like shapes, contribute to very high external surface areas that are complemented in some cases with even more extensive internal surface areas.  These surfaces characteristically carry negative and/or positive charges that influence the attraction and repulsion of the particles toward each other and that attract (adsorb) swarms of ions of the opposite charge along with numerous water molecules.  The adsorbed ions are subject to exchange with plant roots thereby providing plants with most of their essential mineral elements.

Nitrogen (N)

The majority of the nitrogen contained in the soil is held in organic matter. This must be mineralized by soil micro-organisms before the nitrogen it contains is released for plant use. Nitrate, because it is anion, can be susceptible to leaching:-

• Approximately 78% of Earth's atmosphere is nitrogen (N2). The strong bond between the atoms of N2 makes this gas quite inert and not directly usable by plants and animals. As nitrogen naturally cycles through the air, water and soil it undergoes various chemical and biological transformations. Nitrogen promotes plant growth. Livestock then eat the crops producing manure, which is returned to the soil, adding organic and mineral forms of nitrogen. The cycle is complete when the next crop uses the amended soil. To increase food production, fertilizers, such as nitrate (NO3) and ammonium (NH4), which are easily absorbed by plants, are introduced to the plant root zone (Many gardeners use Growmore as does agriculture, but - as the next sentence states - this form of nitrate is lost to the plants reasonably quickly and it has negative impacts on the soil food web - as shown in the table on the right, whereas a mulch of organic matter releases its nitrate more slowly as the organisms in the soil eat it and release their waste products over a longer time period). However, soils do not absorb the excess NO3 ions, which then move downward freely with drainage water, and are leached into groundwater, streams and oceans.

Potassium (K)

Potassium is an essential element in many enzymes; it also regulates the opening and closing of the plant stomata, as well as being essential for photosynthesis and disease resistance. Exchangeable potassium can readily exchange with the soil solution to buffer changes (Cation Exchange capacity in Soils). Plants use a lot of potassium and this means that it can be in short supply.

Phosphorus (P)

Phosphorus is essential for photosynthesis, nitrogen fixation, growth and root development. Phosphorus occurs as the phosphate anion, but, unlike nitrate, potential leaching losses are reduced because it forms insoluble compounds when the pH is outside the narrow band of 6-7 (see Acid and Alkaline Soil in the Soil Formation - What is Soil Texture? Page) and ligand bonds with other soil chemicals that modify its anionic character.

As roots remove nutrients from the soil solution, losses are compensated for by buffering, as nutrients held on exchange sites are exchanged with the soil solution. In well-buffered soils, despite plant absorption and leaching losses, the concentration of nutrients in the soil solution is maintained.

A MORE IN DEPTH EXPLAINATION OF HOW SOIL WORKS AND PLANT NUTRIENTS ARE CREATED IS SHOWN IN THE SOIL SITE MAP PAGE.

The normal method by which home gardeners provide nutrients to their garden plants:-

In English gardens, you weed the ground in your beds and prune your plants including hedges and remove the weeds and prunings before donating them to the council or putting them into a 36 x 36 x 36 inch (90 x 90 x 90 cm) compost bin. The amount you put into that bin each day is so small that it never reaches more than 25 degrees Centigrade and therefore does not consume or destroy the seeds or the pests. The liquor produced goes into the ground below and unless your plants have accessed that ground area; is leached to the subsoil and therefore lost. If the compost bin was 180 x 180 x 180 inches (450 x 450 x 450 cms) and it was filled in one go, then it would reach about 55 degrees Centigrade which would consume and kill those seeds and pests/diseases. The end result of the compost bin is normally not much use, unless you use worm bins and then you can use the bottom tray compost on your garden beds.

Then, if you are generous; you spread some general fertilser once a year like Growmore round your plants,

or

Rose Food round your roses again once a year and expect a marvellous year of flowers etc.

As I have explained elsewhere this does not work for at least 2 reasons:-

• Rain will dissolve these nutrients and successive rain will leach this through the top soil and into the subsoil where no roots go within a fairly short time. Some of this instant food gets eaten by the plant and bypasses their roots normal exchange program - sugar produced by plant is exchanged with the outside earth for nutrients required by that plant to grow. The normal exchange program not being used correctly tends to die off and unless this artificial food is applied throughout the growing season, the plants will run out of food and perish.
• Artificial fertiliser is rather like sugar, it gives you energy but not everything you require and since you might require 3 atoms of molybdenum for one of the plants processes, you will not provide that and therefore the plant has problems.
  If you only ate Marmite sandwiches, then if you got sick then your body will not have all the neccessary components for you to fight it and you might then die as well - you also require a balanced diet.

Have you ever been to an unmanaged forest and walked through it?

You will find litter on the ground and the following article from Wikipedia will explain where that forest gets its nutrients from:-

"Litterfall, plant litter, leaf litter, tree litter, soil litter, or duff, is dead plant material, such as leaves, bark, needles, and twigs, that have fallen to the ground. This detritus or dead organic material and its constituent nutrients are added to the top layer of soil, commonly known as the litter layer or O horizon ("O" for "organic").

Litterfall is characterized as fresh, undecomposed, and easily recognizable (by species and type) plant debris. This can be anything from leaves, cones, needles, twigs, bark, seeds/nuts, logs, or reproductive organs (e.g. the stamen of flowering plants). Items larger than 2 cm diameter are referred to as coarse litter, while anything smaller is referred to as fine litter or litter. The type of litterfall is most directly affected by ecosystem type. For example, leaf tissues account for about 70 percent of litterfall in forests, but woody litter tends to increase with forest age. In grasslands, there is very little aboveground perennial tissue so the annual litterfall is very low and quite nearly equal to the net primary production.

In soil science, soil litter is classified in three layers, which form on the surface of the O Horizon. These are the L, F, and H layers:

L - organic horizon characterized by relatively undecomposed plant material (described above).

F - organic horizon found beneath L characterized by accumulation of partly decomposed organic matter.

H - organic horizon below F characterized by accumulation of fully decomposed organic matter mostly indiscernible.

Surface detritus facilitates the capture and infiltration of rainwater into lower soil layers. Soil litter protects soil aggregates from raindrop impact, preventing the release of clay and silt particles from plugging soil pores. Releasing clay and silt particles reduces the capacity for soil to absorb water and increases cross surface flow, accelerating soil erosion. In addition soil litter reduces wind erosion by preventing soil from losing moisture and providing cover preventing soil transportation.

Many organisms that live on the forest floor are decomposers, such as fungi. Organisms whose diet consists of plant detritus, such as earthworms, are termed detritivores. The community of decomposers in the litter layer also includes bacteria, amoeba, nematodes, rotifer, springtails, cryptostigmata, potworms, insect larvae, mollusks, oribatid mites, woodlice, and millipedes. Their consumption of the litterfall results in the breakdown of simple carbon compounds into carbon dioxide (CO2) and water (H2O), and releases inorganic ions (like nitrogen and phosphorus) into the soil where the surrounding plants can then reabsorb the nutrients that were shed as litterfall. In this way, litterfall becomes an important part of the nutrient cycle that sustains forest environments.

As litter decomposes, nutrients are released into the environment. The portion of the litter that is not readily decomposable is known as humus. Litter aids in soil moisture retention by cooling the ground surface and holding moisture in decaying organic matter. The flora and fauna working to decompose soil litter also aid in soil respiration. A litter layer of decomposing biomass provides a continuous energy source for macro- and micro-organisms."

This is why when I am maintaining a client's garden, I weed and put the weeds under the hedge or inside the base area of groundcover shrubs/trees, remembering to take out the weed roots as well. I spread a 4 inch (10 cms) depth of Spent Mushroom Compost on the weeded area before going to the next area to be weeded the following visit. When I have weeded and mulched the garden beds/hedges, then on each subsequent visit I go round all the garden beds and hoe any weed that pushes its way through the mulch and leave it to dry off and wither away on top of the mulch. I then prune the shrubs/hedges or remove bedding plants etc as required and place those on the lawn before mowing them and the lawn and putting a 0.5 inch (1 cm) layer of grass mowings/prunings on top of the mulch. This then mimics the same process as detailed above in the Wikipedia article to feed my client's plants and reduce their water consumption.

Negatative impacts on the soil food web

Chemical fertilizers, pesticides, insecticides, and fungicides affect the soil food web, toxic to some members, warding off others, and changing the environment. Important fungal and bacterial relationships don't form when a plant can get free nutrients. When chemically fed, plants bypass the microbial-assisted method of obtaining nutrients, and microbial populations adjust accordingly. Trouble is, you have to keep adding chemical fertilizers and using "-icides", because the right mix and diversity - the very foundation of the soil food web - has been altered.

It makes sense that once the bacteria, fungi, nematodes and protozoa are gone, other members of the soil food web disappear as well. Earthworms, for example, lacking food and irritated by the synthetic nitrates in soluble nitrogen fertilizers, move out. Since they are major shredders of organic material, their absence is a great loss. Soil structure deteriorates, watering can become problematic, pathogens and pests establish themselves and, worst of all, gardening becomes a lot more work than it needs to be.

If the salt-based chemical fertilizers don't kill portions of the soil food web, rototilling (rotovating) will. This gardening rite of spring breaks up fungal hyphae, decimates worms, and rips and crushes arthropods. It destroys soil structure and eventually saps soil of necessary air. Any chain is only as strong as its weakest link: if there is a gap in the soil food web, the system will break down and stop functioning properly.

Gardening with the soil food web is easy, but you must get the life back in your soils. First, however, you have to know something about the soil in which the soil food web operates; second, you need to know what each of the key members of the food web community does. Both these concerns are taken up in the rest of Part 1" of Teaming with Microbes - The Organic Gardener's Guide to the Soil Food Web by Jeff Lowenfels and Wayne Lewis ISBN-13:978-1-60469-113-9 Published 2010.

This book explains in non-technical language how soil works and how you can improve your garden soil to make it suitable for what you plant and hopefully stop you using chemicals to kill this or that, but use your grass cuttings and prunings to mulch your soil - the leaves fall off the trees, the branches fall on the ground, the animals shit and die on the land in old woodlands and that material is then recycled to provide the nutrients for those same trees, rather than being carefully removed and sent to the dump as most people do in their gardens leaving bare soil.