Plants for Soil Regeneration

An Illustrated Guide

Sally Pinhey, Margaret Tebbs

• Publisher: CAB International
• Year: 2022
• ISBN:
  9781789243628
• Topic: Technology & Engineering
• Subtopic: Agronomy
• Index: Technology & Engineering

This book is a comprehensive, beautifully illustrated colour guide to the plants which farmers, growers and gardeners can use to improve soil structure and restore fertility without the use and expense of agrichemicals. Information based on the latest research is given on how to use soil conditioning plants to avoid soil degradation, restore soil quality and help clean polluted land.There are 11 chapters: 1 to 6 cover soil health, nitrogen fixation, green manures and herbal leys, bacteria and other microorganisms, phytoremediators and soil mycorrhiza (plant-fungal symbiosis). Chapter 7 has plant illustrations, with climate range and soil types, along with their soil conditioning properties and each plant is presented with a comprehensive description opposite a detailed illustration, in full colour. Chapters 8 to 10 examine soil stabilisers, weeds and invasive plants, and hedges and trees and the final chapter, contains 5 case studies with the most recent data, followed by an appendix and glossary. The book allows the reader to identify the plants they need quickly and find the information necessary to begin implementation of soil regeneration.

1Soil Health

We generally think of the vigour of our plants as being the main test of soil health, and this is right. There are also many additional ways in which the health of soil can be tested. In the same way as animals alter the environment they live in by taking out nutrients and leaving waste matter, every growing plant will also alter the condition of the soil in some way. At the minimum, it will improve the organic content with its decaying form.

Starting from the very earliest plant life in the form of algae, mosses and lichens, photosynthesizing organisms survived on air, moisture, light and minerals in rock surfaces to grow and reproduce gradually. Over millennia, they created soil with their own decaying matter until it was deep enough to support larger plants that required foundations for their roots. This process can still be seen on any bare rock surfaces that have been newly created by quarrying or volcanic activity, for example. It is quicker, however, where there is already a surrounding community of plants or active construction. Soil is therefore composed of substantially dead plants with traces of minerals from rock substrate. Good soil consists of a mix of 40–45% inorganic matter, 5% living and dead organic matter, 25% water and 25% air. There is some flexibility in these percentages.

The depth of cultivable topsoil is also affected by its gradient and position with regard to the surrounding geology. Soil on steep slopes is prone to erosion and is thinner, while areas where sediment collects will differ from the bedrock, be deeper and consist of smaller particles. The activity of living organisms in the soil defines its quality. Living organisms affect soil structure by creating channels, with animals and microorganisms producing pores and crevices. Plant roots can penetrate into crevices to create friability, and strong, deep-rooted plants can break up compacted earth. Plant secretions promote the development of microorganisms around the roots. Leaves and other plant materials decompose and add to soil composition. Clearly, we do not know the entire story of soil dynamics, and there may be other approaches that our forebears, more conscious of natural cycles, understood better. Biodynamic systems (see Appendix) are demonstrably worthy of scientific study.

Microscopic organisms, so small that there would be millions in one teaspoon of soil, keep the soil alive with a variety of activities. They maintain the balance of life on earth by fixing gases and breaking down organic matter. These processes include breaking down bare rock into soil particles, cycling nutrients, transforming nutrients into different forms for plant uptake, helping the plants to absorb the nutrients, degrading toxins, both causing and preventing disease in plants, and both helping and hindering water penetration into the soil. A good balance of microbes in the soil is ideal for plant health, and this can be maintained by crop diversity, or at least small areas of monoculture together with crop rotation. Monoculture limits the type of microorganisms that can survive, and an imbalance permits the development of dominant pathogens.

As recently as 1996, glomalin was discovered by Sara Wright. It is a sticky substance that binds soil particles and sequesters carbon. It is thought to be produced mainly by mycorrhizal fungi that use it to protect their hyphae and strengthen the fragile fibres as they penetrate the soil and bridge air gaps, while converting nitrogen into a form that plants can assimilate. The network of glomalin from decaying hyphae binds the organic soil particles, improving both aeration and drainage. This process is improved by mulching but damaged by digging and ploughing.

Hermaphrodite, blind, deaf and supersensuous worm populations are both indicators and guardians of healthy soil. There are 29 earthworm species in the UK, of which there are four main types. Composter worms, usually bright red and striped, live in rotting vegetation and are excellent recyclers. Epigeic worms are reddish brown and live close to the soil surface, breaking down leaf litter. Endogeic worms live in deeper soil, are grey, pink, blue or green, and eat earth. Anecic worms have red or black heads with paler tails and live in burrows in the upper layers of the soil, pulling leaves down into their burrows from the surface. Following up Gilbert White’s observations 100 years later, Darwin calculated that anecic worm castings add 5 cm of quality topsoil to the surface every 10 years. The worms do this by dragging dead leaves into their burrows, digesting them together with swallowed soil, and ejecting a fine calcareous and nitrogenous mix in their castings on the surface. Glands near the worm’s gizzard add nitrogen and phosphates to their castings, while glands at the rear end add calcium, making the soil affected by their casts less acidic. Worm burrows also help to drain and aerate the soil, while waterlogged soils or those compacted by heavy machinery will have few or no worm populations.

Regular ploughing also chops up surface worms and exposes them to predators. Synthetic fertilizers and pesticides reduce the microbes that make the soil a healthy habitat for worms. Antiparasitic medicines given to grazing animals can reduce the number of microbes in the soil because of contaminated droppings.

A good mix of insects is an essential component of healthy soil. They are preyed on by spiders, which are a keystone species of the arthropods and a top predator, keeping the balance of pests and beneficial insects at an advantageous ratio.

Soil compaction is an increasing problem as farm machinery gets larger and heavier. Clay soils are particularly prone to this and are the hardest to restore. Even deep and strong-rooted plants like chicory, yarrow and sweet clover can sometimes fail to penetrate a clay pan. To some, the obvious solution may seem to plough it, but, paradoxically, not ploughing and leaving the soil organisms to do the work for you works better in the long run. Ploughing compounds the problem by extra heavy machinery passing over and chopping up the worms.

At the Allerton Project in Leicestershire, UK, where ploughing has been minimal over 25 years, the worm population has increased from single figures to an average of 800 per m². This is key to keeping clay soils profitably cultivable.

Knowing how long it takes for this precious earth covering to accumulate naturally, it follows that we should try to take good care of it, as it is the very foundation of our being. Farmers who have largely given up tilling say they can restore soil depth by up to 15 cm in as little as 2 years. The organic component of soil binds it, returns nutrients to the plant, stores moisture, makes soil friable and provides energy for soil microorganisms. Most soil microorganisms – bacteria, algae and fungi – are inactive when dry but become active once moisture is available. Good humus can hold water up to 75% of its volume without becoming waterlogged, rendering the land more drought resistant and preventing excessive runoff. Not all organic matter added to the soil is necessarily beneficial. In Secrets of the Soil (2004), Tomkins and Bird pointed out that cow manure is 25% microbes. Most of these are likely to be beneficial, but some may not be. Most microbes suppress diseases, but others may not. The effects will depend on the pathogens present and the amount and quality of the carbon:nitrogen ratio in the organic material, as well as moisture and temperature.

Insects expert Dr Jonathan Lundgren estimates that there are between 3000 and 15,000 species of potential pest insects, but for every pest species there are between 400 and 1700 species that are beneficial to humans. This is a good ratio, and is better undisturbed by indiscriminate insecticide use. On the whole, however, addition of organic material will suppress disease and increase the variety and activity of invertebrates and microorganisms in the soil. Careful selection of green manure plants is key to ensuring the best-quality organic additions. Soil damaged by repeated synthetic fertilizer applications can regenerate remarkably quickly if kept covered by a diverse mix of cover crops, which can also be grazed. The variety is essential, as plants with different requirements and different length roots coexist without competing and encourage a wider range of microorganisms. At Honeybourne Farm in the Cotswolds, UK, it was found that a sowing of mixed varieties produced a 50% increase in yield over a monoculture, known as the Darwin effect, having been noted by Darwin over 200 years ago.

We know that plants need water, light and nitrogen to grow. Nitrogen exists in the soil in two main forms: organic and inorganic. Crops cannot use organic nitrogen, but microbes in the soil convert it into inorganic nitrogen by a process called mineralization. Hungry plants, for instance those not fed with synthetic fertilizers, form stronger bonds with mycorrhizal fungi and symbiotic microbes, thereby enriching the soil with more diverse and vigorous microorganisms.

The speed at which organic matter breaks down in the soil is dependent on the carbon:nitrogen ratio of the matter. Soil composition will generally settle at a carbon:nitrogen ratio of around 12:1. The best ratio for decomposition is 24:1, which is the optimum for soil microbes, as too much carbon in the soil does not suit them. High-carbon plants such as rye with a ratio of 80:1 take a long time to break down, while low-carbon legumes provide a better ratio for rapid decomposition. This is an important consideration when choosing cover crops.

Plants also need trace elements in smaller quantities. The principal elements are phosphorous, magnesium, calcium, zinc, selenium and microscopic amounts of molybdenum. Molybdenum is essential for nitrogen fixation. In Australia, 28 g spread over 0.4 ha restored fertility for over 10 years. Trace minerals are absorbed by plants, largely through mycorrhizal fungi, becoming part of their structure and an essential element of human food. They are only required in minute quantities, but the band between a deficiency and toxic overload is also very small, and is measured in parts per million. This means that where naturally occurring elements are augmented by synthetic fertilizers, the potential for toxicity arises. A case in point is the use of phosphate fertilizers containing cadmium, or cadmium spread on farmland in contaminated sewage. The cadmium taken up by crops enters the food chain. Incidentally, smokers also inhale additional cadmium from cigarette smoke, tobacco being a plant that readily absorbs toxic minerals.

Food quality follows naturally from soil quality. Diversity of microbes in the soil enables plants to absorb micronutrients such as magnesium, calcium, zinc, sulphur and selenium, whereas synthetic fertilizers can only provide plants with nitrogen and phosphorous. The role that microorganisms play in the nitrogen cycle and in converting nutrients for plant use will be explained in Chapter 2 (this volume).

Nutrient data from 1930 to 1980 in the UK measured calcium, iron and potassium content in 20 different vegetables. Over these 50 years, the calcium content decreased by 19%, iron by 22% and potassium by 14%. Samples taken by government biochemists over a period of 51 years from 1940 to 1991 found potatoes had lost 47% of their copper, 45% of their iron and 35% of their calcium. Carrots, broccoli and tomatoes were in some respects worse. At the University of Texas, biochemists analysed the US Department of Agriculture nutritional data from 1950 to 1999 for 43 different vegetables and fruit, and recorded reliable information in loss of proteins, calcium, phosphorous, iron, vitamin B₂ and vitamin C over 50 years of increased use of artificial fertilizers. It now takes eight oranges to supply the same amount of vitamin C as was found in one orange 50 years ago. It is simple and cheap to measure the nutrient density of the food you grow with a Brix tester. Improvement in readings can be a guide to measuring the success of the planting on your soil. This has long been the tester of choice for the wine trade.

Bibliography

Brown, G. (2018) Dirt to Soil. Chelsea Green Publishing, Chelsea, Vermont.

Darwin, C. (1881) The Formation of Vegetable Mould through the Action of Worms. John Murray, London.

Rainbow, P.S. (2018) Trace Metals in the Environment and Living Organisms. Cambridge University Press, Cambridge.

Stoate, C., Leake, A., Jarvis, P. and Szczur, J. (2013) Fields for the Future. Game and Wildlife Conservation Trust, Fordingbridge, UK.

Tomkins, P. and Bird, C. (2004) Secrets of the Soil. Rupa Publications, New Delhi.

Tree, I. (2018) Wilding. Pan Macmillan, London.

White, G. ([1788] 1981) The Natural History of Selborne. Penguin, London.

Wright, S.F. and Nichols, K.A. (2002) Glomalin: hiding place for a third of the world’s stored soil carbon. Agricultural Research magazine, pp. 4–7. Available at: https://agresearchmag.ars.usda.gov/2002/sep/soil/ (accessed 23 September 2021).

Dr Jonathan Lundgren is Director of the Ecdysis Foundation, and CEO for Blue Dasher Farm in South Dakota.

2Nitrogen-fixing Plants

Nitrogen comprises 80% of the air we breathe. Nitrogen exists in many forms, both inorganic and organic, and is essential in an inorganic form in the air and soil for healthy plant crops. It is an essential part of all nucleic and amino acids required for cell division and growth. Plants that can fix nitrogen in the soil, in a symbiotic relationship with bacteria, are therefore especially valuable. Atmospheric nitrogen must be processed, or ‘fixed’, into a usable inorganic form to be taken up by plants. Between 5 and 10 billion kg of nitrogen year^–1 are fixed by lightning strikes, but most fixation is done by bacteria. Bacteria in a symbiotic relationship with plants have the enzyme nitrogenase, which combines gaseous nitrogen with hydrogen to produce ammonia, which is then converted by the bacteria into other compounds.

The relationship between the plant and the bacteria may be very specific, whereby the plant may only host one type of bacterium, or the possible combinations of hosts and bacteria may be more general. In the process of ‘biological nitrogen fixation’, soil bacteria respond to a chemical message from a nitrogen-starved plant root and fix on to the root hairs. They colonize the cells and produce a nodule in which the nitrogen from the air spaces in the soil is converted into ammonia with other organic compounds that the plant can use, giving it an advantage in poor soil, while also enriching the soil. There are two types of bacteria, symbiotic bacteria, which form nodules, and cyanobacteria, which are effectively algae, making their own nutrients by means of photosynthesis. Cyanobacteria fall more into the category of green manures, and are explored further in Chapter 4 (this volume).

Members of the genus Azotobacter are bacteria that fix nitrogen in the soil and may be used commercially to make biofertilizer. Azotobacter also synthesize some biologically active substances, such as the phytohormone auxin, which stimulates plant growth. This increases crop yields.

Azotobacter also contain antibiotics, which help the seedling to resist disease, and can help to rid the soil of heavy metals such as cadmium, lead and mercury. If the bacteria are not present in the soil, this cannot happen, so seed for commercial growers can be inoculated with the appropriate bacteria to encourage a good rate of nodule formation. This is a very complex interaction, currently the subject of much research. Nevertheless, experiments to date are promising, showing a future for biofertilizers and the reduction of fertilizer application costs and associated fertility loss. Azotobacter species have been found to fix atmospheric nitrogen, dissolve plant nutrients such as phosphates and stimulate plant growth by the production of plant growth hormones such as auxin, cytokinin and gibberellins. Bacteria can be added to the soil in a sterile and neutral carrier. Maize, mustard, rapeseed, sorghum and sugarcane, as well as tomatoes and lettuce, have produced enhanced yields under controlled conditions.

Root hairs absorb nitrate or ammonium from the soil that can be used by the plants. The nitrates absorbed are reduced first to nit...

Table of contents

• Cover
• Title page
• Copyright
• Contents
• Preface
• Introduction
• Acknowledgements
• List of Plant Illustrations in Alphabetical Order of Latin Names
• 1 Soil Health
• 2 Nitrogen-fixing Plants
• 3 Cover Crops, Green Manures and Herbal Leys
• 4 Bacteria and Other Microorganisms
• 5 The Role of Fungi
• 6 Phytoremediators
• 7 Illustrations, with Information on Each Plant
• 8 Soil Stabilizers and Coastal Plants
• 9 Weeds and Invasive Plants
• 10 Hedges and Trees
• 11 Case Studies
• Appendix
• Glossary
• List of Plant Illustrations in Alphabetical Order of English Names
• Index
• Back Cover