- Plant nutrients in the soil
- Major elements
- Gardening FAQ
- Garden Soil Tips
- K-12 Soil Science Teacher Resources
- SOIL BIOLOGY
- WHAT DO SOIL BIOLOGISTS STUDY?
- The Influence of Soils on Human Health
Plant nutrients in the soil
Soil is a major source of nutrients needed by plants for growth. The three main nutrients are nitrogen (N), phosphorus (P) and potassium (K). Together they make up the trio known as NPK. Other important nutrients are calcium, magnesium and sulfur. Plants also need small quantities of iron, manganese, zinc, copper, boron and molybdenum, known as trace elements because only traces are needed by the plant. The role these nutrients play in plant growth is complex, and this document provides only a brief outline.
Nitrogen is a key element in plant growth. It is found in all plant cells, in plant proteins and hormones, and in chlorophyll.
Atmospheric nitrogen is a source of soil nitrogen. Some plants such as legumes fix atmospheric nitrogen in their roots; otherwise fertiliser factories use nitrogen from the air to make ammonium sulfate, ammonium nitrate and urea. When applied to soil, nitrogen is converted to mineral form, nitrate, so that plants can take it up.
Soils high in organic matter such as chocolate soils are generally higher in nitrogen than podzolic soils. Nitrate is easily leached out of soil by heavy rain, resulting in soil acidification. You need to apply nitrogen in small amounts often so that plants use all of it, or in organic form such as composted manure, so that leaching is reduced.
Phosphorus helps transfer energy from sunlight to plants, stimulates early root and plant growth, and hastens maturity.
Very few Australian soils have enough phosphorus for sustained crop and pasture production and the North Coast is no exception. The most common phosphorus source on the North Coast is superphosphate, made from rock phosphate and sulfuric acid. All manures contain phosphorus; manure from grain-fed animals is a particularly rich source.
Potassium increases vigour and disease resistance of plants, helps form and move starches, sugars and oils in plants, and can improve fruit quality.
Potassium is low or deficient on many of the sandier soils of the North Coast. Also, heavy potassium removal can occur on soils used for intensive grazing and intensive horticultural crops (such as bananas and custard apples).
Muriate of potash and sulfate of potash are the most common sources of potassium.
Calcium is essential for root health, growth of new roots and root hairs, and the development of leaves. It is generally in short supply in the North Coast’s acid soils. Lime, gypsum, dolomite and superphosphate (a mixture of calcium phosphate and calcium sulfate) all supply calcium. Lime is the cheapest and most suitable option for the North Coast; dolomite is useful for magnesium and calcium deficiencies, but if used over a long period will unbalance the calcium/magnesium ratio. Superphosphate is useful where calcium and phosphorus are needed.
Magnesium is a key component of chlorophyll, the green colouring material of plants, and is vital for photosynthesis (the conversion of the sun’s energy to food for the plant). Deficiencies occur mainly on sandy acid soils in high rainfall areas, especially if used for intensive horticulture or dairying. Heavy applications of potassium in fertilisers can also produce magnesium deficiency, so banana growers need to watch magnesium levels because bananas are big potassium users.
Magnesium deficiency can be overcome with dolomite (a mixed magnesium-calcium carbonate), magnesite (magnesium oxide) or epsom salts (magnesium sulfate).
Sulfur is a constituent of amino acids in plant proteins and is involved in energy-producing processes in plants. It is responsible for many flavour and odour compounds in plants such as the aroma of onions and cabbage.
Sulfur deficiency is not a problem in soils high in organic matter, but it leaches easily. On the North Coast seaspray is a major source of atmospheric sulfur. Superphosphate, gypsum, elemental sulfur and sulfate of ammonia are the main fertiliser sources.
Plants, as well as all living things, need nutrients and minerals to thrive. These chemical elements are needed for growth, metabolic functioning, and completion of its life cycle.
Most soil conditions will provide many plants adequate nutrition. Testing is available to see what nutrients are in the soil and if the PH is correct, which is necessary for the plant to absorb the nutrients. Nutrient deficiency requiring fertilizing and/or soil amending has many causes. Growing certain crops or plants might deplete the soil. In agriculture, crops are rotated from year to year with increasing soil fertility in mind. Some plants, like roses, are considered “heavy feeders” and need frequent fertilization. A problem that a plant exhibits can often be diagnosed as having a deficiency of a certain element. Iron chlorosis, a deficiency in iron, may be one reason why a plant’s leaves turn yellow or brown between the veins.
Replenishing nutrients in the soil can be done by adding organic material such as compost, dehydrated manure, compost tea or fish emulsion. Traditionallly, most “all-in-one” chemical fertilizers have an “NPK” rating (Nitrogen-Phosphorus-Potassium) and can be purchased in liquid or granule form. In addition to other properties, Nitrogen helps plant foliage to grow strong. Phosphorous helps roots and flowers grow and develop. Potassium (Potash) is important for overall plant health. The plant needs will determine the ratio of each nutrient needed.
HOWEVER – too much of some nutrients, such as phosphorus, can jeapordize the quality of neighboring water bodies from run-off or leaching into the ground when not absorbed by plants. This can contribute to livestock and aquatic animal death, increase the difficulty of water purification, as well as affect recreational use of the water. Many states have banned the use of certain nutrients. Phosphorus has been banned in NY under certain circumstances since 2012. Check with your local extension service for specific information about your state.
Growers of hydroponic plants, grown in water, have found ways for their plants to receive all the necessary nutrients usually found in the soil.
Garden Soil Tips
Basically all types of soil will benefit from the addition of organic matter. Check out the following tips to learn about adding nutrients to improve your soil and ultimately encouraging better plant growth.
- Add a thick layer of mulch and let it rot to improve the soil of existing gardens. Minerals, released as the mulch is degraded into nutrient soup, soak down into the soil and fertilize existing plants. Humic acid, another product of decay, clumps together small particles of clay to make a lighter, fluffier soil. For best success, remember these points:
- Woody mulch such as shredded bark uses nitrogen as it decays. Apply extra nitrogen to prevent the decay process from consuming soil nitrogen that plants need for growth.
- Don’t apply fine-textured mulches, like grass clippings, in thick layers that can mat down and smother the soil.
- Use mulch, which helps keep the soil moist, in well-drained areas that won’t become soggy or turn into breeding grounds for plant-eating slugs and snails.
©2007 Publications International, Ltd.
Add more soil or organic material
to keep shrub or tree roots under cover.
- Get local compost from your city or town hall service department. Made from leaves and grass clippings collected as a public service, the compost may be free or at least reasonably priced for local residents. To find other large-scale composters, check with the nearest Cooperative Extension Service; they are up-to-date on these matters. Or try landscapers and nurseries, which may compost fall leaves or stable leftovers for their customers, and bulk soil dealers, who may sell straight compost or premium topsoil blended with compost. Don’t give up. Yard scraps are discouraged or banned in many American landfills, so someone near you is composting them.
- Plan ahead for bulky organic soil amendments — compost, manures, and leaves — that may be added by the wheelbarrow-load to improve the soil. This will raise the soil level, at least temporarily. As the organic matter decays, the soil level will lower.
- If soils rich in organic matter drop to expose the top of a newly planted shrub or tree roots, add more soil or organic matter to keep the roots under cover.
- If your garden is beside a house or fence, keep the soil level low enough so it won’t come in contact with wooden siding or fencing that isn’t rot resistant.
- When planting around existing trees, shrubs, and perennial flowers, avoid covering the crown — where stems emerge from the ground — with organic material. This helps prevent disease problems.
Where your garden is located and how often you till your soil can also affect its quality. Learn how garden maintenance can improve your soil on the next page.
- Livestock manure
- Straw Grass clippings
- Salt hay
- Shredded bark
- Bark chunks
- Shredded leaves
- Seedless weeds
- Peat moss
- Kitchen vegetable scraps
- Mushroom compost
- Agricultural remains such as peanut hulls or ground corn cobs
Want more gardening tips? Try:
- Gardening Tips: Learn helpful hints for all of your gardening needs.
- Annuals: Plant these beauties in your garden.
- Perennials: Choose great plants that will return year after year.
- Gardening: Discover how to garden.
During the international year of soil in 2015, global attention was drawn to the importance of a healthy soil. One of the key facts emphasized by the food and agricultural organization was that sustainable management of soils can lead to a 58% increase in food production (FAO, 2015). Soil nutrient composition plays a key role in determining the goodness of a soil. A healthy soil will have all the essential elements in the right proportions to support healthy plant growth throughout its life cycle. However, when a soil has been cultivated it may require the addition of organic or inorganic fertilizers.
This article will discuss the soil nutrient composition, the sources of these nutrients and factors influencing their availability.
Soil Mineral Elements
Soil is a major source of nutrients for plant growth. Nutrients supplied by the soil are called mineral nutrients. The non-mineral nutrients such as carbon (C), hydrogen (H) and oxygen (O) come from air and water during photosynthesis. Soil mineral nutrients are separated into two groups the macro and micronutrients. The macro nutrients are further broken down into two groups the primary and the intermediate nutrients. The primary nutrients are required by plants in relatively large proportions. These are the most famous; the nitrogen (N), phosphorus (P) and potassium (K) commonly referred to as NPK. The intermediate nutrients are required by plants in medium quantities, these are calcium (Ca) magnesium (Mg) and sulphur (S).
The micronutrients are required in relatively small proportions. They include the iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl). It is important to note that though the soil nutrients are separated into different groups (based on the quantity required by the plant), each nutrient is equally important. A shortage of any nutrient can limit the growth and yield of a plant. This is in accordance with Liebegs law of the minimum.
Table 1. Forms by which nutrient elements are taken up by plants
Soil Nutrient Sources
Sources of soil nutrient include:
1. Organic matter decomposition
3. Biological nitrogen fixation
4. Inorganic fertilizer application
5. Weathering of soil rocks and minerals
What affects the availability of nutrients in soil?
Factors that affect the availability of soil nutrients include leaching, soil erosion, soil pH, denitrification, volatilization, nitrogen immobilization and crop nutrient uptake. This article will discuss some of these factors. For an exhaustive discussion of all the factors, there is a list of publications or weblinks at the end of this article.
Soil Erosion disrupts the soil structure, washes away organic matter in the soil and therefore reducing soil fertility. This often increases the need for additional and costly fertilizers to compensate for nutrient losses.
Some causes of soil erosion include:
1. Soil gradient: The steeper the gradient of a soil, the more vulnerable it is to soil erosion.
2. Soil erodibility: The more erodible a soil, the more sensitive it is to erosion. This is influenced by the soil characteristic and nature. A soil that has experienced previous erosion will have higher chances of erosion.
3. Vegetative cover: Soil vegetation can protect the soil from wind or water erosion by creating vegetative cover to the soil.
This is the washing downward of nutrients in the soil below the root zone. Some of the factors influencing leaching include:
1. Mobility of nutrients: When there is sufficient water in the soil, nutrients in soil solution can be easily washed down beyond the root zone. An example is nitrogen present in the form of nitrate; a highly mobile negatively charged ion.
2. Soil texture: Leaching occurs in soils which have high water infiltration rates and low ability to hold nutrients. Examples of such are the sandy soil and clay soil.
This process results in the gaseous loss of nitrogen to the atmosphere. Denitrification occurs in warm and anaerobic (saturated) soils usually having high nitrate levels. During this process soil microbes break down nitrate to obtain oxygen for their respiration. The end product is nitrogen gas released to the atmosphere. Ways of avoiding loss of nitrogen through denitrification include proper timing of organic or inorganic fertilizer application; such that the soil receives it when it really needs it. Crop producers are advised to apply fertilizers in splits in order to match the crop demand for nitrogen with supply.
Figure 1: Stages of denitrification reaction
This process also involves the gaseous loss of nitrogen to the atmosphere. When nitrogen fertilizers are applied in urea form (this could be inorganic fertilizers of animal manure), an enzyme urease catalyzes the reaction of urea with water resulting in ammonia gas released to the atmosphere. Volatilization occurs in warm and moist soil conditions. Volatilization also increases with high soil pH. To reduce nitrogen loss through this means, crop producers are encouraged to adapt methods of applying urea or ammonium based fertilizers below the soil surface.
Bibliography and Further Reading
Morgan RPC (2005) Soil Erosion and Conservation. 3rd Ed, Blackwell publishing, UK
Havlin J, Tisdale SL, Beaton JD, Nelson WL (2014) Soil Fertility and Fertilizers: An Introduction to Nutrient Management 8th ed. Boston: Pearson
K-12 Soil Science Teacher Resources
Everything that we eat has nutrients. They are needed for strong teeth and bones, strong heart and blood vessels, and help your brain and nerves work. These nutrients come from the food we eat. We get these from the plants that grow, and the animals that we eat that eat the plants. Most soils have a large supply of nutrients in them, and they get taken up by plants when plants absorb water. Soils need to be healthy to grow large quantities of plants, and animals need plants to grow strong.
Nutrients get into the soil many different ways: from decomposed animal waste and dead plants, the atmosphere, weathering of rocks and bacteria conversions. When soils are used to grow foods, the soils need to be kept healthy, as a lot of nutrients are taken up by plants and not replaced. Nutrients need to be added to replace what is taken out, and the best way to do this is test the soil. Too many nutrients, and it pollutes streams and groundwater, and too little, the plants may die.
This lesson plan is appropriate for grades 4+. There are three different powerpoints that are downloadable for this lesson, Soil is the Ultimate Nutrient, which refers to how all soil is jam packed with nutrients, including Nitrogen, Calcium, and Potassium. The PowerPoint You Eat Dirt! Takes the students through a hamburger, and how the entire cheeseburger relates back to the soil. The third presentation is called Soils Need Food too, and talks about how soils need to be recharged if they are used for continuous food production.
1) Identify nutrients that plants get from soils
Which mineral nutrients are important to plants?
How do plants get these nutrients? à plant roots take up water in the soil (soil water), nutrients are in soil water.
Which nutrients in soil water come from organic materials in soil…from minerals in soil?
2) How do nutrients in plants provide nutrients to people?
Which nutrients needed by people?
What does a “nutrition label” tell you?
Where do animals used in human food get their nutrients
3) Describe how plants tell us their nutrient needs
Plant nutrient deficiency symptoms
Soils can be tested for their nutrient content
4). How do we add nutrients to soil?
Glossary of terms:
In current glossary:
Other glossary words:
Activities or information:
1) Nutrients in soils:
Important Nutrients in Plants – North Carolina Agriculture (Grades K-6) simple description of nutrients essential for healthy plants, also describes influence of soil pH and texture on nutrient supply to plants.
The Great Plant Escape- University of Illinois (Grades K-4) Basic information on nutrients using “mystery solving detective” activities assist learning. Excellent for other basic soil properties information.
2) Nutrients needed by people:
Necessary Minerals- Kids Health (All Grades) Describes the benefits, sources, and quantities of minerals needed in the diet. Teacher can edit to meet grade level need
Kids World Nutrition – North Carolina Agriculture (Grades K-6) excellent activities for kids on understanding nutrition.
Nutrients for Life Teachers Curriculum (All Grades) fully developed curriculum materials for all grade level on nutrients – FREE. Source also gives a $50 grant for lab materials.
Choose My Plate – USDA (Grades K-6) Activities and lesson plans related to the food pyramid.
3) Plant nutrient deficiency symptoms:
Visual Nutrient Deficiencies-University of Bristol (All Grades) Very complete source for nutrient deficiency symptoms of many plants including grains, fruits, and vegetables.
Why Soil Test? – The Compost Gardener (All Grades) General description of soil testing process, website is expanding to other related areas.
4) Nutrients sources
IPNI Academy- International Plant Nutrition Institute (Grades K-6) Activities and games for kids to understand where nutrients come from, includes teacher lesson plans.
How to Home Garden – The Learning Channel (Grades 4-8) teacher source for definitions and function of nutrients in plants.
Soil Composting and Fertilizers – Living a Whole Life Blog (All Grades) Provides a description of nutrients sources with links to each source including organic sources, use a teacher resource.
Composting for Kids – USDA ARS (Grades K-6) discussion of composting
1) Which plant nutrients are macro- and micronutrients and why are they classified this way?
2) Which nutrients are essential for plants and people?
3) Which nutrients primarily come from organic matter and/or mineral materials in soil?
4) How does a plant indicate that it might be deficient in nitrogen? (describe visual symptoms for other nutrients).
5) How can I test the soil for its ability to provide nutrients to plants?
6) What is contained in a bag of fertilizer? (e.g. 18-10-5)
7) How do composts provide nutrients to plants?
Soil is full of life. It is often said that a handful of soil has more living organisms than there are people on planet Earth. Soils are the stomach of the earth, consuming, digesting, and cycling nutrients and organisms.
On first observation, however, soil may appear as a rather inert material on which we walk, build roads, construct buildings, and grow plants. On closer observation, we observe that soil is teeming with living organisms. Living organisms present in soil include archaea, bacteria, actinomycetes, fungi, algae, protozoa, and a wide variety of larger soil fauna, including springtails, mites, nematodes, earthworms, ants, insects that spend all or part of their life underground, and larger organisms such as burrowing rodents. All of these are important in making up the environment we call soil and in bringing about numerous transformations that are vitally important to life.
WHAT DO SOIL BIOLOGISTS STUDY?
The links between soil organisms and how they impact soil chemical and physical properties is complex. Soil biologists study a variety of things.
Microbial Consumers and Decomposers
There are thousands of different types of bacteria, that can both help and harm people.
Only 5% of what is produced by green plants is consumed by animals, but the 95% is consumed by microorganisms. One gram of fertile soil can contain up to one billion bacteria. There are many different types of bacteria, and most of them have not even been discovered yet! Most of these bacteria are aerobic, meaing that they require oxygen from the soil atmosphere. However, other bacteria need to live without oxygen, and other types can live both with, and without oxygen. The growth of these bacteria is limited by the food that is available in the soil.
Soil fungi are also large component of the soil that come in various sizes, shapes, and colors. Mushrooms have underground roots (mycelium) that absorbs nutrients and water until they are ready to flower in the mushroom form. They tolerate acidity, which makes them very important to decompose materials in very acidic forests, that microbes cannot do, they can also decompose lignin, which is the woody tissues for decomposing plants.
Soil animals are consumers and decomposers because they feed on organic matter and decomposition occurs in the digestive tract. Some animals feed on roots, and others feed on each other. There are several types of worms. Earthworms are the easiest to identify. They eat plant material and organic matter, and excrete worm castings in the soil as food for other organisms. They also leave channels that they burrow in, which increases infiltration. Earthworms can weigh between 100-1,000 pounds per acre! There are also microscopic worms called nematodes, or roundworms. These worms live in the water around soil particles. There are several different types of nematodes, some of them eat dead materials, others eat living roots, and some eat other living organisms. Some nematodes are bad, and can cause severe root damage or deformation.
Aside from worms, another large body of insects are arthoropods that have exoskeletons and jointed legs. These include mites, millipedes, centipedes, springtails, and grubs.
Carbon and Nutrient Cycling
Nurtient Cycling is the exchange of nutrients between the living and nonliving parts of the ecosystem. Soil biologists measure how plants and microbes absorb nutrients, and incorporate them into organic matter, which is the basis for the carbon cycle. There are two main processes. Immobilization is when soil organisms take up mineral nutrients from the soil and transform them into microbial and plant tissues. The opposite process is mineralization, which is what happens when organism die and release nutrients from their tissues. This process is rapidly changing, and very important in providing nutrients for plants to grow. The carbon cycle and nitrogen cycle are both very important to soil microbiologists.
Soil Microbe and Organism Interactions
Plant roots leak a lot of organic substances into the soil from dead materials. These provide food for the microorganisms, and create zones of activity around the root called the rhizosphere. In this zone, plant growth or toxic substances can be produced, but most of these organisms are benefical.
This photo is a magnified interaction of fungal and root interactions
Other scientists study soil diseases of plants and animals found in the soil. Bacteria and fungi can cause plants to wilt or rot. The Great Potato Famine in Ireland in 1845 was caused by a fungus that caused the potato blight! These organisms don’t just impact plants. Humans can get sick if certain types of bacteria, like E-Coli, are present in our waste, and that waste isn’t treated properly.
Some fungi “infect” plant roots, but the relationship is symbiotic, meaning that it is beneficial to both the plant and the root. These are called mycorrhiza, and they help plants absorb more water and nutrients, increase drought resistance, and reduce infection by diseases.
Another symbiotic relationship involves nitrogen. There is a lot of nitrogen in the atmosphere, but it is not easy for plants to get. There are certain species of bacteria that absorb nitrogen gas from the atmosphere, and form a nodule. These are called nitrogen fixing bacteria. When the die, the nitrogen that they used are released for plants.
The Influence of Soils on Human Health
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Brevik, E. C. The potential impact of climate change on soil properties and processes and corresponding influence on food security. Agriculture 3, 398-417 (2013b). doi: 10.3390/agriculture3030398.
Brevik, E. C. & Burgess, L. C., eds. Soils and Human Health (Boca Raton: CRC Press 2013b).
Burgess, L. C. Organic pollutants in soil. In Soils and Human Health, eds. Brevik, E. C. & Burgess, L. C. (Boca Raton: CRC Press, 2013) 83-106.
Committee on Minerals and Toxic Substances in Diets and Water for Animals, National Research Council. Mineral Tolerance of Animals, 2nd revised ed. Washington, DC: The National Academies Press, 2005.
Feron, V. J. et al. International issues on human health effects of exposure to chemical mixtures. Environmental Health Perspectives 110(S6), 893-899 (2002).
Food and Agriculture Organization of the United Nations. Trade Reforms and Food Security: Conceptualizing the Linkages. Rome: United Nations (2003).
Hamilton, A. J. et al. Wastewater irrigation: the state of play. Vadose Zone Journal 6, 823-840 (2007). doi:10.2136/vzj2007.0026.
Hubert, B. et al. The Future of Food: Scenarios for 2050. Crop Science 50, S33-S50 (2010). doi: 10.2135/cropsci2009.09.0530.
Lal, R. Soil degradation as a reason for inadequate human nutrition. Food Security 1, 45-57 (2009). doi: 10.1007/s12571-009-0009-z.
Pettry, D. E. et al. Soil pollution and environmental health. Health Services Reports 88(4), 323-327 (1973).
Pimentel, D. & Burgess, M. Soil erosion threatens food production. Agriculture 3, 443-463 (2013). doi: 10.3390/agriculture3030443.
Selinus, O. ed. Essentials of Medical Geology. (New York: Springer, 2013).
SOIL ORGANISMS IN ORGANIC AND CONVENTIONAL CROPPING SYSTEMS
ABSTRACT: Despite the recent interest in organic agriculture, little research has been carried out in this area. Thus, the objective of this study was to compare, in a dystrophic Ultisol, the effects of organic and conventional agricultures on soil organism populations, for the tomato (Lycopersicum esculentum) and corn (Zea mays) crops. In general, it was found that fungus, bacterium and actinomycet populations counted by the number of colonies in the media, were similar for the two cropping systems. CO2 evolution during the cropping season was higher, up to the double for the organic agriculture system as compared to the conventional. The number of earthworms was about ten times higher in the organic system. There was no difference in the decomposition rate of organic matter of the two systems. In general, the number of microartropods was always higher in the organic plots in relation to the conventional ones, reflectining on the Shannon index diversity. The higher insect population belonged to the Collembola order, and in the case of mites, to the superfamily Oribatuloidea. Individuals of the groups Aranae, Chilopoda, Dyplopoda, Pauropoda, Protura and Symphyla were occasionally collected in similar number in both cropping systems.
Key words: soil microorganisms, organic agriculture, microartropods, cropping systems, environmental impacts
ORGANISMOS DO SOLO EM SISTEMAS DE CULTIVO ORGÂNICO E CONVENCIONAL
RESUMO: Apesar do crescente interesse pela agricultura orgânica, são poucas as informações de pesquisa disponíveis sobre o assunto. Assim, num Argissolo Vermelho-Amarelo distrófico foram comparados os efeitos de sistemas de cultivo orgânico e convencional, para as culturas do tomate (Lycopersicum esculentum) e do milho (Zea mays), sobre a comunidade de organismos do solo e suas atividades. As populações de fungos, bactérias e actinomicetos, determinadas pela contagem de colônias em meio de cultura, foram semelhantes para os dois sistemas de produção. A atividade microbiana, avaliada pela evolução de CO2, manteve-se superior no sistema orgânico, sendo que em determinadas avaliações foi o dobro da evolução verificada no sistema convencional. O número de espécimes de minhoca foi praticamente dez vezes maior no sistema orgânico. Não foi observada diferença na taxa de decomposição de matéria orgânica entre os dois sistemas. De modo geral, o número de indivíduos de microartrópodos foi superior no sistema orgânico do que no sistema convencional, refletindo no maior índice de diversidade de Shannon. As maiores populações de insetos foram as da ordem Collembola, enquanto para os ácaros a maior população foi a da superfamília Oribatuloidea. Indivíduos dos grupos Aranae, Chilopoda, Dyplopoda, Pauropoda, Protura e Symphyla foram ocasionalmente coletados e de forma similar entre os sistemas.
Palavras-chave: microbiota do solo, agricultura orgânica, microartrópodos, sistemas de cultivo, impacto ambiental
Contamination of the water-soil-plant system with pesticides and fertilizers, in addition to breaking up the soil structure due to inadequate use of machinery and implements, is one of the main problems caused by intensive agriculture. The implementation of integrated cropping systems and the reduction of the external energy requirements have been suggested to minimize these problems. The organic cropping system is defined as a production system that is sustainable in time and space, by means of management and protection of the natural resources, without the use of chemicals that are aggressive to humans and to the environment, retaining fertility increases, soil life and biological diversity. Thus, the use of highly soluble fertilizers, pesticides and growth regulators must be excluded in this system (Paschoal, 1995). Not only does the system have to satisfy the need for reducing the environmental negative-impact problems caused by intensive agriculture, it must also be economically competitive. In comparing the organic and the conventional cropping systems, an important step is to establish which social, economic and ecological factors influence the production systems the most. Besides, a knowledge of those factors allows for a better understanding of how the production systems are structured and how they work.
With respect to the biological activity, in studies to compare the conventional, integrated and organic cropping systems, Bokhorst (1989) found that the number of worms in a soil planted with sugar beets was five times higher in the organic system than in other systems, and that the percentage of wheat and potato roots infected with arbuscular mycorrhizae was twice as high in the organic as compared to the conventional and integrated systems. Gliessman et al. (1990, 1996), working with similar objectives, compared conventional and organic strawberry cropping systems in areas where farmers became organic producers, and verified an increase in the number of plants infected with mycorrhizae. Swezey et al. (1994) found higher microbial biomass in the soil and in arbuscular mycorrhizae in the organic system than in the conventional, in an area being changed from conventional into an organic apple growing area. All these studies emphasize the biological elasticity in the organic systems as a fundamental characteristic, influencing the occurrence of pests and diseases.
With regard to soil organisms, Brussaard et al. (1988, 1990) verified that the total biomass of soil organisms was higher for the integrated than for the conventional cropping system, with figures averaging 907 kg C ha-1 and 690 kg C ha-1, respectively. Of these biomasses, bacteria accounted for over 90%, fungi represented approximately 5% and protozoa were less than 2% of the total biomass. El Titi & Ipach (1989) studied the effect of a cropping system with low input rate index as well as the conventional system on the soil fauna components and observed there were smaller populations of nematodes pathogenic to plants, higher worm biomass, and larger populations of collembolans and Mesostigmata mites in the system with low input index. Collembola is a microarthropod related to the soil’s capacity to suppress Rhizoctonia solani (Lartey et al., 1994). Rickerl et al. (1989) found that populations of this organism were 29% larger in soils under minimum tillage as compared to soils under conventional tillage. Ladd et al. (1994) verified that the C biomass of microbial populations was greater in soils under crop rotation than in soils under continuous monoculture; greater in soils where plant residues were incorporated or remained on the soil surface than where they were removed; and smaller in a nitrogen-fertilized soil than in non-fertilized ones. This information is important because these are characteristics that contribute to soil biological equilibrium, nutrient mineralization and suppressive capacity toward plant pathogens, among others, making the system less dependent on external input.
The objective of this work was to evaluate the influence of the organic and the conventional cropping systems, for tomato and corn, on the community of soil organisms.
MATERIAL E METHODS
The experiment was set up as randomized blocks with six replicates, and plots measuring 25 x 17 m. Tomato planting pits were spaced 0.5 m apart with 1.20 m between rows. Each plot was split in two halves, the first 12.5 x 17 m-half being planted with the variety Débora and the other planted with the variety Santa Clara. Therefore, each of the twelve rows contained 17 planting pits for each variety. The edging between plots was 10 m wide and was planted with sorghum. Two tomato plants were transplanted per pit. The tomato crop was conducted using the stake system, with one or two stems/plant. The number of stems was determined based on the successful establishment of the seedlings. Furrow irrigation and plant pruning were performed as often as necessary.
In the conventional system, 0.15g/pit of active ingredient of the insecticide carbofuram were applied before planting. According to the procedures utilized by conventional local growers, a blend of insecticides, fungicides and miticides was sprayed twice a week, after planting. Active ingredients of fungicides sprayed during the crop cycle were metalaxyl, mancozeb, chlorothalonil, copper oxychloride, kasugamycine, cuprous oxide, methyl thyophanate, iprodione, benomyl, cymoxamil, maneb and monohydrate zinc sulphate, at the rates recommended by the manufacturers. Insecticides used were deltamethrin, permethrin, methomyl, methamidophos, acephate, avermectin and cartap, also at the recommended rates.
Extracts of black pepper, Eucalyptus, garlic and fern; Bordeaux mixture, and biofertilizer were applied twice a week (Bettiol et al., 1997; Abreu Junior, 1998) to control diseases and pests in the organic system. These applications were performed according to the program adopted by organic producers in the region.
Weed control was carried out by mechanical weeding and with the herbicide glyphosate (directed spray) on post-planting in the conventional system, and with mechanical weeding in the organic system.
After harvesting the tomato the area was planted with ‘BR 201’ corn; sowing occurred 178 days after planting the tomatoes. The organic system plots received an application of 4 m3 of organic compost and single superphosphate at the rate of 20 g per meter; in addition, the biofertilizer was sprayed at 10% as sidedressing. In the conventional system fertilization consisted of 500 kg ha-1 of the 4-14-8 NPK rate applied pre-planting and 15 g m-1 urea as sidedressing. Weed control used the herbicide paraquat (directed spray) in the conventional system, and mechanical weeding was used in the organic.
After harvesting the corn, ‘Débora’ tomatoes were again cultivated, as previously described. Transplantation was made 401 days after the initial tomato planting.
A sample composed of 20 sub-samples of soil taken at the planting row from the 0-7 cm-depth layer was obtained for each plot. Samples were placed in plastic bags and immediately transported to the laboratory. Assessments were performed within 24 hours after collecting the samples.
Populations of fungi, bacteria and actinomycetes: The populations of fungi, bacteria and actinomycetes were quantified through the serial-dilution method, followed by plating in culture medium. Martin’s culture medium (Tuite, 1969) added of 100 mg mL-1 streptomycine was used for fungi; for bacteria, the agar nutrient medium added of nistatin (42 mg L-1) was used; for the actinomycetes, the alkalized agar-water medium was utilized. Aliquots (0.1 mL) from three dilutions, for each soil sample, were transferred to the culture media in three replications. Assessments were performed by counting the number of colonies per Petri dish and expressed as colony-forming units/g of dry soil (CFU g-1 dry soil).
Total respiratory activity: Total microbial respiration was evaluated according the method described by Grisi (1978). Soil samples (200 g) were incubated for 10, 20, and 30 days within tightly sealed containers holding 10 mL of a 0.5 mol L-1 (10 mL) KOH solution. At 10-day intervals, the solution was substituted and titrated with 0.1 mol L-1 of HCl. Incubation was conducted in the dark, at 25°C. This parameter was expressed as g CO2 (g dry soil-1) (day-1). Since the more substantial changes happened in the first days, only readings up to the tenth day were used to determine mean values. For the statistical analysis, data were transformed into square root (x + 0.5) and subjected to analysis of variance and Duncan’s mean comparison test.
Soil microarthropods: Collecting was made with a Uhland-type, stainless steel auger 5 cm in diameter and 10 cm in height, totaling four samples per plot. Samples were placed in plastic bags and taken to the laboratory. Collecting was between 8:00 and 10:30 h, 82 days before and 325 days after the first tomato seeding, for a total of 16 evaluations. Extraction was according to Tullgren’s modified method, which uses heat and desiccation to force the animals to leave the soil. Samples remained in the extractor for 72 hours. An alcohol:glycerin (1:1) aqueous solution was used for specimen preservation. After extraction, the animals were counted and separated into groups with the use of a stereoscopic microscope. Mites and other smaller animals were fixed on permanent slides for identification. Data were expressed as number of individuals per 785 cm3 soil. Shannon’s diversity index (Shannon & Weaver, 1949) was calculated for a better understanding of the variations in the soil microarthropod populations.
Organic matter decomposition rate estimate: The decomposition rate was estimated via loss of organic content from leaf litter confined in nylon bags, 20 x 20 cm, with a 1 mm mesh, where 10 g of elephant grass dried at 60°C for three days. The field-collected samples, were collected every 20 days and transported to the laboratory, dried at 105°C for 24 hours and ashed at 600°C for 4 hours. The loss of organic matter estimate was calculated using the equation described by Santos & Whitford (1981), which corrects for the adhesion of soil particles to the organic matter.
Evaluation of earthworms in the soil: The first evaluation was carried out 81 days before the first planting, i.e., before plowing and liming. A hand excavator was used to collect samples; two samples were collected from each plot, up to a depth of 20 cm, with 20 cm diameter. Shortly after planting the tomatoes, and 90 days later, samples were taken at about 40 cm depth, with a diameter of 10 cm. Three samples were collected from the compost: one from the pile surface; another at a layer up to 35 cm, and the third at a depth of 90 cm. The worm populations were determined 370, 407, and 471 days after the first tomato planting.
RESULTS AND DISCUSSION
The populations of fungi, bacteria and actinomycetes were similar for the two cropping systems over the entire period of study, with populations of fungi varying from 104 to 105, whereas populations of bacteria and actinomycetes varied from 105 to 107 CFU g-1 dry soil (Figure 1). Similar results were obtained by Castro et al. (1993), when several types of soybean management were compared, and by Cattelan & Vidor (1990) on soils cultivated with different crop rotation systems. Grigorova & Norris (1990) justified not adopting this method for evaluating soil microorganisms, because only a small fraction of microbial biomass could be cultivated on a selective medium. However, Cattelan & Vidor (1990) demonstrated the effectiveness of the method in studies with different cropping systems. In spite of a similar behavior in regard to microbial populations, starting 145 days after planting the tomatoes, the bacteria populations (Figure 1 C) were higher in the organic system as compared to the conventional. This could be due to soil plant cover, like Cattelan & Vidor (1990) who found a smaller bacterial population on naked as compared to cultivated soil.
Soil total respiratory activity continued higher in the organic system during the crop cycles, showing in some evaluations twice as much as the evolution observed in the conventional system (Figure 2). Differences were found during the intermediate period, that is, between 142 and 400 days after planting. There were no statistical differences between treatments at the initial periods or at the end. The higher respiratory rate in the organic system could be due to the addition of an exogenous source of organic matter to the soil and the consequent stimulation of heterotrophic microorganisms (Lambais, 1997).
Observed organic matter decomposition rates ranged from 15 to 45% of organic carbon loss in a 20-day period. Rodrigues et al. (1997) observed, in corn cultivated during the summer, values reaching 70% of carbon loss in a period of 30 days. There was no difference among results from the organic and the conventional systems (Figure 3). However, regardless of the system, there was an influence of time on the organic matter decomposition rate was, although no interaction between time and the treatments was found. This suggests that variations found during the study period could be related to the humidity and temperature fluctuations that occur in the field, thus providing no evidence that the adopted management forms influenced decomposition rate.
The CO2 release method used in this study to evaluate respiratory activity favors the microorganism population, since soil manipulation can eliminate the majority of the microarthropod community. Several authors have, in microcosmos studies, demonstrated the role microarthropods in soil organic matter decomposition process. A low fungivore density (Collembola) has a stimulating effect on microbial respiration, whereas high densities inhibited microorganism respiration Barsdate et al, 1974; Hanlon & Anderson, 1979).
Mites and insects, belonging to various families, were the two main groups of arthropods found in the soil in 1993 and 1994 (Tables 1 and 2). In general, rates and numbers of individuals from these groups were higher in the organic cropping system, reflecting on Shannon’s diversity indices, which were higher in the organic system on all sampling dates (Figure 4), but not on the soil organic matter decomposition (Figure 3).
The largest populations of insects were from the Order Collembola, and the number of individuals found in the organic system was three times as high as that in the conventional system, during the first nine months (Table 1). During the following six months, the number of collembolans remained 20% higher in the organic cropping system than in the conventional (Table 2). These data agree with El Titi & Ipach (1989), who verified larger populations of collembolans for the low-input system than for the conventional. Collembolans contribute to the soil’s abilitity of suppressing plant pathogens such as Rhizoctonia solani, Fusarium oxysporum f. sp. vasinfectum, and Pythium (Wiggins & Curl, 1979; Curl et al., 1985a, b; Rickerl et al., 1989; Lartey et al., 1994), because these organisms are, for the most part, mycophagous, modifying the community of fungi. Because in this work the practices in the organic system stimulated the community of collembolans, it can be inferred that these organisms are responsible, at least in part, for the suppression ability in soils enriched with organic matter. Still, in regard to insects, the number of individuals was low for the rest of the orders (Tables 1 and 2).
During the first nine months of evaluation (Table 1), for both cropping systems, the largest mite population was of the superfamily Oribatuloidea, followed by the family Galumnidae and by the superfamily Passalozetoidea, all in the suborder Oribatida and with similar behavior between cropping systems. In the suborder Gamasida the most abundant population was Laelapidae and in Actinedida the most abundant was Pygmephoridae, both more numerous in the organic system. Populations in the suborders Acaridida and Ixodida were very small. In the six subsequent months (Table 2), when only the families of mites were quantified, the largest population was of Scheloribatidae followed by Galumnidae, with similar behavior between the systems. The expressive number of individuals in the families Galumnidae and Scheloribatidae for both cropping systems is due to the characteristic these families exhibit toward occupying space in agroecosystems. In the orders Actinedida and Gamasida, families Cunaxidae and Laelapidae were the largest, respectively. In general, mite population densities in the classes Gamasida and Actinedida were higher in the organic system. The fact that the Gamasida showed high numbers is possibly due to a large Collembola population, because these organisms are a source of food for this class of mites. El Titi & Ipach (1989) verified the existence of larger populations of collembolans and Gamasida mites in the low-input system than in the conventional.
Due to the more abundance of microarthropods in the organic system, it was believed that the organic matter decomposition rate would be higher in this system, because these organisms contribute for organic matter degradation and stimulate microbial activity in the soil (Nosek, 1981). Accordingly, when the presence of Oribatida and Collembola in litterbags incorporated into the organic and the conventional systems was evaluated, a larger number of individuals in the litterbags was found for the organic system (Melo & Ligo, 1999), indicating that this system contributes for an increase in biological diversity. Since the presence of these organisms in larger numbers was not accompanied by a higher decomposition of organic matter, one can say that the differences in arthropod density found in the soil between the organic and the conventional systems did not reflect on the organic matter decomposition rate, as evaluated by the litterbag method. The community of microarthropods in the soil might have, among other factors, influenced microbial activity, since the organic system showed a higher microbial activity potential than the conventional system. The influence of the soil fauna on the organic matter decomposition rate of forest soils is well documented, but this is not true for agricultural ecosystems (Crossley et al., 1989). In agroecosystems the effect of the fauna on the organic matter decomposition rate seems not to be very significant and consequently, there are many points that need to be clarified when it comes to the role of fauna in agricultural soils. Occasionally, and similarly among the crop systems evaluated, individuals belonging in the groups Aranae, Chilopoda, Diploploda, Diplura, Pauropoda, Protura and Symphila were collected. In addition to these, individuals of the insect orders Dermaptera, Hemiptera, Homoptera, Isoptera and Thysanoptera were found in limited numbers.
The higher biological diversity in the organic system is important because it contributes to keeping the biological equilibrium, essential in an agroecosystem. This equilibrium may bring about greater stability for the system and consequently fewer problems with diseases and pests.
With respect to the worm community, after a one-year period of cropping, the soil in the organic system showed at least a ten-fold higher number of specimens per 3140 mL soil sample than the conventional system. After 370, 407 and 471 days from planting a total of 18, 24 and 101 specimens were found in the organic cropping system, and 1, 2 and 12 specimens were found in the conventional system, respectively. These data agree with Bokhorst (1989), who found that the number of worm individuals per square meter, in a soil planted with sugar beets, was five times higher in the organic system as compared to the conventional. Also, El Titi & Ipach (1989) observed the existence of greater worm biomass in a low-input system than in the conventional. The higher number of species in the organic system is possibly due to the availability of organic substrates for them to breed on and the absence of pesticides. On the other hand, the presence of pesticides explains the small number of species in the conventional system, since the worms are sensitive to the products used in the conventional system (Lee, 1985). These organisms are important because they not only improve the physical properties (Lee, 1985), but also contribute to the soil’s ability to suppress pathogens, such as R. solani, among others (Stephens et al., 1993). No worm specimens were found in the recently plowed soil (81 days before planting) and in the evaluations carried out at planting time and 90 days after planting the seeding, as well as in the organic compost.
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Received July 29, 2001