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COG Organic Field Crop Handbook
1.3 The Soil Ecosystem
Understanding the role of the soil in the farm ecosystem, and knowing how to manage the land, are critical and difficult tasks facing the organic farmer. The soil's biological and electrochemical processes cannot be observed directly since they take place at a microscopic and sub-molecular level. Changes in fertility, tilth and structure may take years to become evident. Early indicators are subtle, and the farmer must be a keen observer to spot them. This chapter reviews the biological, chemical and physical properties of the soil as a background to sound management. Much of the material will be familiar; the key difference is the recognition of the vital role soil micro-organisms play in recycling, releasing, and storing plant nutrients. Organic farmers use techniques that support and enhance the biological life of the soil, which in turn nurtures the crop and maintains soil structure.
1. Soil biology
Most of the farm's life exists out of sight, beneath the surface of the soil. Billions of organisms inhabit the upper layers of the soil, where they break down dead organic matter, releasing the nutrients necessary for plant growth. The micro-organisms include bacteria, actinomycetes, algae and fungi. Macro-organisms include earthworms and arthropods such as insects, mites and millipedes. Each group plays a role in the soil ecosystem and can assist the organic farmer in producing a healthy crop. Micro-organisms can be grouped according to their function: free-living decomposers convert organic matter into nutrients for plants and other micro-organisms, rhizosphere organisms are symbiotically associated with the plant roots and free-living nitrogen fixers.
In an undisturbed soil, leaves and other organic debris accumulate on the surface, where they are broken down by the decomposers. Aerobic bacteria and certain small animals begin the process. These organisms are joined by actinomycetes and fungi. Mites, springtails, small insects, other arthropods and earthworms assist the process by consuming, mixing and transporting materials. The rate of decomposition is affected by soil temperature, moisture and food availability. The main by-products of the decomposition process are soluble plant nutrients and microbial remains that bind the soil particles together, giving a stable crumb structure. Since biological activity is greatest when the soil is warm, nutrient availability is highest during summer, when crop needs are greatest. The decomposers are most active in the upper layer of the soil, i.e. the top 8 cm (3 in.). Organic farmers incorporate organic matter into the surface layers when conditions are favorable to stimulate decomposition and thereby provide plant nutrients.
Plant roots leak or exude a large number of organic substances and continually slough off root caps into the soil. These materials are food for the many micro-organisms living in a zone of intense biological activity near the roots called the rhizosphere. Bacteria benefit most from the food supplied in the rhizosphere and may form a continuous film around the root. Roots form the microbial highways of the soil. Other micro-organisms liberate nutrients from the clay and humus colloids (a colloid is a mass of fine particles).
Symbiotic organisms in the rhizosphere
The best-known symbiotic relationship occurs between nitrogen-fixing Rhizobia bacteria and legumes. The Rhizobia inhabit small pea-like lumps on the roots, extracting carbohydrates from the plant and providing the plant with soluble nitrogen compounds synthesized from nitrogen gas in the soil atmosphere. Mycorrhizal fungi have similar symbiotic relationships with the roots of many plants. By extending the surface area of the roots by as much as 400 times, the fungi help the plant with the absorption of water and nutrients and with its ability to withstand heat and drought. These symbiotic relationships begin at germination when the young sprout exudes toxins to kill pathogens and hormones to attract beneficial organisms.
The most important group of larger soil animals are the earthworms, of which there may be a dozen species in a healthy Canadian soil. Earthworms perform the final task of humification -- the conversion of decomposed organic matter to stable humus colloids -- and mix the humus with material from the lower soil horizons. The digestive tract of the earthworm has a remarkable capacity to literally alter the chemical and physical nature of soil. Earthworms are major agents in the process of soil creation through the formation of clay-humus complexes and they play a key role in the management of calcium. By inoculating their castings with intestinal flora, earthworms distribute microbial populations throughout the soil. Earthworms can increase the availability of phosphorus from rock phosphate by 15-39 per cent. They act as mini-subsoilers, their burrows increasing soil aeration, drainage and porosity. In the process of burrowing, earthworms mix the subsoil with the topsoil and deposit their nutrient-rich castings on or near the soil surface. The presence of a large earthworm population indicates good soil fertility. They can be encouraged by adding lime when needed to correct soil acidity and organic matter to provide the worms with food. Note that the red wriggler, or manure worm, prefers an environment higher in organic matter and cannot survive in most soils; inoculating a field with these worms will not improve soil fertility.
[Illustration earthworm, no caption]
Mites are the most abundant of the soil arthropods. Most mites are beneficial, feeding on micro-organisms and other small animals. They assist with decomposition by browsing on preferred fungi, thus preventing any one species from becoming dominant, and by transporting the spores through the soil. Springtails perform similar functions. Larger arthropods, slugs and snails burrow through the soil and feed on dead plant material. By maintaining a suitable environment for the hundreds of species of soil creatures, large and small, organic farmers provide their crops with an abundant supply of plant nutrients.
2. Soil chemistry (fertility)
Organic matter and humus
Organic matter (OM) is the term used to describe the component of the soil made up of the remains, residues or waste products of any living organism. OM comes primarily from plant residue, but also includes soil micro-organisms and animal remains. The amount of OM in a soil depends on its type and how it is managed. OM content can range from one per cent, in the case of a sandy soil in which no special management practices have been used to build OM, to more than 30 per cent in a muck soil.
Soil life depends on the continual replenishment of OM. Most organic farming practices, such as crop rotation, composting, green manuring and keeping the soil covered, help to increase the soil's OM and hence its biological activity. Including a three- to five-year grass-legume sod in the rotation is an effective way of increasing OM because losses are minimized when there is no tillage. It is important to understand that OM alone does not guarantee fertility or biological activity. Peat moss, for example, is made up entirely of OM but contains few nutrients. Excessive moisture will produce anaerobic conditions in which OM will rot and will favor the development of pathogens that may infect the crop. The soil must be managed so that the OM produces the intended results, namely an increase in available plant nutrients, improved soil structure, and increased nutrient reserves.
[Illustration - Grass-legume sod]
When fresh OM is added to the soil, the micro-organisms immediately start to decompose it. The partially-decomposed residue is called effective humus. It holds nutrients by absorption, releasing them to plants as needed and prevents their loss by leaching. Micro-organisms that decompose residues with a high carbon content will utilize some of the available nitrogen, making it temporarily unavailable to a crop that is seeded immediately afterwards.
Stable humus is the final product of the decomposition process. It can be recognized by its dark color, crumbly or slightly gelatinous texture and characteristic "earthy" smell. Stable humus, or colloidal humus, provides long-term nutrient reserves and improves soil structure and cation-exchange capacity.
Benefits of humus:
supplies nutrients, especially nitrogen (N), phosphorus (P) and sulphur (S), when the plant needs them;
holds nutrients, thereby reducing nutrient leaching;
binds soil particles together, stabilizing loose soils against erosion;
increases the friability of heavy soils; and
improves porosity, thereby facilitating air and water movement, and increases the soil's water-holding capacity.
Cation-Exchange Capacity (CEC)
Plants obtain many of their nutrients from soil by an electrochemical process called cation exchange. This process is the key to understanding soil fertility. Cation exchange requires very small particles with a large surface area to hold electrically-charged ions. Humus colloids are ideal; clay colloids also have a good CEC, but sand particles are too big. The finely-divided platelets of the humus and clay colloids produce a large surface area -- one gram of the clay mineral bentonite has been estimated to have a surface area of 800 square metres. The surfaces are coated with a thin film of water, which contains dissolved nutrients. Each platelet has an extra electron, which gives it a negative charge. This negative charge attracts positively-charged nutrient ions from the nutrient solution such as ammonium (NH4+), calcium (Ca++), magnesium (Mg++) and potassium (K+). These nutrient ions can be absorbed by the plant root, by exchanging them for other ions such as hydrogen (H+). Many soil micro-organisms carry a negative charge, which enables them to attract nutrients, and to move freely about the humus and clay colloids.
CEC measures the quantity of potentially-available cation nutrients that are in a stable, accessible form. It is measured in milliequivalents (me) per 100 grams of soil. Typical values are 6.3 me/100g for sand and 27.2 me/100g for clay/loam. The higher the CEC, the greater the potential fertility of the soil. This is why clay soils tend to be more fertile than sandy soils, and why the fertility of sandy soils can be improved by the addition of clay and humus. The cation-exchange process can however only store and release positively-charged nutrients; the availability of nutrients in anion form, such as phosphorus and sulfur is not affected by CEC. Soil organisms play a key role in conserving and releasing these nutrients.
Soil pH and the role of calcium
The term pH refers to the acidity or alkalinity of a soil. It is important because it influences soil nutrient availability and biological activity. pH ranges from 0-14. A pH level below 7 (the neutral point) is acidic, and above 7 is alkaline. Soil pH ranges from 4-9; fertile soils are usually between 6.0 and 7.0.
Acid soils have, by definition, a large number of free H+ ions. Acidity reduces bacterial activity and therefore decomposition and nutrient release. Nitrogen-fixing Rhizobia and legumes generally do not do well in acid soils. Excess H+ ions displace nutrient cations attached to the soil colloids, thus depleting the soil's nutrient reserves. An acid soil may, therefore, have a high CEC but be low in fertility.
The addition of crushed limestone (CaCO3) corrects an acid soil. An acid soil with a high CEC needs a greater amount of limestone than a low CEC soil of the same pH, because of the very much greater number of reserve H+ ions held in the soil with the high CEC.
Lime not only corrects soil pH, it also supplies the plant nutrient calcium. Its double electrical charge, Ca++, lets it function as a link, binding clay and humus colloids together in clay-humus complexes. The resulting soil has improved structure, is less subject to erosion and has improved nutrient-holding capacities. Dolomitic limestone functions in a similar way to calcitic limestone, but in addition contains magnesium (Mg++). It should only be used in areas that are low in magnesium. If magnesium levels are high compared with calcium it will have adverse effects on the crops and on the breakdown of organic residues in the soil.
Excessively alkaline soils have few free H+ ions and an excess of sodium (Na+) ions. Biological activity is suppressed and associated nutrient availability decreased. Additional problems include destruction of OM, saline seepage, soil crusting and the accumulation of toxic levels of sodium, selenium and other minerals. Alkalinity can be reduced somewhat by the addition of gypsum (calcium sulphate) or, in extreme circumstances, sulfur. Gypsum is used to reduce magnesium and supply calcium and sulphur without raising the pH.
[Figure 2: General relationship between soil pH and availability of plant nutrients: the wider the bar, the more availability]
The vegetative growth of plants (leaves, stems, and roots) is especially dependent on nitrogen. The atmosphere contains 78 per cent nitrogen by volume, yet it is the element that most often limits plant growth. Plants cannot use gaseous nitrogen, but require nitrogen in the form of nitrate (NO3-) or ammonium (NH4+). Atmospheric nitrogen is converted into NO3- and NH4+ in the soil by nitrogen fixation, which is performed by certain soil micro-organisms. These include the symbiotic Rhizobia bacteria associated with legumes, and the non-symbiotic bacteria Clostridium and Azoterbacter which are free-living in the soil.
Once gaseous nitrogen is incorporated into plant material as proteins and amino acids, it may be recycled many times through the activity of the soil decomposers. Young plants are especially rich in nitrogen and, when they are incorporated in the surface layers of the soil as green manure, this nitrogen is released by biological activity. The ammonium (NH4+) ions can be stored on the clay-humus complex for long periods. The nitrate ions (NO3-) are subject to leaching if not taken up by the crop.
Deficiencies of nitrogen may occur not because there is not enough entering the system but because of the way it cycles round the system. Cycling is increased by maximizing biological activity which is determined by the way different components of the system, such as residues, manure, weeds and drainage, are managed.
Carbon is the building block of life. Plants obtain carbon from atmospheric carbon dioxide (CO2) through photosynthesis, during which the chloroplasts in the plant cells convert CO2 to carbohydrates. It is the cycling of carbon from the atmosphere through plants and algae, to animals and micro-organisms and back to the atmosphere, that maintains earth's atmosphere and climate in its current balance. The greenhouse effect, or warming of the planet, is a consequence of an excess of atmospheric CO2 caused by deforestation (reduced CO2 consumption) and compounded by excessive fossil fuel energy use (increased CO2 production). Keeping the soil covered with growing plants can make a contribution to reducing global warming.
Carbon is a critical element in the formation of stable humus. The carbon:nitrogen (C:N) ratio of the organic matter supplied to the soil is a controlling factor in this process. A ratio of about 20:1 is considered ideal. If greater amounts of carbon are present, decomposition slows as micro-organisms become nitrogen-starved and compete with the plants for available nitrogen. Nitrate nitrogen practically disappears from the soil because microbes need nitrogen to build their tissues. If there is too much soil nitrogen, the decomposers produce soluble nutrients in the form of effective humus, but little stable humus. These conditions can give the advantage to weeds rather than the crop. A good C:N ratio will result in the formation of both effective humus and stable humus. As decay occurs, the C:N ratio of the plant material decreases since carbon is being lost as CO2, and nitrogen is conserved. This process continues until the micro-organisms run out of easily-oxidized carbon. The exuded, undecomposed carbon persists as stable humus.
Phosphorus (P) is important in plant-cell division and growth. It is a difficult nutrient to manage because, although abundant in the soil, it is often in a form unavailable to plants. In acidic soils (pH below 5) the phosphorus gets tied up with iron and aluminum, and in alkaline soils (pH above 7) it gets tied up with calcium. Even with a favorable pH, phosphorus readily becomes immobilized by other soil minerals. Phosphorus anions may also be physically trapped in the clay-humus complex. Phosphorus is lost from soils through soil erosion, often at a greater rate than it can be replaced from the underlying subsoils. It accumulates in lakes and slow-flowing rivers, causing eutrophication. The elimination of soil erosion is the first step in phosphorus conservation. The addition of powdered rock phosphate or colloidal phosphate is a precautionary measure which, used in conjunction with the biological measures described below, can avoid phosphorus deficiency.
The release of P to plants depends on soil biological activity, particularly that of certain bacteria and mycorrhizal fungi. Soil acids, produced by these micro-organisms and by OM decomposition, release phosphates. Phosphorus availability is therefore dependent on the maintenance of high levels of biological activity and stable humus in the soil. Under these conditions, phosphorus is continually recycled through the processes of OM decay. Some plants produce acidity around their roots which assists in the uptake of P; examples of these are legumes actively fixing nitrogen, rapeseed, oilradish and buckwheat.
[Illustration, line drawing: Buckwheat]
Potassium (K) is important as an enzyme activator in plants. It is involved in facilitating membrane permeability and translocation of sugars. Potassium is also needed for photosynthesis, fruit formation, winter hardiness, disease resistance, and amino acid and protein formation. Potassium builds plant stalk strength. It does not, however, form a permanent part of plant tissues, but is translocated to the stems and roots during ripening. Thus, potassium is readily available in crop residues -- roots, straw and corn stalks. Very little potassium is removed with a grain crop at harvest if the straw is left on the field. Repeated cutting for hay or silage without returning potash in the form of manure or crop residues will quickly induce K deficiency.
Soil potassium is present in minerals that dissolve slowly, thereby limiting its availability. Potassium availability is regulated by cation exchange. Potassium leaching increases as the amounts of clay and humus decrease and therefore may be a problem in sandy soils. A deep-rooting green manure will help prevent losses. Increased biological activity and colloidal humus formation will increase potassium availability by enhancing the CEC in the soil. The addition of powdered basalt, green sand and clay minerals has been found to correct potassium deficiencies in a biologically active soil. Manure is a good source of K if care has been taken to minimize leaching during storage.
It has been reported that in some organic systems, low available potash levels, according to soil analyses, are not necessarily associated with plant deficiencies or lower yields. This may be because available K is immediately taken up by the growing plant.
About one hundred elements have been found in living plants. Carbon, hydrogen, and oxygen are the most abundant and are derived from water, oxygen and carbon dioxide. The nutrients N, P, K, calcium and magnesium have been discussed above. Of the other elements, we know that sulfur, iron, copper, manganese, zinc, molybdenum, boron and chlorine are required by plants in trace amounts. They are not constituents of the plant structure, but contribute to plant growth and development. Other elements, such as iodine, are essential to the animals that eat the plants. Deficiencies occur in soils that lack an inherent source of an element, or they can be caused by an imbalance in soil pH. Conversely, if certain micronutrients exceed trace levels, they can be toxic to plants. The range between deficiency and excess is very small. Therefore, micronutrients should not be applied unless a deficiency is shown by leaf analysis or by visible plant symptoms. Micronutrients are best applied via compost, or by a foliar spray. Either of these methods is preferable to applying a trace mineral directly to the soil. In a biologically-active soil with good CEC and balanced pH, micronutrient deficiencies are rare. Products based on seaweed (kelp) contain more than 80 elements, and organic farmers feed kelp meal as mineral supplement to their livestock, or incorporate small amounts of kelp products into compost as a precautionary measure against micronutrient deficiency.
Water, air and drainage
Fundamental to soil ecology is the cycling of water to the soil through precipitation and its return to the air through evaporation and transpiration. Biological activity is dependent upon the balance of air and water in the soil. Too much water causes aerobic decomposition to cease and anaerobic bacteria to take over, with damaging effects. For example, nitrification, or the breakdown of nitrate nitrogen to gaseous nitrogen, occurs as a result of anaerobic biological activity in the soil. Too little water also causes biological activity to slow down and hence reduces the availability of nutrients.
The water available to plants is the moisture held mostly by capillarity in small soil pores. A soil with a large number of small pores, such as a clay-loam, will withstand drought much better than a sandy soil, which has few capillary pores. Large pores allow drainage and air flow that supplies oxygen and nitrogen for root and microbial growth. Both types of pore space are important for soil fertility, and both can be maintained and enhanced by the addition of organic matter and humus to the soil.
An ideal soil has a high infiltration rate, and fairly slow hydraulic conductivity. The infiltration rate is the rate at which water soaks into the ground; if the infiltration rate is slower than the rate of precipitation, the excess water will become surface run-off, with attendant erosion and pollution hazards. Hydraulic conductivity is the rate at which water drains through a saturated soil. This action transports nutrients from the surface layers to the rhizosphere. If the hydraulic conductivity is too fast, nutrients will be leached out of the soil and groundwater may become polluted. Organic matter in the form of cover crops or mulch improves the infiltration rate. When converted into humus through biological activity, organic matter can lower the hydraulic conductivity of sandy soils.
Wet soils, if caused by high groundwater levels, tend to be unsuitable for organic field crops and are often better left as permanent pasture, or allowed to revert to natural habitat. If the water problem is caused by compaction or hardpan, chisel plowing or subsoiling may correct the situation. Earthworms, and crops with long tap roots such as alfalfa, can then help to maintain the field in improved condition. Solutions such as ditching or tile drainage should be very carefully assessed for their environmental implications.
3. Soil physical properties
The term soil structure is used to describe the way soil particles are grouped into aggregates. Soil structure is affected by biological activity, organic matter, cultivation and tillage practices. Soil fertility and structure are closely related. In an organic production system soil management techniques are designed to enhance soil structure.
An ideal soil structure is often described as granular or crumb-like. It provides for good movement of air and water through a variety of different pore sizes. Plant roots extend down and soil animals, including small earthworms, travel through the spaces between the aggregates. An ideal soil structure is also stable and resistant to erosion. The clay-humus complex, in combination with adequate calcium which helps to bind the aggregates together, forms the basis of this structure. The glutinous by-products of soil bacteria and the hair-like threads of actinomycete and fungi mycelium add to soil stability. Plant roots also play a role in maintaining soil structure.
All tillage operations change soil structure. Excessive cultivation, especially for seedbed preparation, can harm soil structure. Working clay soils when wet leads to compaction and subsequent soil puddling. The soil is easily puddled by rain, easily eroded and will have poor aeration. Tillage, when too dry, shatters the aggregates. Careful cultivation, growing sod crops and returning crop residues can enhance soil structure. Organic matter and the humification process improve structural stability, and can rebuild degraded soil structures. Therefore it is vital to return organic material to the soil and to maintain its biological activity.
Tilth and tillage
Tilth is the term used by farmers to describe how easy it is to till the soil. It is determined by soil structure, presence or absence of hard-pans, soil moisture and aeration. Tilth determines the soils fitness as a seedbed, especially for root penetration and shoot emergence. However, if the deeper soil layers are compacted or cemented, plant roots will be prevented from getting to the stored water in these layers and plant growth will be affected regardless of upper soil tilth.
[photo 3.1: Spring tillage]
Tillage should be carried out under conditions that preserve good tilth, that is, when soil moisture conditions are optimum and there is enough water to allow separation of the soil aggregates, but not so much as to induce puddling or compaction. The soil should not stick to your boots when you walk on it and it should break easily and crumble at the deepest depth it is being tilled. This rule is more crucial for fine-textured (clay) soils than for coarse-textured (sandy) soils.
A tillage system should work residues into the top 8 cm of the soil where it can be digested by the micro-organisms. It should also leave some residue on the surface to reduce erosion potential. Annual use of the moldboard plow can create a hardpan and bury organic matter and living topsoil in an anaerobic zone. On many farms its use has been replaced by the chisel plow which loosens, aerates and mixes the soil without burying all of the crop residue. If wisely used on soil in good tilth, the moldboard plow need not create problems and it is still useful to turn a heavy sod. However, plowing should be kept as shallow as possible. Overuse of offset discs in the spring can lead to compaction problems and, in some cases, the S-tine cultivator is more appropriate for seed bed preparation. In recent years, equipment modifications and new combination tools have been developed to minimize the adverse effects of tillage on soil structure and to reduce the number of tillage operations required.
[photo 3.2: Chisel plow]
Soil texture is a classification system based on mineral particle size. It is a relatively permanent feature of the soil that does not change appreciably over a human lifetime. Soils are classified according to the percentages of oven-dry weights of sand, silt and clay. For example, a sandy soil is composed principally of large sand particles, whereas a loam contains more or less equal amounts of clay, sand and silt. Organic matter is excluded from the texture classification. Soils with a high silt content and those with a high clay content have greater capacities for retaining water and available nutrients than sandy soils. By adding small amounts of clay minerals to the soil and by encouraging the activities of earthworms to reduce the size of soil mineral particles, organic farmers can modify soil texture to a small degree, but the greatest effect of these amendments is on structure, as discussed above.
[Figure 3: Percentage of sand, silt and clay in major soil textural classes]
4. Evaluating your soil
Soil evaluation is an ongoing process for the organic farmer. Regular observation of the crops and of weed growth provides vital information. The simple act of digging a hole in the field can reveal the following information, which should be recorded:
soil profile, which describes the depth and color of the different soil horizons, or layers;
soil structure, including stoniness and hardpan formation at the various soil horizons;
earthworm populations and other soil life; and
root structures, noting whether roots enter the soil structure, follow fissures made by a chisel plow or subsoiler, or are obstructed in any way.
The information gained from test holes can be used to find out why one part of a field yields differently from another and to compare soil conditions from one year to the next. The plant populations under these soil conditions should be described at the same time, including the density, vigor and composition of the weed population.
Conventional soil tests are useful indicators. Soil samples should be taken at the same time each year, preferably under the same conditions. The results, compared from year to year, enable the farmer to evaluate the effectiveness of the management practices used and determine what changes are required. It is also important to use the same testing laboratory because different procedures can give different results. Soil samples taken during the growing season will give more information on availability of nutrients than those taken when the micro-organisms are not active. Most labs give information on texture, pH, phosphorus, potassium and magnesium but other information useful to organic farmers such as OM, CEC, calcium and micronutrient levels may have to be specifically requested. Soil nitrate profiles are used to determine nitrogen levels in the drier soils of the Prairies. A nitrogen test is now available in eastern Canada. Tissue analysis should be used if micronutrient deficiency is suspected. New tests are currently being developed which will help the organic farmer to gauge soil biological activity.
Some labs record results in ppm, others in lbs/acre. To convert ppm to lbs/acre, multiply by 2. If results indicate low nutrient levels, check to see if factors such as pH are limiting availability. Rotation plans may need to be modified to include more soil-building crops and more emphasis given to increasing organic matter. Very low levels suggest the need for soil amendments such as finely-ground rock powders or increased compost applications.
Percent base saturation of the exchangeable cations calcium, magnesium and potassium is given by some labs. It is claimed that this provides a guide for soil mineral balance with desired levels being potassium 2-7 per cent, magnesium 10-20 per cent and calcium 60-70 per cent. However, there is research that shows this is not appropriate in Ontario especially where calcium content is naturally high. Organic matter levels of 4-5 per cent are considered good.
An example of a soil test report is given below.
[Figures 4, much reduced]
Another way of assessing soil management practices is to look at the quality of the crops grown. Some consultants are using the refractometer to measure sugar content of the plant cell sap. The sugar concentration is measured on the Brix scale. Plants under stress due to lack of moisture or nutrients will give lower readings. Plants with high readings are found to be more resistant to pests and disease.
"There is no soil without plants, and no plants without soil."
"The good farmer can tell how biologically-active the soil is, not by counting bacteria, but by the tilth and the workability of the soil."
Albrecht, W., The Albrecht Papers I & II, Acres, USA, Raytown MO, 1975
Balfour, E., The Living Soil and the Haughley Experiment, Faber and Faber, London, 1975, 382 pp.
Belanger, J., Soil Fertility, Countryside Press, Waterloo,WN, 1977, 160 pp.
Doram, D., "Measuring Crop Quality with the Refractometer", Synergy, Vol. 3, No. 2, Spring 1991, pp. 32-34
Gershuny G. & Smillie J., The Soul of Soil, Gaia Services, Erle, Que., 1986, 109 pp.
Waksman, S., Humus, Wiley & Sons, New York, NY, 1948
Waksman, S., Soil Microbiology, Wiley & Sons, New York, NY, 1952
Copyright © 1992 Canadian Organic Growers. Inc
Reprinted with permission. All rights reserved.
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