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The SoilOrganic matter is the source of humusPlant nutrients Organic matter - a source of nutrients Humus as a nutrient bank Soil aeration Soil moisture retention Soil colour and light absorption Soil acidity (pH) Is the effects of organic matter a miracle? Application of organic matter and digging Mulching Breaking new soil SoilmaniaMany gardeners see the preparation of the soil as a necessary evil, in the worst case a hard and sweaty work with almost unbreakable clods of clay. It is certainly something that they want to get over and done with as quickly as possible. Organic gardeners are not in such a hurry. This is not because organic gardeners find special pleasure in self-torment. It is when you notice over a few years how your efforts vitalise the soil that you will see it differently. Good soil is simply a pleasure to work with. It feels good in your hands. It smells good. It looks good and the plants love it. Good soil is nothing less than getting off to a really good start.
Since the quality of the soil is of such importance to the success of the organic gardener, I will explain the properties of a good soil in some detail, and how soils can be improved and maintained.
As discussed in the previous section water drainage is essential. A good soil also is sufficiently rich in nutrients, well aerated, water retentive, heat absorbing and has a balanced pH. All these important properties are affected by the addition of organic matter and the resulting formation of humus and other substances.
Organic matter is the source of humusOrganic matter consists mostly of carbon, oxygen and hydrogen. These constituents are of little interest with respect to plant nutrition. Plants absorb carbon from the air in the form of carbon dioxide, not from the soil. The oxygen and hydrogen that plants require they need in the form of water. The other elements that plants need are actually a minute fraction of organic matter. So what is the significance of the bulk of it? One year after the incorporation into the soil most of the organic matter has been transformed into carbon dioxide and water. The carbon dioxide goes of to the air and the water wets the soil, but about 30 percent of the carbon is still present in the soil in organic form. Some of it is incorporated into the bodies of soil organisms. The rest is mostly humus. Humus is a heterogeneous mixture of complex organic molecules. It consists of both finely divided residues of organic matter and compounds that have been newly made by soil organisms from that material.
Sometimes other gardeners tell me that it's pointless to add organic matter because it disappears so quickly. To some degree they are right. Most of the organic material added has turned into carbon dioxide and water after one year. The remainder is humus. Humus particles are so small that they cannot be seen by the naked eye or even with the aid of a light microscope. So in a sense this fraction is also gone, but although the humus particles cannot be seen directly it's quite easy to see the effects of humus especially on clay soils if one knows what to look for. Some components of humus are very long lived. It has been shown in long-term experiments that the addition of organic matter can have beneficial effects on soil fertility after more than a hundred years.
Plant nutrientsWhen vegetables are harvested and other plant parts removed the nutrients that they contain are also taken away from the soil. To keep the soil fertile these nutrients must in some way be replenished. So far we know that at least 16 elements are essential to plants. If one of these elements is lacking plants cannot complete their life cycle. Shortage of one nutrient will restrict plant growth even if there is plenty of the others. All of the essential nutrients must be at adequate levels in the soil. Most of the essential elements are usually found in large amounts in soils, but in forms that are not immediately available to plants. Through weathering and biological breakdown they are slowly transformed into the small water-soluble inorganic molecules which plants can absorb and utilise. Some elements have a faster turnover. Nitrogen levels can be much reduced in a few years and the gardener who does not take care to add nitrogen to the soil may see a marked reduction in productivity from one year to the next.
There may be some soils that could be gardened for decades without the addition of any nutrients, but nitrogen. One way to add nitrogen is to grow clovers or other leguminous plants. The air is 80% nitrogen in gaseous form that is not available to plants, but clovers can, with the aid of certain soil bacteria, fix this nitrogen into usable forms. The clovers are dug into soil to enrich it. Most gardeners would not rely on this sole source of nutrients. They feed the soil with all the nutrients that plants need. Organic gardeners add generous amounts of organic matter, while others use chemical fertilisers. The nutrients supplied with organic matter are not better than those in chemical fertilisers. Actually they are exactly the same. The superiority of organic fertilisers rests on the many positive side effects of adding organic matter to the soil.
Organic matter - a source of nutrientsOrganic material is the food of a rich collection of soil organisms like earthworms, insects, bacteria and fungi. When they are at work nutrients are released slowly and plants can absorb most of them. When conditions are good for growth and the plants need lots of nutrients the organic matter in the soil is also more rapidly broken down and more nutrients are released. As long as nutrients are fixed in organic matter they are stuck. There will never be a large surplus of nutrients dissolved in the soil water, which could be washed away by heavy rains or watering. This gives the gardener better control over the nutrient status of the soil without resorting to chemical testing. Only if the vegetation is sparse is there an increased risk that some of the goodness will be lost. Soil organisms will continue to break down organic matter even if there are no plants to absorb the mineralised nutrients. It is therefore important to keep some kind of plant cover for the entire growing season. In the spring weeds can be tolerated in-between rows as long as the growth of seedlings is not impeded. When plants get bigger and their roots penetrate the entire soil the weeds should be removed to reduce competition for water and nutrients. Some plants like potatoes and kohlrabi will be lifted from the soil long before the season is over. Try to have seedlings of a catch crop ready for transplantation, or let the weeds grow back again to protect the soil and conserve nutrients. The nutrients released will be taken up by the weeds and recycled to the soil by way of the compost bin or directly as the weeds are dug into the soil. Nutrients lost from garden soils may eventually turn up in waterways, ponds and lakes where they cause massive growth of algae and other disturbances of the aquatic ecosystems. This is an environmental problem in many parts of the world.Top Humus as a nutrient bankPlants absorb nutrients in the form of very simple inorganic molecules, which are all soluble in water. Nutrients dissolved in water are more or less subject to leaching. Soils have a buffer capacity when it comes to water. A dry soil soaks water up, but eventually the level is reached where the soil cannot absorb more. Water then starts to percolate down to deeper horizons. Heavy rains and watering may cause such mass flow of water and when it happens the nutrients dissolved in the soil water may be transported away from the root zone. However, clay and humus have the ability to attract and temporarily bind many of the dissolved nutrient molecules. Clay and humus particles typically have negative electric surface charges and will therefore attract everything in the soil that is positively charged. Many of the dissolved nutrients are actually positive ions. When clay and humus molecules hold these ions they are protected from leaching. Plant roots can withdraw these nutrients by releasing hydrogen ions. Hydrogen ions are more strongly attracted to clay and humus particles than other ions and can knock these ions loose to take their place. Once the nutrient ions are in the water solution plant roots can absorb them. In many soils clay particles make up a large fraction, but in a sandy soil there is very little clay. Humus, on the other hand, is a small fraction of most soils, but humus has a much greater capacity to hold nutrients than clay. This means that even a small increase in the humus content of a soil can dramatically increase the capacity of the soil to store and release nutrients. This beneficial effect will be largest in soils with low clay content.Top Soil aerationThe humus, gels and other substances formed from organic matter stimulate the formation of aggregates in the soil. Good aggregates vary in size, but are typically a few millimetres or less across. Aggregation and the physical activity of soil organisms (especially earthworms) make the soil loose, crumbly and full of pores that allows the free movement of air and water. This property is very important as plant roots need air (O2) for nutrient uptake and survival as do most of the organisms responsible for the breaking down of organic material and thus for the supply of nutrients to plants.
The pore space of a soil is the volume occupied by air and water. A sandy surface soil under cultivation has a pore space around 40 percent. It may come as a surprise, but fine textured soils (clays) typically have more pore space. Pores vary in size. Very small pores are mostly filled with water under normal conditions and do not permit much air movement, whereas larger pores (larger than 0,05 mm) allow the ready movement of air and water. In sandy soils the larger pores dominate and insufficient aeration is seldom a problem. The pore space in clays, on the other hand, may be dominated by very small pores. The addition of organic material tend to increase total pore space somewhat, but the most important effect is that aggregation and granulation greatly raises the proportion of large pores in fine textured soils. If no organic material is added fine textured soils will go in the opposite direction since the substances that promote aggregation and the stability of aggregates will be broken down and disappear with time. The loose structure resulting from good aggregation and granulation does not only improve aeration but also makes root penetration easier.
Soil moisture retentionWater is immensely important to plant growth. All the important biological and chemical processes in soils and plants take place in water. Water is the transport medium of nutrients and storage products within plants. By the transpiration of water plants can cool themselves on a hot day. Transpiration (and water loss) is also an inevitable effect of opening the stomata (small openings usually found on the underside of leaves). It is through the stomata that carbon dioxide enters the air spaces within leaves. Carbon dioxide is together with water the main molecule from which all plant tissue is made. When there is a shortage of available water in the soil plants will close the stomata to conserve the water that they have absorbed. Plant surfaces are essentially gas and water tight. With the stomata closed the little carbon dioxide within the leaves is quickly depleted and the production of plant tissue comes to a halt.
The ability of a soil to hold water is increased by organic matter. The humus and gels which are formed during decomposition has a sponge-like action soaking water up when there is plenty and serving as a reservoir when the soil dries out. The ability of sandy soils to hold water is greatly increased by organic matter. Fine textured soils like clay can store more water before it percolates down to the water table than sandy soils and the beneficial effect on fine textured soils is smaller.
Soil colour and light absorptionMany humus components are dark in colour. When organic matter is added to a soil it gradually gets darker as humus is formed. A darker soil reflects less of the sunlight that falls on it. Instead more of the light is absorbed and its energy transformed into heat. Most of the year the beds will be covered with plants, which will shade the soil, but in the spring and early summer seedbeds will heat up quicker if the soil is dark, decreasing the risk of frost damage or making earlier sowing possible.Top Soil acidity (pH)It has been shown that the solubility of nutrients, and thereby the ease at which plants can lay their roots on these valuable assets, depends on the acidity of the soil. Acidity is measured in units of pH. If the soil is too acid (low pH) some nutrients will react chemically with other molecules to form compounds that are useless as plant nutrients. The process is in most cases reversible, which means that if the pH, is raised the nutrients will dissolve again and be available to plants. It works in the same way when the pH is too high. This also makes some nutrients largely unavailable to plants.Nowadays acidification of soils is seen as a natural consequence of growing a crop whether you do it in a small garden like mine or in huge fields. Rarely can you open a book on gardening and not find the advise to lime your soil at certain intervals to maintain optimum levels of soil acidity. The advice is usually to maintain the pH around 7. Lately I red some scientific papers on the subject that challenge this view. It seems that acidification is a problem that is strongly linked to the use of chemical fertilisers. The effect is twofold. First it's very likely that chemical fertilisers is the primary cause of acidification. Second plants are more negatively affected by acid conditions if the soil is low in organic matter. Loss of organic matter also typically results from the use of chemical fertilisers. Organic matter on the other hand tends to stabilise pH at a suitable level and to decrease the negative effects of departures from the optimum value. I measured the pH of my soil for the first time in the fall of 1997. I have not added any chemical fertilisers or liming materials since then and the pH has been very stable. The pH was 6.7 in 1997 and it still was in 2003 after six years of intensive vegetable gardening. It may not be universally true that the addition of organic material and composts stabilises soil pH, but if you use the organic method you should not automatically assume that you have to lime the soil. Liming has at least one serious negative side effect. It can increase the loss of valuable nitrogen from garden soils and composts. (The nitrogen is lost to the air in the form of gas.) Some soils are naturally alkaline i.e. the pH is high. (This is common in arid regions and where the bedrock is limestone) Addition of lime to such a soil to raise the pH even more is absolutely not a good idea. If you are concerned about the pH of your soil - test it first. Initially you may want to test the soil to see what you have got. When testing the soil you should know that taking just one or two samples would result in very unreliable test results. The acidity of the soil can vary widely on a small scale. The minimum is three samples from different parts of your vegetable plot. If the three samples give roughly the same pH then the result can probably be trusted. If they differ - take more samples. In Sweden some garden shops will give you the opportunity to have your soil pH-tested for free in the spring. You bring your samples, which are tested by a person who is most likely to be a representative for some company in the liming industry. These people may have a tendency to recommend heavy liming whatever the test results are. It's safer if you know yourself what you should be aiming for. If the pH 6.5 or more no liming is called for. If the pH is lower, read the instructions on the product you bought and use the amount of lime that is needed to raise the pH to the 6.5 level or slightly more. Lime should never be combined with the addition of organic matter because this may cause some of the nitrogen in the organic matter to be lost to the air in the form of ammonia. Be sure to allow an interval of at least three warm months (May to October in Stockholm) between the addition of organic material and lime. For the same reason you should not lime the compost heap until the compost is well broken down. Generally composts don't need liming. Most compost heaps will end up with a pH around 7 or higher without liming. I you decide that your soil needs liming there are many different products to choose from. I would recommend products that contain both calcium and magnesium e.g. dolomite. Beside their effect on pH calcium and magnesium are both among the 16 essential plant nutrients. Other good alternatives are products made from calcified algae or clamshells. These products contain very little magnesium, but many of the nutrients that plants require in only small amounts e.g. boron. The best recommendation might be a mix of dolomite and crushed clamshells. This will also be one of the cheapest solutions.
Some people may wonder why the seemingly small difference between pH 5 and 7 can be of such importance. Actually the difference is not small. Every step down the scale represents a tenfold increase in acidity. At pH 5 the soil is a hundred times more acidic than at pH 7.
Is the effects of organic matter a miracle?At this point I'm sure that some readers are suspicious. Is organic matter really such a miraculous cure that can fix virtually any type of soil problem? The best answer is that it's not a miracle at all. In almost every natural habitat where plants grow, whether it is grassland, forest or tundra, plants or plant parts will die and fall to the ground together with animal faeces. These and other organic materials are then incorporated into the soil and broken down by soil organisms. This is the environment in which plants have evolved for millions of years and it is a small wonder that they are well adapted to it. In the garden situation we remove lots of weeds and hopefully a bumper crop of vegetables. We try to balance this export of organic matter by an equally large import just to keep the system running at the natural balance. There is nothing mysterious about the good effects of organic matter. By adding it we simply recreate the conditions that plants are adapted to.
It doesn't matter what kind of soil you have. The best strategy is to add nutrients in the form of organic material. If the material is plant parts in various degrees of decomposition it's likely to contain all the nutrients that plants need and the risk that there will be deficits of some nutrients is very small. There are soils that are mainly of organic origin. If your soil falls in this category it's unnecessary to add large amounts of organic matter. Use composted poultry manure and add other organic fertilisers that are high in phosphor and potassium e.g. bone meal and wood ashes and maybe some seaweed for micronutrients.
Application of organic matter and diggingOrganic matter can be incorporated into the soil or placed on top of it. Well decomposed organic matter that is beginning to look like soil can safely be dug into and mixed with the soil at all times of the year, but adding fresh material at the wrong time can be harmful to plants for several months although it will be beneficial in the long run. The addition of fresh manure may also be hazardous as it may contain organisms that cause disease in humans. Manure that has been composted is generally safe, but be careful to observe good hygiene when you handle fresh manure.Many of the components of fresh organic matter are easily broken down by microorganisms and initial decomposition will be very fast if the soil is not too cold, compacted, dry or wet. The speed of decomposition will increase until some factor becomes limiting. If the nutrient balance of the material is right, intensive breakdown can continue until the more easily broken down components have been consumed and only more resistant components remain. The danger in this case is that the activity can be so high that the oxygen content of soil air is drastically reduced which is bad for plant roots, which need oxygen for metabolism and nutrient uptake. If, on the other hand, the incorporated organic matter is low in some of the nutrients that microorganisms require the rate of decomposition will increase until those nutrients are depleted in the soil. When nutrients are bound in the cells of bacteria and fungi they are unavailable to plants. The mass of microorganisms must be reduced before these nutrients can benefit the plants, but this will not happen until the easily broken down components are consumed which will take time since microbiological activity is checked by lack of some nutrients. The problems can be especially severe if the added material is relatively rich in carbon. Hay or wood shavings are examples of organic matter that contain a lot of carbon, but not so much of the other nutrients. Composted organic matter does not produce nearly as high levels of microbial activity since the easily broken down components have already been consumed. Composting will also much reduce the ratio of carbon to nitrogen and other nutrients in organic matter because much of the carbon goes of to the air as carbon dioxide during composting. If you are already in trouble and think that the added matter was too rich in carbon you could try to alleviate the problem by adding a fertiliser like composted poultry manure or blood meal which are both low in carbon in relation to other nutrients. If the matter was to fresh and microbial activity tend to deplete the supplies of oxygen this calls for an effort to keep the soil open and breathing. Work the surface soil often and especially after watering and rain. Nutrients are less concentrated in most organic materials than in chemical fertilisers. Lots of organic matter is needed to supply an adequate amount of nutrients. If you use farmyard manure from horse and cow or garden compost about 5 litres per square meter is enough. Household composts can contain more nutrients depending on the input. Bagged organic fertilisers usually give some guidelines on the bags. Only a fraction of the nutrients in organic fertilisers is used by the first year's crop. Of the nitrogen supplied typically 50 percent is used. The rest will benefit the crops of the years to come. If you are starting from scratch it will take a few years to build the organic nutrient reserves of the soil. You should therefore add about twice as much as recommended the first year (e.g. 10 litres of farmyard manure per square meter) and about 50 percent more the second year. Supplement this with an organic fertiliser rich in phosphorus e.g. bone meal. If the soil is denuded by land levelling, exhausted or low in organic matter, initial applications of 20 litres of composted farmyard manure or more per square meter may be worked into the soil.
I usually add organic matter in the spring. The weeds that grow in the autumn after the cropping season protect the soil and absorb mineralised nutrients. It's seems a little daft to remove the weeds only to leave the soil naked and bare. I work the soil in the spring so that it is weed free when I sow. Adding organic matter in the autumn is always associated with some loss of nutrients. It's better to keep organic materials under cover during the winter to conserve nutrients. If you live in an area where the soil freezes in the winter fellow gardeners will sometimes tell you that the major benefit of autumn digging is that the large clods formed will freeze and crumble during the winter. This is said to be especially important if you have clay soil. This is probably a good tip if you use chemical fertilisers. The loss of organic matter that this leads to will certainly give a clay soil the properties of concrete. If you have concrete clay and refuse to add organic matter - work the soil in the autumn. The clods of my soil, which is originally a very heavy clay, crumble like sand when I dig and rake the soil. I prefer to work in the sweet and inspiring sunshine of the spring. Digging in the spring has the further advantage of drying the soil, which will make it warmer. It's a good idea to add organic matter a week or two before sowing to allow the soil a chance to settle a little. If access to irrigation water is a problem it may be better to conserve soil water by digging in the autumn and restrict spring work to the raking and weeding necessary to prepare a seedbed. If the soil is low in organic matter it may also be better to dig the soil in the autumn the first one or two years. The addition of organic matter reduces the need to dig the soil. As a matter of fact, many organic gardeners never use a spade on their soil. They add organic matter on top of the soil and then the earthworms do the digging with great efficiency. If you need or want to dig it will be much easier in the loose soil. The best results are achieved if the soil is dug at medium moisture. If the soil is to wet clods will form, if it's to dry soil aggregates will pulverise. To dig wet or dry soil will also be much harder on your back. When a soil is dug we are usually talking about the top 25-30 cm. The roots of vegetables in most cases grow much deeper than that. It is important that they can do so unrestricted. Sometimes there is a hard and impermeable soil layer just beneath the topsoil. All such hardpans should be broken up. This will be hard work since the topsoil has to be removed first and then put back agian.
As plants grow bigger their demand for nutrients increase. Many organic gardeners add lawn trimming or composted poultry manure in the summer to top the soils fertility. If growth seems to be slowing down and other reasons like drought, cold weather or disease can be ruled out it's usually a good idea to give the plants a booster, but we are not talking heavy fertilisation here. A few centimetres of lawn trimmings or a small handful of composted poultry manure for every square meter will be sufficient. Bury the poultry manure in the top 4 centimetres of the soil to conserve nutrients. For sanitary reasons, use only well composted manure. As pointed out earlier fresh manure may contain organisms, which are harmful to you, and these can remain in the soil for months and contaminate your vegetables.
MulchingIf you want to add fresh organic material to your garden it's better to put it on top of the soil as mulch. It's also possible to mulch with organic matter in various states of decomposition, or even nonorganic materials. When fresh organic matter is put on the soil instead of being dug in, it will be incorporated into the soil by soil organisms little by little over a long period of time. This avoids the hazards of digging fresh organic matter into the soil, but is still an efficient way to add nutrients. In the spring of 1999 I put 10 centimetres of relatively compact and rotted hay on heavy clay. When I dug the soil one year later the hay had largely disappeared. The structure of the clay had also changed. It was loose and crumbly down to a dept of 25 centimetres.To cover the soil with organic mulch has several effects provided that the mulch is not too soil like. Compost that looks like soil will have the properties of soil rather than those described below.
breaking new soilA little planning ahead reduces the effort necessary to clear a patch of soil for vegetables. The basic method is to cover the soil to kill all vegetation. If the soil is rich in organic matter the easiest way is to cover it with anything that blocks the sunlight and does not rot to easily. An area covered with carpet or black plastic sheets will be totally free of vegetation after a few months and the soil will be surprisingly loose and easy to dig. If the soil is low in organic matter add a thick layer of organic material. For denser materials like half rotted farmyard manure 12 centimetres may be sufficient. If you use hay that is partly rotted and packed down twice as much is the minimum.Top
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