Soil Fertility

The products of agrotechnology are displacing the source upon which the technology is based. It is analogous to taking stones from the foundation to repair the roof”.

-- Garrison Wilkes

The major difference between the two extremes of modern agricultural practise lies in the attitude toward soil. The chemically oriented extreme sees the soil as merely an inert medium to hold up the plants that grow in it. Taken to its utmost, the soil is eliminated, as in hydroponics. The organic extreme sees the soil as the only proper source of plant nutrients and that nothing should be imported onto the farm, or sprayed onto the crop. The vast majority of farms operate somewhere between these limits.

There are now few agronomists who would argue with the proposition that a high level of organic matter in the soil is desirable from several points of view. The first is that soil well-supplied with organic matter is easier to till. As organic matter levels in the soil have declined, tractor power needed to increase to achieve the same amount of work. As well, organic matter is a sponge for water, the major crop-limiting  factor. Not only is organic matter a source of slow-release nutrients, it also can have the capacity to hold on to nutrients that were not an original component of the organic matter. Such a soil has its constituent particles held in aggregates, called crumbs. The air gaps between the crumbs allow better aeration, more rapid infiltration of water and better drainage.

Organic matter in the soil takes several different forms. The most important from the point of view of farming are humus and its related compounds. Organic matter in the soil can remain undecomposed, ferment, or humify. Raw organic matter ties up nutrients in a form that plants cannot access until it has decomposed. Fermentation results in the production of alcohols and other substances that inhibit plant growth. Humification produces the essential materials that qualify soil as fertile.

The factors that promote humus formation are: a source of lignin, cellulose, a nitrogen source, water, warmth and oxygen. Lignin and cellulose are the structural part of plants. The lignin component is transformed by bacteria into humus. The cellulose is the energy source for those bacteria. The nitrogen is required for the formation of the bacterial protein. If this nitrogen is already in the form of protein, such as animal manure, or a leguminous green manure, then the humifying bacteria can use it directly. Water soluble nitrogen sources must first be converted to protein by other bacteria before they can be used. Water soluble nitrogen is not only susceptible to leaching, but also seems to promote the proliferation of undesirable fungi. Moisture levels should be those sufficient to allow plant growth. The optimum temperature range is between 15 and 25C. Below 10C there is very little bacterial activity. Above about 30C humus tends to oxidise at a greater rate than it is produced. Oxygen is required by humifying bacteria. In the absence of oxygen, anaerobic bacteria and fungi decompose organic matter and this is the fermentation process that results in production of plant inhibiting chemicals, such as alcohols.

Another factor that affects humus formation is the balance of the major fertility elements calcium, magnesium, potassium and sodium. This was Albrecht’s discovery. The relative percentages to optimise humification and consequently protein formation in most crops are:

Calcium

60 - 75%

Magnesium

10 - 20%

Potassium

2 - 5%

Sodium

0.5 - 3%

All other cations

5%

When the soil is balanced in this way, not only is humification enhanced, but the pH (acidity) of the soil tends to stabilise between 6 and 7. This happens to be the level of acidity that optimises the availability of all the essential plant nutrients in the soil. It is also the range that is preferred by earthworms.

The role of earthworms in soil fertility cannot be ignored; their tunnels provide aeration, rapid infiltration of water and improved drainage. Their guts digest soil, some from as deep as two metres below the soil surface, releasing locked up phosphorus and a special gland secretes calcium into their casts. Some agronomists believe that man’s move from hunter-gathering to farming could not have occurred prior to the evolution of modern earthworms.

In one study of water infiltration rates, 100mm of water was poured onto two soil samples. The sample with earthworms absorbed the water in less than a minute. The worm-free sample took more than two hours.

Earthworms also assist in restructuring soil into aggregates, or crumbs. Not only are the aggregates more prevalent in soil with good earthworm activity, the aggregates are more stable and longer lasting.

The bacterial activity associated with humification releases many essential nutrients from the silt fraction of the soil. Thirty year duration trials of soil fertilisation in Germany and Switzerland showed that, with no return of crop residues, more than 90% of the nutrients taken up by crops came from the silt, not the applied fertilisers. This research would appear to justify the claim of many organic farmers that well maintained organic farms require very little in the way of off-farm fertiliser inputs.

Silt particles, like clay, are very small indeed. Unlike clay, silt carries no negative electric charge and the particles are flattened, which is why silty soil feels silky smooth, rather than sticky like clay. Silt is created by glacial action on rocks and the bulk of the world’s silt was produced during the great ice ages. Over time, silt is depleted of its nutrients. Consequently, the young soils of the planet, such as those in Europe, are much more fertile than those of greater age, such as those in Australia. Australia is known as the “trace element desert” because of the shortage of many essential micronutrients.

Pulverised basalt and granite have been promoted as one answer to this problem. It is a relatively inexpensive material to obtain since it is a by-product of quarrying. The material must be very finely divided, to present the largest possible surface area for the bacteria to work on. You can test the potential of a sample by placing a pinch in a glass of distilled water and leaving it on a sunny window sill for a few days. The more vigorous and rapid the formation of algal bloom, the better the sample is for use as fertiliser. Some unscrupulous fertiliser companies are selling rock dust mixed with clay. While it can be said that the clay adds to the cation exchange capacity of the soil, increasing exchange capacity by increasing humus levels would appear to make better economic sense.

The Fertility Elements

We are now going to discuss the individual fertility elements, their role in plant growth and animal health, and the implications of using various fertilisers to supply them.

Phosphorus

Phosphorus encourages root development and is essential for the formation of protein in the plant. As well, it increases palatability of the plants as it promotes the formation of fats and convertible starches. By stimulating rapid cell development, phosphorus increases the plants’ resistance to disease. Many plants respond to a phosphorus deficiency by showing a reddish, or purple colour in their leaves. Heavy feeders are stunted. Phosphorus toxicity symptoms include the margins and interveinal areas of older leaves dying. Younger leaves show interveinal chlorosis, particularly tomatoes, celery and sweet corn.

The most popular fertiliser source of phosphorus in recent decades has been superphosphate. The response of crops to super has declined over time, more and more being necessary to achieve satisfactory yields. On average, only 30% of the phosphorus in super becomes available to plants. While a tiny amount leaches out of the soil through irrigation and rainfall, the bulk is chemically locked up in the soil. Phosphorus from farmland appearing in rivers and streams is generally carried there through erosion of the soil, rather than phosphorus in water solution. Humic acids, earthworms and associated beneficial bacteria and fungi in a fertile soil unlock the phosphorus in reactive phosphate rock, chemically inactivated superphosphate and silt, making it available to plants.

Fertiliser recommendations followed by most farmers results in the application of more phosphorus than is removed by the crops. As a result, many farmers have built up phosphorus reserves in their soils that are sufficient for decades, and in some cases centuries, of cropping. Where low soil phosphorus levels are a problem, some farmers are using reactive phosphate rock (RPR) as an alternative to super. RPR is cheaper than superphosphate as well as containing a higher percentage of phosphorus and trace elements. Under typical soil conditions, the phosphorus is only readily available when the soil pH is around 4.5 to 5.5. However, the organic acids associated with bacterial activity are capable of unlocking the phosphorus when the soil pH is a more acceptable 6.0-6.5.

Many Australian organic farmers are exploiting the phosphorus residues locked up from earlier superphosphate applications. The question arises how long will those reserves last? Is there sufficient phosphorus in these residues and the silt fraction of the soil for economic, long-term production? For conventional farmers, the questions that arise are, does it make economic sense to leave 70% of the applied phosphorus in superphosphate unused? How can farmers exploit the reserves they have built up? And how long will the world’s fossil phosphate deposits last? We do not have answers for these questions at this time. Nevertheless, it should be apparent that fossil phosphate reserves will continue to dwindle, driving the price higher. As well, it would appear to be sensible to maximise the availability of any applied phosphate, rather than letting the bulk become chemically locked up to the detriment of the soil biology and the farmers’ input costs.

Nitrogen

Nitrogen stimulates the production of plant tissue and influences the protein content. Nitrogen applied as nitrate produces a blue-green colour in plant leaves. When applied as protein, the colour is noticeably a more golden-green. Excessive nitrate levels are associated with increased fungal disease, delayed maturity of plants and weakening of plant tissue leading to lodging. As well, nitrates in the plant sap are reduced by bacteria to nitrite which is toxic to the consumer of the plant, animal, or man. In livestock, excessive amounts of nitrite in the diet cause abortion, hay poisoning, grass tetany, and reduced haemoglobin count in the blood (anaemia). Nitrogen deficiency symptoms in crops include, edges of leaves turn brown, smaller leaves and yellow-green foliage. Nitrogen toxicity symptoms include rotting of roots and delayed maturity. Young leaves are dark green and older leaves yellow with necrotic spots.

Nitrogen, can be supplied as protein (animal manure, legume green manure, fish etc.), or as water soluble artificial fertiliser (Nitram, urea, ammonium sulphate etc.). While a pasture can supply its own nitrogen needs through fixation of atmospheric nitrogen by clover, horticultural crops have a much higher requirement. As little as 10% of applied water soluble nitrogenous fertilisers are taken up by the crop. The remainder leaches into groundwater and streams. While this may please the fertiliser manufacturers, it is not so great from the point of view of the farmer. As well as wasting money, adverse impacts on the environment can lead to stiff penalties.

As protein slowly decomposes, it supplies the plants with nitrogen at the rate generally needed by the crop. Leaching becomes a non-issue. Where short-term nitrogen needs are not being met by the soil, liquid fish, with, or without urea, as a foliar spray is preferable to urea alone. Foliar sprays of water-soluble nitrogen encourage fungal disease. In contrast, liquid fish has been observed to reduce the incidence of fungal disease by many farmers.

While mainstream agricultural scientists have been slow to investigate, Dr James Wong and Tony Allwright of the Tasmanian Department of Primary Industry have research under way. Hopefully, this work will provide a better understanding of the reasons why organic fertilisation reduces the ability of fungal organisms to proliferate. This should enable us to enhance the degree of control. In the meantime, we know that using organic fertilisers enhances the effectiveness of chemical fungicides as well as being a worthwhile control mechanism in their own right.

In pasture, both pelletised poultry manure and liquid fish have increased clover nodulation. This is a clear indication that the clover is fixing more nitrogen that increases the protein content of the pasture. As well, palatability is reported to increase, with more even grazing. There is a downside to this, with some graziers reporting problems with stock breaking into grazed out paddocks treated with proteinaceous fertiliser, in preference to lush pasture grown on super.

Pome fruit has responded well to foliar applications of liquid fish and liquid seaweed. Lateral growth has been good with an increase in leaf size. The leaves seemed to be somewhat thicker and glossier. Red Fujis, prone to russet, which makes them unmarketable, showed a marked decline of this problem. In the writer’s orchard, a trial of foliar applications of liquid fish as the only control for black spot was a limited success in a season of overcast, drizzly weather. Some varieties remained clean, but the less resistant varieties were a disaster.

In cropping, the pelletised poultry manure was applied at a rate calculated to supply 50% of the usual artificial nitrogen application. This rule of thumb has worked well in supplying the nitrogen needs of most crops. One grower applied a soil drench of 60 litres per hectare of liquid fish to a crop of brassicas. They responded as well as they did to artificial, even though the nitrogen content of the fish emulsion was a mere 2.8%. This anomalous result needs investigation.

Potassium

Potassium is essential for starch formation in the plant and the development of chlorophyll. Unlike phosphorus and nitrogen, which are part of the structure of the plant, potassium is more of a catalyst involved in plant processes. Deficiency symptoms include lowered resistance to disease, low yields and mottled, speckled, or curly leaves, especially older leaves. Potassium toxicity symptoms include marginal necrosis on the oldest leaves and in celery, blackheart.

More and more farmers are coming to appreciate deep rooting plants to bring potassium from deep in the subsoil to supply their crops’ potassium needs. Such plants are called biological ploughs because they serve much the same purpose as a ripper, leaving deep channels in the soil when they decompose. In pasture, New Zealand graziers use chicory developed for this purpose. In China, vegetable growers use Pawlonia trees whose large succulent leaves decompose to humus when they fall in autumn. The roots of lucerne and comfrey are capable of diving two metres or more into the soil.

Potassium is used to excess in many crop fertiliser programs. For instance, the recommended application rate on potatoes is twice the amount removed from the soil. This leads to reduced availability of calcium and many trace elements. As well, the most commonly used potassic fertiliser is potassium chloride (muriate of potash). This material is deadly to earthworms, as it burns holes in their skin. Frogs, Nature’s vastly underrated pest controllers, are also decimated by its use. Continual overuse of potassium chloride can lead to toxic levels of chloride and a consequent decrease in yields. Potassium sulphate (sulphate of potash) is a much better source of potassium, particularly as it includes sulphur which is often in short supply. It is unfortunate that it is much more expensive than muriate.

When the writer commenced his organic market garden ten years ago, a soil test was taken that showed a deficiency of potassium. This was “corrected” with the recommended amount of muriate of potash. In the ensuing ten years, only compost has been applied and this is regarded as only a fair source of potassium. Nevertheless, a recent soil test showed that the potassium level was slightly excessive.

New Zealand dairy farmer, Brian Gordon of Katikati, had his soil tested in 1989 (see table). The recommendation from the local extension officer was the usual 300-500 kg of potash-super per hectare on his 123 hectares, plus lime. Instead, he applied 20 litres per hectare of Vitec fish emulsion annually.

As these tests show, the available nutrients in the soil increased dramatically. Taking the potassium result as an example, the increase was nearly 3,000%. The quantity of potassium applied was a mere 432 grams per hectare. If the response to fertilisers is only due to the NPK content, then the extension officer’s recommendation should have been for 1.27 kg per hectare of potash-super, not 500 kg!

 These results would appear to indicate a need for great caution interpreting soil test results when introducing organic fertilisers into soils that have previously received high levels of potassium.

Calcium

Calcium is often applied to the soil to release other nutrients by altering the soil acidity (pH). It is said, on this account, not to be a fertiliser. Calcium is a structural part of the walls in plant cells. As well, it is essential for the proliferation of soil bacteria. Clay soils often become sticky if there is an excess of sodium. Calcium displaces sodium attached to clay particles and since it is a much bigger atom, the clay becomes more friable. While gypsum (calcium sulphate) is recommended to break down sticky clay, it will only work if the reason for the stickiness is excessive sodium. If the soil is also acidic, it is cheaper to use limestone (calcium carbonate). An excess of calcium relative to magnesium is generally accompanied by insect problems in the crop.

Sap tests of potatoes grown on pelletised poultry manure showed much higher levels of calcium than those grown on artificial fertiliser. Part of the reason for this could well be the very high level of potassium in the artificial fertiliser. Excessive potassium is known to produce calcium deficiency symptoms in some crops. These include deformed terminal leaves, buds and branches, poor plant structure, such as weak stems, celery black heart, lettuce tip burn, internal browning of cabbages, cavity spot in carrots and bitter pit in apples.

Calcium is generally applied as ground limestone (calcium carbonate), or dolomite (a mixture of calcium carbonate and magnesium carbonate). As referred to earlier, calcium and magnesium in the soil must be in appropriate ratio. Liming to merely adjust pH will generally lead to excess calcium, or worse, if high magnesium dolomite is used exclusively, excess magnesium.

Sometimes, calcium hydroxide is used for a quick response. The bulk of this is rapidly converted to calcium carbonate when it reacts with dissolved carbon dioxide in the soil water. It is more economical to use very finely ground limestone if a faster response is needed.

When the soil is badly out of balance, it is not a good idea to lime heavily. This has a very bad effect on the soil microbiology. It is much better to apply more frequent, lighter applications.

Magnesium

Magnesium is the companion to calcium in mineral deposits. The carbonates of both are used as lime. However, in plant nutrition it is the companion to phosphorus and stimulates the assimilation of phosphorus by plants. It is essential for the formation of chlorophyll. Magnesium deficiency causes chlorosis in plants, analogous to anaemia in animals. An excess of magnesium relative to calcium results in too high a pH and consequent deficiency of many trace elements. In an emergency, Epsom salts (magnesium sulphate) can be applied as a foliar source of magnesium, but this is an expensive source of magnesium. Where the use of even high magnesium dolomite will still leave an excess of calcium over magnesium, there are several magnesium sources; Kieserite (16%), Magnesite (25%) and magnesium oxide (50%).

Sulphur

Sulphur is a neglected element in farming. This is difficult to understand as it is essential for the formation of chlorophyll, proteins and vitamins. Perhaps it is because we rely too much on research conducted in the Northern Hemisphere, where sulphur compounds generated as pollution by industry arrive in the rain. These compounds, sulphuric and sulphurous acids, as well as hydrogen sulphide (rotten egg gas), are a fortunate rarity in Australia’s relatively unpolluted atmosphere.

Sulphur can be applied to the soil as elemental sulphur. The usual source of sulphur for Australian farmers is superphosphate, which contains more sulphur than phosphorus. However, elemental sulphur is a much cheaper source when the phosphorus is not needed.

Hopefully, more work on necessary levels in the soil for particular crops will be conducted in the future. A high level of sulphur in a soil test is generally a symptom of poor soil aeration.

Trace Elements

Trace elements are those required in minute amounts for essential plant processes. Their availability is optimised when the soil pH is between 6 and 7, the major nutrients calcium, magnesium, potassium and sodium are in balance and the soil humus level is more than 3%. Absence, or deficiency of particular trace elements may mean that enzyme cycles cannot be triggered into action, resulting in reduced crop performance, or failure. Some trace elements are required for animal and human health without having any obvious influence on plant health, or production.

The assessment of trace elements through soil testing is an uncertain procedure. Measured levels that have been thought to indicate deficiency, have been contradicted by the measurement of adequate levels in the plant tissue and vice versa. Part of the problem is the fact that certain elements stimulate, or suppress, other elements. This is an area of soil science that is very poorly understood and needs much more research. While tissue and sap testing offer the potential for better assessment of crop needs, they too have their difficulties.

Trace elements are only poorly taken up by plants when they are in salt form. This has led to increasing use of chelated trace elements. Chelation (KEY-LATION) means combined with an organic molecule. The compounds generally used are EDTA and ligno-sulphamate with the latter preferred. (EDTA is a suspected carcinogen). Of course, the trace elements in organic fertilisers, such as pelletised poultry manure, liquid fish and seaweed, are already chelated, and often these materials contain sufficient trace elements for crop needs.

Manganese

Manganese is required in very small amounts and is very important, for without it, the production of amino acids and proteins suffer. It also works alongside magnesium in eliminating chlorosis. Soil with an excessive amount of magnesium and/or calcium locks up manganese.

Iron

Iron is essential for the formation of chlorophyll in plants and the prevention of anaemia in animals. Nearly all soils contain a lot of iron, mostly in unavailable form. Soils treated with excessive amounts of superphosphate often have excessive available iron, which reduces the availability of other trace elements. Maintaining good humus levels is beneficial in optimising the availability of iron.

Boron

Boron is implicated in the resistance of plants to diseases and is necessary for the formation of amino acids and protein. It is needed in only tiny amounts and many crops have benefited from the discovery that their potential was being limited by a deficiency. In the sap tests referred to earlier on potatoes grown under pelletised poultry manure, the boron levels were deemed excessive, whereas the sap tests from the conventional plot were deficient. The implications of this are unknown at this stage.

Copper, Cobalt and Zinc

There remains much to be learned about this group of trace elements. Their deficiency is implicated in a number of animal diseases, steely wool in sheep and infertility in cattle among them. Plants short of copper show abnormal growth and stunted young branches. Zinc is essential for the formation of chlorophyll, but copper and cobalt also appear to play a lesser role. Zinc deficiency is implicated in poor stock fertility.

Iodine, Chlorine, Fluorine, Sodium and Lithium

Iodine, chlorine and fluorine are all halogens. Iodine is well known as an essential ingredient in human and animal health as a regulator of metabolism. It is readily taken up by plants from foliar applications of liquid fish, or seaweed. It appears to have no major role in plant nutrition, or health.

Chlorine deficiency in plants is extremely rare. What is not rare is an excess caused by over-reliance on muriate of potash as a source of potassium. Excess chloride in soil tests is invariably accompanied by reduced availability of trace elements. Members of the rose family, rosaceae, which includes pome fruit, are particularly sensitive to excessive amounts of chloride.

Fluorine is not considered essential for plant growth, but has an important role in animal nutrition. Both an excess and a deficiency are implicated in poor tooth development.

Sodium and potassium play complementary roles in plant and animal nutrition. Where potassium is deficient, sodium is absorbed in its place. Sodium is more often in excess than deficiency. Excessive sodium makes clay sticky. Gypsum (calcium sulphate) is often used to supply calcium, which displaces the sodium, allowing it to leach, making the clay more friable. Lime (calcium carbonate) is cheaper and can also be used where an increase in pH is desirable.

Lithium needs further study, but appears to be a companion to sodium and potassium. It has been applied to tobacco crops with the benefit of improving the quality of leaf grown for cigar wrappers.

Aluminium and Molybdenum

Aluminium is known more for the toxic effects of an excess than for any role in plant or animal nutrition. The conditions leading to toxicity are excessive soil acidity, reduced aeration and biological activity and needless to say, low humus levels.

Molybdenum is essential for many plants. It serves as a catalyst in the early development of brassicas and appears to be essential in the fixation of nitrogen by bacteria. It is required in very small amounts. Deficiency is often caused by excessively acid soil and low humus levels. Excessive levels of molybdenum cause reproductive problems in livestock.

Cadmium and Lead

Cadmium and lead appear to play no role in plant nutrition, nor do they appear to be required for animal health. They are discussed here because they are toxic in excess, generally causing chronic disease, rather than outright poisoning. They are particularly problematic because the animal, or person consuming them can only eliminate them slowly. This means that they tend to accumulate in the body over time.

Superphosphate, until recently, was made from phosphate rock that was very rich in cadmium and lead. This means that soils heavily fertilised with this super contain elevated levels of lead and cadmium and it is a cause for great concern that they are taken up by crops. The level of cadmium in sheep and beef kidneys has led to their being banned for human consumption in Western and South Australia.

In animal nutrition it is known that cadmium uptake is determined by food quality. Where the diet is deficient in zinc, cadmium absorption is increased. Other predisposing factors to increased cadmium absorption include periods of low nutrient intake and lack of high quality protein in the diet.

It is a matter for conjecture at this stage, but some organic farmers believe that increasing humus levels and bacterial activity in the soil reduces the uptake of heavy metals by crops.

Enzymes

Enzymes are catalysts used by plants to manufacture cell tissue, trigger hormone reactions (flowering, leaf-drop etc.) and take up nutrients. Most enzymes contain a trace element. An example is the use of molybdenum by the cauliflower. The enzyme requiring this element is only created in the first few days of the plant’s existence. Application of molybdenum after this period has no effect on the deficiency symptom of “whip-tail”.

Auxins

These plant hormones regulate cell division and elongation (ie. plant growth and development). They are relatively unstable and are most readily created from complex organic compounds, such as those found in animal manures, fish and seaweed. They require enzymes for their formation.

Acidity

Soil acidity is the measure of the number of hydrogen ions in the soil (pH). When there are a lot of hydrogen ions, the soil pH is a low number. When there are few, the number is high. The neutral point is 7. Thus, pH less than 7 is acid, more than 7 alkaline.

Soil that is too acid, or too alkaline, locks up essential nutrients. A soil in which the calcium, magnesium, potassium and sodium are in appropriate ratio12 will have a pH between 6 and 7. This level of acidity is optimum for the availability of nutrients for most crops. A few crops prefer a pH between 5 and 6 and a small number tolerate alkaline conditions.

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TC Jonathan Sturm 2002 - 2011

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