Plant Nutrition

While conventional growers feed the crop, the organic grower feeds the soil and the many micro and macro organisms living in it. These organisms in turn provide the crop with its nutritional needs. The reasoning behind the first approach is artificial fertilisers are said to be much cheaper than organic fertilisers. The reasoning behind the organic approach is that organic fertilisers produce healthy crops that need little if any expensive pesticide, or fungicide, considered essential by conventional growers.

The health of the living fraction of the soil is dependent on an adequate level of humus, optimally between 5 and 10%, a constant source of fresh carbohydrate and lignin (dead plant material), a smaller constant supply of protein (such as animal manure and legumes), and mineral nutrients at adequate levels and appropriate ratio. The most important nutrients are the anions nitrogen and phosphorus, and the cations calcium, magnesium, potassium and sodium. Also of importance are the nutrients needed in only small amounts, the trace elements.

The crop nutrient in shortest supply for the plants' needs is referred to as a crop-limiting factor.

Major Elements

Carbon, hydrogen and oxygen

Somewhat surprisingly, you won't usually see these elements listed as crop nutrients in many books on gardening. It's surprising to The Git because the greatest bulk of plants consists of these three elements. Carbon as carbon dioxide plus hydrogen and oxygen as water are converted by the chloroplasts in green plant tissue to carbohydrates. The simple carbohydrates are called sugars, more complex carbohydrates starches and the most complex of the lot, cellulose. The latter accounts for the bulk of plant tissue.

Animals eat the carbohydrates and convert them back into carbon dioxide and water extracting the stored solar energy in the process, ready for the plants to use more solar energy to start the next round of the never-ending cycle. Carbon dioxide then is not a pollutant as is so often the description these days, but an essential part of life. In the geological timescale, carbon dioxide levels are very near the lower limit. Plants evolved in an atmosphere with far greater amounts of carbon dioxide than today which is around one percent of the total atmospheric gases.

While carbon dioxide levels are usually not perceived as a crop-limiting factor, greenhouse growers can gain substantial increases in yields by increasing the amount of carbon dioxide available to crops. Permaculturists recommend keeping chickens, or making compost in the greenhouse in order to achieve this. A level of 2-3% carbon dioxide provides worthwhile benefits.

Water is the single greatest crop-limiting factor, but not because of its nutrient content. Rather, water is the transport system for nutrients and hormones, the chemical messengers that regulate the growth of all living tissue. Water is so important, it is dealt with separately as a major topic.

Phosphorus

Phosphorus encourages root development and is essential for the formation of protein in the plant. As well, it increases palatability of the plants since 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. The common weed, fat hen, is a good indicator plant for this condition. 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. Since this latter condition is usually caused by excessive use of superphosphate, the organic grower is unlikely to see it.

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 locked up in the soil through chemical reaction with iron. Phosphorus from farmland appearing in rivers and streams is generally carried there through erosion of the soil, rather than the phosphorus being in water solution. Humic acids, earthworms and associated beneficial bacteria and fungi in a fertile soil unlock the phosphorus in reactive phosphate rock, superphosphate residues 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 determined to be a problem, reactive phosphate rock (RPR) is the organic 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 in RPR is only readily available when the soil pH is around 4.5 to 5.5. However, the organic acids associated with bacterial activity in a fertile soil are capable of unlocking the phosphorus when the soil pH is a more acceptable 6.0-6.5.

Many Australian organic producers 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 to 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, particularly juveniles. 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 (rotted animal manure, legume green manure, fish, blood 'n' bone etc.), or as water-soluble artificial fertiliser (Nitram, urea, ammonium sulphate etc.). It is worth noting that raw animal manure contains much of its nitrogen as water-soluble chemicals such as ammonium carbonate. Raw animal manure is, from the point of view of the plant, very similar to artificial fertiliser in its effects. 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 manures made from fish, comfrey leaves, or nettles are popular. Lucerne chaff also works well, with the added advantage of containing plant growth hormones.

In cropping experiments 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 commercial grower applied a soil drench of 60 litres per hectare of liquid fish to a crop of broccoli. The plants responded as well as they did to artificials, even though the nitrogen content of the fish emulsion was a mere 2.8%. The artificial fertiliser salesman said to the grower: "You'd have been better off pissing on the crop than using this fish emulsion!" The grower responded: "Maybe that's what I'll do for the next crop!"

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 growers 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 varieties 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, since the particles burn holes in their skin. Frogs, Nature's vastly underrated pest controllers, are also devastated 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. Overseas books recommend greensand and other sources of potassium not generally available to organic growers in Australia. Wood ash is also often quoted as a good source of potassium. However, it is only the twigs that supply significant amounts potash - tree trunks are not overly endowed with this mineral.

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.

Short-term potassium deficiency can be met with liquid manure made from plants that concentrate potash, such as lucerne, comfrey leaves, or bracken fern.

Seaweed is often recommended as a good potassium source. Because seaweed contains potent plant growth hormones and auxins, large amounts can temporarily reduce plant growth. This means when seaweed is used for this purpose, it is better used in the compost heap where these substances can decompose without affecting plant growth rate.

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 and deficiency is associated with poor keeping quality. As well, it is essential for the proliferation of beneficial 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, holds the clay particles further apart and the clay becomes more friable. While gypsum (calcium sulphate) is recommended to break down sticky clay, it will only work if the cause of 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 elsewhere in this book, 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, builders' lime (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 frequent, lighter applications allowing the soil biota to gradually adjust to the changing environment.

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. Where the use of even high magnesium dolomite will still leave an excess of calcium over magnesium, there are several suitable 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. Perhaps it would be better named supersulphate! 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 compost, 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. Plants readily take it up 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. Excessively acid soil and low humus levels often cause deficiency. 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.

Important Compounds

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 (i.e. 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 ratio 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.

Soil Testing

Soil testing as a method of predicting crop yield is fraught with difficulty. Most soil tests ignore the importance of the balance between the major cations and the contribution of the organic fraction of the soil. You will find a crude organic matter figure derived by igniting the sample, but this does not tell us what proportion is humus. However, if you use the soil test result to bring the calcium, magnesium and potassium into balance, you will experience a long-term benefit to both the crops you grow, and the health of the consumers of your produce.

A major 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%

We have seen soil test results that show all other cations account for 40% of the total cations. This was a dairy farm and the stock were in extremely poor health. Bringing the major cations into better balance with each other reduced the availability of the minor cations and stock health improved quickly and dramatically.

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Jonathan Sturm 2003 - 2011

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