Teachers Guide #4.5 Soil Nutrients Refers to FACT CARD 4.5 Soil Nutrients

Teachers Guide #4.5 – Soil nutrients refers to FACT CARD 4.5 Soil nutrients

Soil nutrients

Some history

Around seven thousand years ago, some civilisations grew alongside rivers where the soil of the alluvial valleys was deep and fertile. This fertility was replenished by the silt deposited during seasonal flooding. The Tigris/Euphrates rivers (Mesopotamia) and the Nile river (Egypt) gave rise to two such civilisations built on the nutrients that the rivers carried with them from distant lands. The depositing of this river-sourced ‘fertiliser’ was the key factor that allowed agricultural-based civilisations to grow a crop surplus, and therefore put more time into developments such as the invention of writing, creating the wheel, writing literature, and developing law books. In ancient times, Egypt was able to export large amounts of food and helped to sustain the Roman Empire. Not all of these civilisations survived though. For example, the Sumerians from Mesopotamia (present day Iraq) could not sustain their agricultural advancements because they caused extensive environmental degradation. The seasonal flooding was less reliable than that in the Nile valley and therefore irrigation became the crux of the agricultural system. Deforestation in the catchment resulted in irrigation channels being blocked up with silt. In time, human labour was insufficient to cope with removal of the silt and the cities were abandoned.

Thousands of years later, in 1842, a young English squire called John Bennet Lawes of Rothamsted (Hertfordshire, England) became interested in why ground-up bones improved the fertility of some soils but not others. Lawes dissolved some bones in sulfuric acid and found the answer was relatively simple – bones provide phosphorus, which is essential to good root growth. Acid soils (those with a low pH reading) could dissolve bone material whilst alkaline soils (with a high pH reading) could not (because calcium phoshate is insoluble in water). He patented a process for treating phosphate rock with sulfuric acid to make the phosphate soluble, creating ‘ superphosphate’ and founded the world’s first artificial fertiliser industry.

The problem of satisfying a plant’s hunger for nitrogen still remained, however, as shipping sodium nitrate deposits from Chile and accumulated seabird dung from Peru was proving expensive. The problem was solved in two ways. Firstly, scientists discovered that certain plants (known as legumes) had a relationship with naturally occurring soil microbes that resulted in the microbes converting nitrogen from the air into nitrogen in the plant in return for a meal. As the importance of these legumes (such as clover) became much better understood, farmers began to grow more of them, e.g. as components of pasture mixes to provide them with a natural source of nitrogen fertiliser. Secondly, in the early 1900s two German chemists – Fritz Haber and Carl Bosch – thought that it should be reasonably easy to combine nitrogen with the hydrogen in the air to make ammonia (as this is done regularly by lightning in nature). They found that a powerful charge of electricity could do the trick and the process involved became known as the Haber-Bosch Process. This discovery ultimately led to factory manufacturing of nitrogen fertilisers and gave way to the modern day nitrogen fertiliser industry.

Soil pH

A soil pH test tells you whether a given soil is acidic or alkaline and this will determine to a large extent the plants that can be grown in a given soil. A soil with a pH of 7.0 is termed neutral; below 7.0 is acidic and above 7.0 is alkaline. It is possible to adjust the pH of your soil by adding appropriate materials. For example, if you wish to increase the soil pH level you will need to add a liming agent such as crushed limestone, dolomite, quicklime or slake lime. The change in soil pH will be proportional to the amount of material that you add. Similarly if you find that the soil pH is too high it can be reduced by adding appropriate amounts of elemental sulfur, aluminium sulfate or even sulfuric acid.

Plants vary widely in their overall tolerance to different soil pH levels – with some better able to cope with pH changes than others. The pH of the soil can also affect the availability of nutrients, with some nutrients becoming inaccessible to plants at certain pH levels.

Soil pH / Impacts
> 7.5 / Many flowering shrubs do well, but not the heather group, azaleas, rhododendrons, lupins or most lilies. There is a reduced availability of phosphate, potassium, manganese and iron.
6.0 - 6.5 / Optimum for most plants. Maximum availability of mineral nutrients.
Phosphates fixed by soil; potassium, calcium, magnesium, and trace elements suffer loss by leaching. Bacteria affected more than fungi.
< 5.5 / Many plants suffer from acidity; roots short, stubby often fanged. Phosphate becomes less available
4.0 / Heaths and moorland plants and rhododendrons do well, but many other flowering plants fail. Soluble aluminium appears in harmful quantities.

Nutrients

Plants need food, just as you and I do, and those grown in poor soils that are low in nutrients will go hungry and are likely to become unhealthy and get sick. Pests and diseases have an uncanny knack of being drawn to weak and sick plants with ‘low immunity’. When it comes to the fertility and health of your garden soil, it is always best to opt for prevention, aiming to keep the soil fertility at an adequate level rather than to wait for signs of deficiencies to occur – ‘prevention being better than cure’. Maintaining a physically and chemically healthy, biologically active soil will aid in preventing problems of ill health. It is this concept that led to the saying ‘feed the soil, not the plant’. Thomas Jefferson was on the mark when he wrote the following in a letter to his daughter regarding a pest problem, in 1793: “When the earth is rich, it bids defiance to droughts, yields in abundance, and of the best quality. I suspect that the insects which have harassed you have been encouraged by the feebleness of your plants, and that has been produced by the lean state of your soil ”.

The nutrients required to provide a balanced meal to the crop must be present in soluble, inorganic forms in order for plants to be able to use them. Whether you provide your nutrients in ‘organic’ or ‘chemically synthesised’ forms, it is important not to neglect the general health of the soil. Soil organic matter contains plenty of nutrients but they are not available to plants until microbes degrade the organic matter and excrete nutrients in inorganic forms that plants can take up. Thus the topic of a soil’s fertility is intimately tied to biological health of the soil. The many armies of microscopic organisms and other soil organisms such as earthworms continuously work away, decomposing organic matter and converting nutrients from forms that are not available to plants into forms that are.

Soils vary in their natural ability to supply plants with essential nutrients. Some soils are deficient in one or more nutrients because they are formed from rocks that naturally contain little of those elements. Other soils are deficient because they are very old and have lost their nutrients as a result of either weathering or overuse. Soils with high amounts of clay and humus will have a good ability to supply nutrients because their overall negative charge holds onto the positively charged nutrients such as potassium and calcium. Too much clay, however, can mean that such nutrients are held on to so tightly by the soil that the plant may have trouble getting access to the nutrients. Similarly, a sandy soil has a low ability to hold on to nutrients, and may benefit from organic matter additions. It is a therefore a good idea to know your soil’s capacity to retain nutrients as doing so can save you both time and money in the long run. Fertiliser needs for gardens also depend on the type of plants that you intend to grow, as some plants have very specific requirements or tolerances.

Where soil nutrient deficiencies do occur, we can get a clue as to what might actually be lacking by looking at the symptoms that appear on the plants (see table below) and can add appropriate materials to correct the identified problem.

Nitrogen (N) / Positive effects: The rate and vigour of growth and colour of leaves. Protein building, enzymes and photosynthesis
Prevalent time: Occurs during early spring owing to the leaching effects of high rainfall and again in late summer when prolonged drought conditions result in soil nitrogen being trapped and concentrated in the dry surface soil above the absorptive root zone ( / Deficiency effects
Stunted growth; small yellow, pale green or possibly bluish leaves. Thin weak stems.
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Phosphorus (P) / Positive effects: Root growth, ripening of seeds and fruit.
Prevalent time:Phosphorus deficiencies are most pronounced in the winter and early spring when soil forms are immobile and relatively unavailable for growth. / Stunted roots and growth. Small purple leaves and stems. Yield of fruit and seed poor.

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Potassium (Potash or K) / Positive effects: Assists photosynthesis and the production of carbohydrates. Protects plants against diseases and environmental stress.
Prevalent time:May occur at any time of the year. It is easily leached from the soil and consequently deficiencies are common during periods of high rainfall.
Note: Close links with nitrogen. When nitrogen is increased, so must potash or deficiency will appear. / Fruits are poorly coloured, lacking in flavour. Leaves will appear scorched at edges, mottled, spotted or curled.

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Calcium (Ca) / Positive effects: Essential for sturdy young plant growth. One of the most important soil foods.
Prevalent time:Occur when plants are water-stressed during dry periods, the former when high levels of soluble salts are present in the soil and the latter when soils are over-limed. / General lack of vigour, growing points die, growth stops. Without calcium, some species are unable to assimilate nitrogen.

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Magnesium (Mg) / Positive effects: Part of the chlorophyll molecule (which makes leaves green). It is necessary for the formation of amino acids and vitamins. Essential to germination of seed and synthesis of sugar.
Prevalent time:These deficiencies are disorders characteristic of acid soils, especially of pumice land or soils of recent volcanic origin as, for example, on the central plateau of the North Island. / Photosynthesis is affected and shows as yellowing of leaves, or purplish brown patches between the veins. Leaves may fall prematurely.

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Element / Positive Effects / Prevalent time /

Deficiency Effects

Nitrogen / The rate and vigour of growth and colour of leaves. Protein building, enzymes and photosynthesis. / Nitrogen deficiencies are most prevalent during early spring, owing to the leaching effects of high rainfall, and again in late summer when prolonged drought conditions result in soil nitrogen being trapped and concentrated in the dry surface soil above the absorptive root zone. Plants under nitrogen stress show rapid and spectacular recoveries when nitrogen supply is restored, although the degree of yield restoration depends on the state of maturity of the crop. / Stunted growth; small yellow, pale green or possibly bluish leaves. Thin weak stems.
Phosphorus / Root growth, ripening of seeds and fruit. / Phosphorus deficiencies are most pronounced in the winter and early spring when soil forms are immobile and relatively unavailable for growth. Plants under phosphorus stress grow slowly, have an abnormally long growing period and only partially recover when the supply is restored through soil warming or other means. / Stunted roots and growth. Small purple leaves and stems. Yield of fruit and seed poor.
Potassium (Potash) / Close links with nitrogen. When nitrogen is increased, so must potash or deficiency will appear. A fruit-forming fertiliser. Assists photosynthesis and the production of carbohydrates. Protects plants against diseases and environmental stress. / Potassium deficiencies may occur at any time of the year. It is easily leached from the soil and consequently deficiencies are common during periods of high rainfall. It is common when plants are growing vigorously, particularly when they are oversupplied with nitrogen. Restored supply leads to good recovery. / Fruits are poorly coloured, lacking in flavour. Leaves will appear scorched at edges, mottled, spotted or curled.
Calcium / Essential for sturdy young plant growth. One of the most important soil foods. / These deficiencies are liable to occur when plants are water-stressed during dry periods, the former when high levels of soluble salts are present in the soil and the latter when soils are overlimed. / General lack of vigour, growing points die, growth stops. Without calcium, some species are unable to assimilate nitrogen.
Magnesium / Part of the chlorophyll molecule (which makes leaves green). It is necessary for the formation of amino acids and vitamins. Essential to germination of seed and synthesis of sugar. / Magnesium and molybdenum: These deficiencies are disorders characteristic of acid soils, especially of pumice land or soils of recent volcanic origin, for example, on the central plateau of the North Island. / Photosynthesis is affected and shows as yellowing of leaves, or purplish brown patches between the veins. Leaves may fall prematurely.
Sulphur / Necessary for chlorophyll synthesis. / Similar to nitrogen deficiency, leaves become light green.
Trace Elements
- Iron, Manganese, Zinc, Copper and Boron / Each of these elements is required by plants only in very small amounts, but they can be vital for certain plant functions, e.g. iron is mainly needed in the formation of chlorophyll, copper for nitrogen metabolism, and zinc for seed and starch formation. / Manganese and iron: These deficiencies also occur in calcareous or overlimed soils at any time of the year.
Copper: Copper deficiencies are largely confined to peat soils. / Wide variety of symptoms possible.

If we bear in mind that ‘what goes in is what comes out’, then it makes sense that if a plant is grown in a soil that is low in phosphorus, the plant will also be low in phosphorus. Similarly if dead plant material that contains a large amount of phosphorus is returned to the soil, then the amount of P found in the soil will increase accordingly. Nitrogen (N), phosphorus (P) and potassium (K) are the nutrients that plants need in the largest quantities and there is a wide range of fertiliser products containing these nutrients available for the gardener. There are also a host of ‘micronutrients’ that, although plants don’t need them in such large quantities (indeed some can be toxic if oversupplied), are still essential for balanced plant diets. The gardener needs to be aware that not all plants have the same nutrient requirements and must match fertiliser and/or compost applications to a given soil with the needs of the type of plants that are to be grown in that soil.

Nitrogen

A lack of N can mean smaller and/or unpalatable plants or crops. It is a good idea to feed small amounts of high N fertiliser often, rather than large amounts infrequently, and only when the plant is growing strongly as N is very mobile in the soil and if too much N is just sitting around in the soil it can be lost to the air or groundwater. Some natural sources of N include hen manure, animal urine, green lucerne hay, fresh grass clippings and legume green manures (i.e. beans, peas, lupins, clover). Other mulches and composts, however, (e.g. cereal straw residue) contain only low amounts of N and their addition to soil may temporarily ‘lock up’ soil N while the material gets broken down. This is because microbes need to access nitrogen to grow while they break down the such material and they will need to `borrow’ it from the surrounding soil if it is not present within the added organic material. The N is not `lost’ forever from the garden, but while the microbes are using it, it becomes unavailable for plant uptake. Once there is no more organic material left for the microbes to eat, the microbes will die and the N contained in their bodies will be released back into the soil. If small amounts of high-N fertiliser are added between layers of mulch, you can prevent plants in the immediate vicinity of a mulch application from becoming N-deficient. Including legumes as part of your crop rotation in the garden is a good natural way to add some N back into your soil.

Phosphorus

Phosphorus is removed from soil when plants and crops are harvested so it must be replaced. There are a number of P sources that can be utilised as all plant waste contains some phosphorus, as do bones (blood and bone is a good source of P), eggshells and animal manures. However, most organic materials contain only low amounts of P. So if a P ‘fix’ is needed, you can buy soluble P fertilisers that will become readily available for plant growth. Phosphorus tends to behave in a somewhat sneaky way in the soil as it likes to form insoluble compounds with calcium and other minerals, but plants need to take up phosphate when it is in the soluble form.

Potassium

Most of the soils in New Zealand have sufficient K reserves, so that providing plant residues are returned to the soil, K fertiliser does not need to be applied so often. Potassium fertilisers are frequently more commonly referred to as ‘potash’.

A balancing act