Macro and Micro-Elements in Soil for Plant Nutrition
The element nitrogen is a constituent of all forms of plant material, and is essential for the formation of proteins. It is taken up by the plant largely in the form of nitrate, but can also be absorbed as ammonia. The effect of nitrogen on plants is to cause an increase in total bulk, which in practical terms generally means larger leaves and larger plants, although they may not necessarily be more productive of fruit and; in fact, the reverse is often the case. Too much nitrogen in relation to other nutrients can also induce a lateness in flower or fruit formation. With , a gross excess of nitrogen can, in the early part of the year when light intensity is low, induce such lush, soft vegetative growth that the plant fails to develop flower trusses, and, even if it does, these can ‘miss’ completely. The amount of nitrogen present naturally in can vary greatly, and soils rich in organic matter can contain up to 1% or more, whereas an infertile soil will contain only 0.05%. A usual figure would be in the order of 0.10 to 0.30%.
The total nitrogen figure in the soil is no real guide to its availability to the plant, a lot depending on the rate of bacterial activity, which in turn depends on temperature, lack of competition from competing organisms such as protozoa, water-logging, lack of air, excess consolidation and other factors. Partial soil sterilization or pasteurization by means of heat will result in a flood of available nitrogen because the pasteurization heat kills off pests and diseases and destroys competing organisms, leaving the thick-walled spore-forming nitrogen bacteria free to carry on with their activities. The breakdown of organic matter in the soil results in the production of CO2 (carbon dioxide) which will form a weak carbonic acid solution which is one of the main dissolving agents for nitrogen and other plant nutrients.
As nitrate is highly soluble it can quickly be leached out of the soil by over-watering, which shows the folly of over-watering. At the same time, flooding out of excess nitrogen can be a useful technique when there is known excess, as is the case following soil sterilization methods which rely on heat. Ammonia is also washed out by flooding.
While an excess of nitrogen generally results in lush unproductive growth, prone to disease, shortage is quite clearly exhibited by pale sickly yellow leaves. Plants can also assimilate nitrogen as ammonium compounds, which can result in problems, a typical example being associated with black bottoms in tomato fruit.
Breakdown of nitrogen
When a fertilizer such as hoof and horn meal is added to a soil, it is necessary for soil bacteria to attack the hoof and horn meal particles and break them down first into nitrite and then into soluble nitrate before the plant can utilize the hoof and horn’s nitrogen content. Soilless media are initially fairly sterile, containing few bacteria, and it takes time before the hoof and horn can be converted to nitrate. If, on the other hand, nitrogen is applied in the form of nitrate (e. g. potassium nitrate) the plant can absorb it almost immediately.
Required for all plants, especially those of a leafy nature. Taken up largely in the form of nitrate, but also as ammonia. Induces lush soft growth in many plants if applied in excess, especiallyand other leafy crops early in the year when light intensity is low. Can also reduce flowering and fruiting potential of plants if over-applied. If in short supply, this results in small and often yellow leaves, along with a general lack of vigour.
Phosphorus (P and P2O5)
Phosphorous is associated with all life and is a constituent of every living cell in plants and animals. It might therefore be expected that large quantities are required by growing plants, but this is not the case; while it is constantly needed, the actual amount required is relatively small in proportion to the nitrogen and potash required.
Natural supplies of phosphorus are in most types of soil, a percentage of 0.15 being the amount present in an average loamy soil (a medium soil with a good proportion of organic matter). In chalky soil areas the amount present rises considerably to the order of 0.2-0.3%, whereas in some types of infertile soil its presence may be minimal.
Phosphorus is frequently associated with successful root growth and early maturity, yet the need for it persists right through to flowering and subsequent fruit formation and ripening, where this is applicable. On the other hand many soils are heavily overloaded with phosphorus, indicating that its actual uptake by the plant can be relatively small. The surplus phosphorus remains in the soil because of its low solubility. Its availability to the plant is in the form of a weak phosphoric acid dissolved in a weak acid solution produced by micro-organism activity.
In a very acid soil, phosphorus combines with iron and aluminium to form even less soluble components. The particular role of calcium in relation to phosphorus is by no means fully understood, as phosphorus would appear to be quite readily available to plants under the acid conditions of a soilless media (pH 5.5). The way in which phosphorus actually becomes available to plants is thought to be linked to the process through which previously insoluble phosphorus becomes available to the plant when more phosphorus is added, but, to confuse the issue still further, plants seem to exist quite happily in soils shown on analysis to have a high phosphorus content without the addition of any additional phosphorus.
The temperature of the soil or growing medium and the temperature of the air seem to be closely associated with the availability of phosphorus and the ability of the plant to assimilate it, this being illustrated quite clearly by the bluish coloration which results when many tender plants are subjected to cold. General lack of vigour and a distinct tendency towards poor rooting and late flowering are further symptoms of phosphorus shortage.
The effect of excess phosphorus on the plant is extremely difficult to define, no doubt because any excess phosphorus applied is rendered insoluble or is possibly lost byin a free draining soil or growing medium.
Fertilizers are stated as having a percentage of soluble and insoluble phosphoric acid, a somewhat confusing statement; it is better if the percentage is stated as phosphorus (P) only.
Required by all plants in relatively small quantities. Has an effect on root development and early maturity. Shortage results in blue coloration; excess difficult to define.
Potassium (K and K20)
Potassium is required in very large amounts, particularly by flower and fruit producing plants. It is present initially in many soils in larger quantities than either nitrogen or phosphorus (1% or more), although only a small proportion of the total quantity may be actually available to the growing plant.
Excess results in hard stunted growth, whereas shortage gives rise to marginal leaf scorch and further encourages the soft lush growth typical of nitrogen surplus. There is therefore a very close relationship between the effects of nitrogen and those of potassium, and successful cultivation of most plants relies on the correct ratio of nitrogen to potash, phosphorus playing little part provided it is present in sufficient quantity.
Potassium is said to afford disease resistance qualities to plants, yet this is invariably associated merely with a hard or balanced growth which, apart from ensuring a good protection, has no surplus of carbohydrates for the disease organisms to plunder readily. Colour pigments develop well in the presence of adequate potash, which means clearer expression of colour in flowers, or better colour in fruit. An excess of soluble potash in the soil can also give rise to soluble salt problems, which can affect the osmotic process adversely, or burn the roots physically.
Required in large amounts for most plants. Shortage is indicated by marginal leaf scorch. Excess results in stunted dark blue tinted growth, and in some cases burning of the leaf tips, no doubt because an excess in the growing medium results in a high soluble salt content.
The importance of magnesium varies according to the species of plant involved, but is essential for the photosynthesis process and the translocation of elaborated foodstuffs round the plant. Where this element is either in short supply or rendered insoluble, browny-orange areas appear between the veins, especially in older leaves. For those plants which make large demands on magnesium, this element is added regularly to the growing medium, often along with lime.
Excess of magnesium will create a salt problem in the growing media before its toxic effects, if any, are exhibited by the actual plant. The unavailability of magnesium to the plant is a complex subject, centred around the quantities of potassium and nitrogen in the growing media, and possibly other factors. Often when there is an excess of potash, or potash is used copiously, as with tomato soils, magnesium deficiency is at its worst. On soils of poor physical structure, potassium seems to be taken up in preference to magnesium, or alternatively the magnesium is rendered insoluble in some way. Magnesium deficiency is also worse in areas of strong light: in the case of tomatoes those along the outside areas nearest the glass seem to be the first to show deficiency symptoms.
Required by all plants and in large amounts by some. Shortage results in interveinal chlorosis, with a yellowing or bronzing of the chlorotic area. Areas most exposed to strong light are usually first affected in a greenhouse. Plants in light soil heavily dressed with potassium are usually prone to magnesium deficiency. Excess is difficult to define, although excess salt concentration with all its side effects will usually result when it is oversupplied.
Many authorities do not discuss calcium under plant nutrients, preferring to give this element a special consideration as a neutralizing agent. Calcium is, however, an clement needed by plants, especially for the cell walls, apart from its other functions — which are best pursued in a work on chemistry.
The exact causes of acidity in the soil or growing media have been studied for many years and are still not completely understood, but the gardener need merely think in terms of the amount of free lime available to the plants enabling the base exchange mechanism to operate whereby plants obtain their nutrients. A soil or growing medium may contain considerable amounts of calcium, but this may be ‘held’ by the organic content or humus content of the soil, a situation by no means unique in greenhouse culture, especially with soilless media.
A growing medium not supplied with lime will become considerably more acid, due to the acid-forming activities of micro-organisms and chemical activity. PH scale The measure of exchangeable calcium in the soil or other media (including of course liquids) is recorded on the well-known pH scale which, for the technically minded, is a logarithm of the calcium ion concentration. The pH scale runs from 1 to 14, most plants falling between about 4.5 and 7 for their particular preferences. However, matters have been further complicated by the lower pH range at which plants, in many cases, seem capable of growing satisfactorily in soilless media, no doubt because of the use of nutrients which are available to the plants without recourse to micro-organism activity. Unfortunately the amount of exchangeable calcium can greatly affect the availability of other nutrients and vice versa. Ammonium and potassium can, for example, immobilize the calcium if present in large quantities, whereas too much calcium can result in insolubility of manganese and iron, and too little can produce calcium toxicity of the same elements. It can be seen therefore that what might appear to be, on the surface, a simple question of acidity and alkalinity with calcium acting as the neutralizing agent, is far from the case. At the same time there is much to be said for ignoring the complexities and pursuing the more simple course of relying on the pH figure.
Required in varied amounts by most plants, especially for cell wall formation. Amounts needed vary and this translates itself into adjusting the soil or growing media to a suitable pH figure. Actual shortage of calcium in the plant will result in yellowing of leaves, rather typical of nitrogen shortage, or in extreme cases a whitening of the growing points. It is difficult to consider calcium shortage by itself, as there are so many associated issues involved, the same being true of an excess of calcium of which the yellow-white leaf coloration of iron deficiency is so typical.
Lesser or micro-elements
This element has a general role in plant nutrition, it being difficult to explain its exact function. Sulphur is present initially in most soils and in addition in many fertilizers used, so its shortage is seldom, if ever, a problem.
This is an important element in greenhouse culture and is greatly concerned with photosynthesis. It is very insoluble and is rendered still more so by high pH figures. Symptoms of shortage are yellowing of the whole leaf, although in extreme cases the leaf can turn white, the older leaves usually being more affected than the younger ones. Deficiency usually arises from an excess of exchangeable calcium in the soil or growing medium. Excess is difficult to detect, no doubt because excess iron combines with phosphorus to form insoluble compounds, but excessive iron application is not to be recommended.
Like iron, this is much concerned with photosynthetic activity, which is why any shortage invariably produces a leaf-mottling effect. High pH figures usually spark off any deficiency of this element, although the mottling of the leaves of young plants caused by the invariably high pH figure of many growing composts usually disappears as the pH figure drops. Toxicity of manganese is a much more common problem, particularly in olderwhich are steam sterilized. Here a blue-black coloration develops on leaf tips, in addition to a general drooping.
Much more has been heard of boron in recent years, largely because its importance in tomato culture has been highlighted. It would appear to have a multifarious role to play in plant growth, as its shortage in many plants causes shrivelling of leaf tips and a blueing of stems and petioles. Tomato fruits develop a corky brown layer beneath the skin. In the majority of cases, and with properly formulated growing media, boron deficiency should not arise, although it is obviously an element which can cause considerable trouble. Care should be taken not to use boron to excess, as the efficiency of boron-based weedkillers is well known.
The other elements referred to earlier do not usually give cause for great concern unless present in excess, or rendered so by general soil imbalance. There can also be a degree of substitution where the micro-element replaces the macro-element, resulting in deficiency symptoms of the latter in some cases, although they may serve admirably; eg silicon can replace phosphorus, and sodium can substitute for potash.