Roses, like most other plants, derive their food from the air and the soil. By far the greatest amount of food taken up is water, which consists of hydrogen and oxygen. Next, in both quantity and importance, comes carbon dioxide, a gaseous compound of carbon and oxygen.

Analysis Of Plant Ash

When plant tissues are burnt in a crucible all the hydrogen, oxygen, and carbon are reconverted to water and carbon dioxide, and driven off. The non-volatile minerals of the plant remain as ash. The burning alters the chemical composition of this matter, but it does so only by combining the same elements in different groupings. The same quantity of each element is still present, but it exists in new compounds, differing from the old in both chemical and physical properties.

From this ash we learn that less than five per cent of the dry weight of a rose plant is mineral matter derived from the soil, and that carbon, hydrogen, and oxygen comprise over ninety-five per cent of it. These come from carbon dioxide of the air and water from the soil and are chemically broken down and re-combined in the foliage. The resulting carbohydrates (same group as sugars and starches), together with amino-acids, are the true food of the plant, the basic building material of all wood, leaves, and blooms. All energy for growth and other plant activities is derived from its oxidation.

When plants are pruned hard, great quantities of food stored in the young branches are lost. When pruned after new shoots have grown there is similar loss. This late  pruning  may conveniently make roses bloom later than usual, but will give smaller blooms and less growth, because of this loss.

Loss of foliage, out of season, forces a plant to live on its reserves, for no more plant food can be made. Any new growth is weak, because it is made from inadequate food. The regaining of normal vigour is a slow process and should the new leaves be lost the plant is usually doomed. The most common causes of abnormal defoliation  of roses are black spot, sunburn, and spray-burn .

Carbon dioxide can be absorbed only by healthy leaves, in sunlight and when temperatures are sufficiently high. It is taken up by the stomata, quickly dissolved, and used in the making of complex substances by the sun's acting on chlorophyll. Smoke and chemical fumes are injurious to plants; the former clogs the stomata, but is much less prevalent in these days of electricity than in former years, when it was impossible to grow any healthy vegetation in or near large cities. The chemical fumes seldom exist in sufficient concentration in the air to affect foliage, but are taken into the soil and are toxic to both soil bacteria and plant rootlets probably an increasing menace.

From root-tip to leaf-tip every plant is an intricate system of water channels. The framework consists of cells, many of which contain as much as ninety-nine per cent of water. With such force do plants draw water from the soil through their roots that this moisture level can be maintained while they are reducing soil moisture to as low as three per cent in very light  sandy loams. Heavier soils will not yield water to plants to this same extent.

Soils contain some uncombined carbon in the form of charcoal or soot ; this is only very slowly, if ever, available to plants. Combined with oxygen, as carbon dioxide, some is carried into the earth by rain, some is liberated from lime  (calcium carbonate), some is generated in the process of decomposition of organic matter, and some is excreted by roots. With water it forms carbonic acid, and acts on many other compounds in the soil, but no carbon is absorbed by the roots. As carbon dioxide escapes from the soil it assists in soil aeration. Soil bacteria need carbohydrates in the soil; these are provided by organic matter. Soil-derived food must be in very dilute solution before it can be taken up by roots. In hot weather rapid evaporation occurs from leaves. Absorption of water by roots must be still faster, for this water contains additional food that will increase the sap concentration unless the newly absorbed solution is extremely weak. Over-concentration of cell-sap causes a demand for more water from the soil; if it not forthcoming, wilting results. If the concentration remains at its normal low level, growth is accelerated by heat, which, in this way, accelerates food intake.

Organic Waste Matter

All soils contain some amount of plant foods in their interstices. Primitive man soon found that plants grew best where organic matter had undergone combustion either in the rapid manner of burning or the slow manner of decay following the depositing of vegetable matter, animal excreta, or dead animals either on, or just under, the soil surface. He did not know how this came about, but it led him to feed his ground regularly with organic waste matter.

Plant and animal waste products contain all the essential constituents of plants, and help perform all the necessary collateral functions. Every garden soil will be improved by them; very light and very heavy soils cannot be rendered fertile and of good tilth without them. Their actual amount of plant food is very small in proportion to their bulk, but the amount of mineral food needed by plants is also very small, and the bulk improves the physical condition of the soil. Organic foods are rendered soluble by soil action, slowly, and over a long period.

After millenniums of soil husbandry, chemists began to investigate animal tissues, plant tissues, soils, animal excreta, air, tap-water and sunlight all factors in plant feeding. Although soil-derived food comprises so small a part of plant ash, it does not mean that any one of its component elements is unimportant. So far at least nineteen elements have been found in the tissues of a healthy rose plant. Next in order of quantity to carbon, oxygen, and hydrogen come nitrogen, potassium, sulphur, calcium, magnesium, phosphorus, iron, manganese, boron, zinc, copper, and others in merest traces, yet of vital importance. Any of these so-called "trace elements" should be added under only expert guidance; a slight excess of any one of them will kill plants. Most organic manures contain them in sufficient quantity.

Thousands of skilful gardeners rely solely on organic fertilizers, and their long-range results are much better than those who use only inorganic chemicals as manures, even though these men may prove, by analysis, that two or three pounds of chemicals, mixed in balanced proportions, contain as much plant food as a dray-load of animal manure or compost. After all, our forefathers had none of these compounds available, but they grew excellent crops of all kinds. Today, we can produce greater quantities of products from the land, in shorter spaces of time, by using chemical manures. We do not need, now, to allow long resting periods for the soil between crops, but these methods of intensive cultivation will ruin the soil quickly, if we do not incorporate large quantities of organic matter from time to time. This is necessary in order to maintain good physical conditions of the soil, its moisture-retaining powers, «nd its aeration. Chemical fertilizers decrease all of these properties, and decrease plant resistance to disease, but, used sparingly, they are a wonderful adjunct to organic manures. In fact, many organic manures produce starvation for one or more elements, and need to be supplemented appropriately. For example, vegetable matter should never be used without the application of a rich nitrogenous manure a couple of weeks later.

With ever-decreasing available supplies of horse and cow manure, gardeners should think of green manuring. Rose plants do not grow very actively from late April to mid August, and during that time a crop of Cape barley or Algerian oats could be grown between them. It will look untidy, but will keep the surface layers of the bed from setting hard, will help to keep the beds from becoming sodden, and will retain plant foods in the surface-soil. It will also provide excellent vegetable waste matter. Legumes, such as peas, beans, or red clover could be used, for their roots go deeper into the soil and so increase aeration, but they are more subject to disease than the barley or oats. Any mulching adds organic matter.

As the presence of humus increases the amount of available plant food, the concentration of nutrients in solution in soil moisture greatly increases unless watering is increased too. For this reason plants wilt quickly in rich soil unless well watered. The moisture-holding capacity of a soil is increased by the incorporation of organic matter.

Frequently, but erroneously, it is stated that a heavy subsoil will ensure continuous replenishment of water to lighter surface soil superimposed upon it. The suction or attraction for water is greater in the fine interstices of clay than in coarse soil. If the roots cannot penetrate sufficiently deeply, the plant will wilt despite the presence of adequate moisture near at hand-Addition of heavy loam to light surface soil will increase water-retaining powers and may even help to raise some water from more sandy lower levels. In addition, there would be better conservation of plant foods, especially potash. Adsorption of dissolved mineral salts on soil particles renders the soil moisture that drains away much lower in concentration of those salts than the moisture in surface levels. Flower-pots adsorb mineral nutrients in this same way, and that is why plants send their roots towards the pot itself rather than massing them in the inner soil.

Insufficient available water-supply can be largely counterbalanced by mulching. With the exception of some of the hill and coastal areas, evaporation greatly exceeds rainfall at most times of the year in Australia.

Major Plant Food Elements

For many years nitrogen, phosphorus, and potassium were known as major soil-derived plant food elements, and all others as minor elements. In more recent times sulphur, calcium, and magnesium have heen added to the former group by some authorities. The distinction in terms between major and minor elements is based on the quantity required by plants and not on their relative importance.

Nitrogen

Popular garden manures are all rich in nitrogen, particularly bird-droppings, urine, blood-and-bone, dung of any type, urea, soot, sulphate of ammonia, and the various nitrates of which sodium nitrate is the one most commonly used. Sulphate of ammonia is very acid; nitrate of soda is alkaline.

There are about seventy-five million pounds of atmospheric nitrogen above every acre of land, but it is absolutely unavailable and useless to all plants except legumes such as the clovers, lucerne, peas, and beans. It is most likely to be deficient in light soils. Nitrogen is absorbed generally as nitrates; nitrifying soil bacteria render it available from organic matter. Nitrogen is needed by a plant before any leaf-growth is apparent; it is a key element in plant proteins. Its general effect is to produce big dark leaves and lush growth. Young plants that show yellowing of the foliage respond almost immediately to light dressings of nitrogenous manures. All forcing manures are high in nitrogen content, but must be used sparingly and in repeated applications rather than in heavy doses.

Inorganic nitrogenous manures are best applied as very weak aqueous solutions. The strength should never exceed one ounce to two gallons of water, and even then should be used on wet soil and be followed by a further watering. By judicious application they can be employed to retard maturity of blooms; exhibitors sometimes use them for this purpose. Urea is becoming the most popular of this type of manure, though sulphate of ammonia and nitrate of soda are still better known.

Nitrogen is needed in the breaking-down of vegetable matter in the soil. Land fed with straw, green crops, seaweed, sawdust, and the like should be given, soon afterwards, a good dressing with highly nitrogenous manure; otherwise there will be a temporary nitrogen deficiency. The nitrogen used in the process of disintegration is not lost; it is later released for plant consumption. Conversely, soils treated with nitrogenous chemicals rapidly decline in humus content, with consequent proportionate loss of fertility.

Potassium

Potassium is needed by plants in large quantities. It appears to increase their resistance to heat, cold, and disease, especially mildew. When deficient in potash, a plant is stunted and its leaves die, first at the tip and later along the margins.

Most heavy clays contain adequate potassium and they do not allow it to leach away. Sandy soils are generally deficient. Gardens regularly dressed with vegetable matter and dung are usually well supplied with potassium. It is very soluble and is apt to wash out of organic substances if they are exposed to wet conditions before spreading. Virile plants retain their potassium, but return it to the soil as they age. When killed they release it and allow it to be washed out quickly.

Potash is most abundant in wood ashes, young vegetable matter, seaweed, fresh dung, and in nitrate, sulphate, and chloride (or muriate) of potassium. Potassium sulphate is the most commonly used potassium salt. Its sulphate radical could raise the sulphur content of the soil to a level that would be toxic to soil micro-organisms and earthworms in the same manner as ammonium sulphate, but it is not used in sufficient quantity to cause any such trouble. It is to be preferred to the chloride. The nitrate is expensive, but the best of the three. It adds no toxic element, and is a strong nitrogenous food.

Minor Plant Food Elements

Sulphur

It is seldom realized that plants contain more sulphur than phosphorus; it is very important in chlorophyll formation. Inadequacy of sulphur in gardens is unknown, for it abounds in dung, blood-and-bone manure, vegetable matter, sulphates, superphosphate (as excess sulphuric acid), flowers of sulphur, and in many sprays, such as Bordeaux mixture, lime-sulphur, iron sulphide, and the various colloidal and wettable sulphur preparations. Some of it oxidizes and escapes as sulphur dioxide. Commonly, in putrefactive processes, some sulphur combines with hydrogen to form sulphuretted hydrogen. Any sulphur added by way of spray, dust, or sulphate manure is usually in excess of plant food requirements. Ample is available from organic matter in steady supplies; putrefaction quickly liberates large quantities. Excessive sulphur in soil is one of the worst calamities that can befall a garden. It means death to many millions of soil micro-organisms and will almost certainly banish all earthworms.

Calcium

Calcium exists in all plant tissues, particularly the older parts. In soils it is most common as limestone, impure calcium carbonate. This is freely acted on by even dilute solutions of weak organic acids, which abound in soils containing much organic matter. Carbon dioxide and a calcium salt are formed; the gas is liberated; the salt is usually very soluble and apt to leach away quickly. This chemical action reduces soil acidity and helps the bacteria that assist in nitrification. This faster metabolism will call for greater supplies of manures. Lime should never be used unless the soil is found by actual test to be unduly acid. A pH value of less than 5.5 would indicate a need for light liming.* Heavy liming can easily be harmful.

Bone-meal contains large quantities of calcium as well as other foods, especially phosphorus, but when calcium is applied in this form it is not as effective in reducing soil acidity or improving the physical condition of the soil as when lime or dolomite is used. Dolomite consists of sixty per cent calcium carbonate and forty per cent magnesium carbonate; it is an excellent corrective for excess acidity. Gypsujn is calcium sulphate; it does not increase or decrease soil acidity but helps to improve the physical condition of soils and is slowly available as a plant food.

In heavy clay, calcium causes a coagulation or flocculation of the fine soil particles and so renders the soil more friable, better aerated, and more easily drained. This good effect lasts for many years.

Calcium stimulates root-growth and deficiency appears to bear some relation to die-back. It helps to make potash and phosphorus more readily available and soluble phosphates less liable to be lost or changed into insoluble unassimilable forms. It renders iron less available-Calcium oxide, or quicklime, is made from limestone. When moistened with water it swells, heats, and breaks down to a fine powder, calcium hydrate, limil, or slaked lime. On exposure to air it slowly absorbs carbon dioxide and is reconverted into calcium carbonate.

* The gauge of acidity (or hydrogen ion concentration) is called the pH scale. A pH of 7.0 is neutral. Less than 7.0 it is acid, and more than 7.0, alkaline. The pH value denotes the intensity of acidity and not the quantity of acid present. A pH of 5.0 is ten times as acid as a pH of 6.0, and a pH of 9.0 is only one-tenth as

Magnesium

Magnesium is the key element of chlorophyll, so essential in Nature's sun-worked food factory. It is present in most soils in adequate quantities, but deficiency is common in our light coastal soils. Heavy clays of the inland areas contain ample available magnesium. Deficiency is marked by yellowing between veins of old leaves and is frequently accompanied by iron deficiency. It can be corrected by adding magnesium nitrate, magnesium sulphate (Epsom salts), or, if the soil is acid as a pH of 8.0, or, in other words, is ten times as alkaline. Roses do best in soils where the pH value lies between 5.5 and 7.0  slightly acid. very acid, dolomite. Magnesium sulphate is much cheaper than the nitrate, but less desirable in large quantities or repeated use because of its sulphur content. The nitrate is doubly useful because of its nitrate radical. Green vegetable matter has a fairly high magnesium content. Compost and most other organic manures would provide some organic magnesium compounds, and they are always to be preferred to inorganic salts if available in adequate quantities.

Phosphorus

Phosphorus is not needed by roses in as great quantities as is commonly believed, but is the most commonly deficient plant food element in Australian soils, especially where available iron is abundant. The brilliant autumn foliage of deciduous trees and the blueness of hydrangea blooms in our mountain districts are related to this proportionate association of these two elements. In these same localities very few roses are grown. They do not seem to do as well there as in most other parts, but this is probably due more to climatic conditions than to soil chemistry. Phosphorus stimulates root-growth and is needed by plants before leaf-growth is apparent.

The most readily available inorganic form of phosphorus is superphosphate, which contains about twenty-two per cent water-soluble phosphate. More of it is soluble in weak acids, particularly citric acid, which is excreted by roots. If the soluble phosphate combines with metallic bases, such as iron or aluminium, the resultant compound is insoluble and the phosphorus is lost to plants.

Superphosphate should be used sparingly and never more freely than one and a half to two ounces to the square yard. Ammonium phosphate ("Floraphos") is another inorganic salt of phosphorus. It is less dangerous than superphosphate and more forcing, due to its nitrogen. It can be used to hasten the maturing of blooms.

Rock phosphate gives up its phosphorus very slowly under the action of acid root excretions. Bone-meal is rich in phosphorus and, in the soil, it and other elements are rendered available slowly by bacterial agencies. The effect of these two fertilizers is seldom seen for at least two years, but it then lasts for many years. It is safe to use them at any time of the year. They may be dug deep into the rooting zone of the roses, and well mixed with the soil a month or so before planting and subsequently worked into the surface soil from time to time.

Bone-dust is often used in combination with blood manure as blood-and-bone manure, the most obvious action of which is due to its nitrogen, but there are many other elements present, including large amounts of phosphorus. The same applies to urine and bird-droppings. Phosphoric acid added carefully to liquid manures is another way of feeding plants with phosphorus.

Iron

Chlorophyll metabolism needs iron, which varies in amount greatly in Australian soils from almost complete absence in some coastal sandy soils to excess in some mountainous parts. It is often present in great quantities, but in forms that are insoluble in water, soil products, or root excretions.

Iron deficiency in plants is evident by pale young leaves and poorly coloured flowers. It can best be corrected by adding large quantities of organic matter, especially if it has been grown on soil rich in available iron. Chemically it can be added as ferric nitrate or ferrous sulphate, preferably the former, wherein both the iron element and the nitrate radical are helpful. It is the more expensive salt. Iron filings are usually easy to obtain, and a very light scattering of them over rose beds every few years will provide a steady supply of available iron to the roses as the filings oxidize and slowly break down. Manganese, zinc, copper, boron, and other elements are found as mere traces in plant ash, but when any one of them is lacking in a soil, the plants are just as handicapped as when starving for a major element. Thousands of square miles of Victoria, South Australia, and scattered coastal districts of Australia have been found to be barren waste land wholly because of deficiency of zinc, copper, and cobalt. At a cost of less than five shillings an acre those deficiencies have been corrected and the land has been converted into fertile, lush pastures. Areas of South Australia are deficient in manganese and copper. Small quantities of zinc have greatly increased some Australian wheat crops, especially in the Wimmera district of Victoria. Areas of the United States of America have been helped by minute doses of copper. Boron is very deficient in some otherwise very rich soils in New South Wales (for example, in the Orange district), and in New Zealand. Mysterious diseases amongst plants and livestock in many parts of the world have been found to be due to a deficiency of one or more of the minor plant food elements. The remedy has been simple once the cause was discovered.

Manganese and boron are the only two minor elements that may be of practical interest to rose-growers. Manganese is best added, chemically, as an extremely light dressing of manganese dioxide, or potassium permanganate (Condy's crystals) in very pale-pink solution or added to liquid manures. Boron is most easily applied as sodium biborate (borax), but must be used in very minute quantities, under scientific supervision.

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