Under natural conditions plants feed themselves. Water makes up 80 to 90% of their weight, and the assimilation of carbon dioxide from the air provides a high proportion of the rest of the tissue material. Three elements only—carbon, hydrogen and oxygen are concerned in this: the other 1 or 2% of the plant however, consists of 60 or more different chemical elements, all of which are taken up from the soil. There are large stores of plant foods in most soils but these are released by the normal soil processes too slowly for high yielding vegetables, fruits and vigorous flower plants that the plant breeder has produced for us. So we have to supplement the natural supplies with fertilizers.
Fortunately you do not have to include carbon, hydrogen and oxygen in your feeding program, nor is there any need to add all of the 60 nutrients, since only 16 elements are known, at present, to be essential for healthy growth. Three only of these, nitrogen, phosphorus and potassium—are required in really large amounts and are likely to be deficient in your soil. It is necessary to add magnesium and even elements such as copper in certain conditions.
The elements that are not essential just find their way into the plant—for better or for worse. Substances definitely harmful, such as sodium chlorate weed-killers, can also get in because plants have no mechanism for rejecting what is bad for them.
So, plants, like us, have to eat to live. It is the minute root hairs just behind the tip of the root and not the large roots that you see when you dig or pull a plant from the soil, that collectively form the `mouth’ of the plant. But of course, they do not eat, but drink their food which is obtained from the soil solution. This is like a thin soup of soil moisture and dissolved salts that surrounds soil particles and partly fills the spaces between them.
But plants require oxygen to release the energy needed to take in foods which are then used to build cell walls, proteins or storage materials such as starch, sugars and fats. So it is most important to see that soils are kept in a well-aerated condition so as to make this oxygen available; a waterlogged soil is one in which there is no air and the uptake of foods is prevented.
Of the 16 elements that are essential three, nitrogen, phosphorus and potassium are taken out of the soil in comparatively large amounts and supplies have to be replenished regularly for best results; these are the `big 3′ in your fertilizer needs. Calcium and magnesium are sometimes lacking and have to be added as lime and Epsom salts. Sulphur, boron, copper, iron,manganese, zinc, molybdenum and chlorine are needed only in very small amounts or traces—hence their name trace elements. They are like the tiny bearings in a watch—very small parts, but essential to its working.
You need to consider them only for special crops and soils. Each of these elements has a particular job to do. For example nitrogen is the leaf maker, phosphorus the root maker and potassium the flower and fruit maker No one element can replace the other; you cannot have normal plants when nitrogen is in short supply even though you have given plenty of phosphates or potash. In fact an excess of one element can easily cause symptoms of deficiency of another element to become apparent: for example, apples and tomatoes often show symptoms of magnesium deficiency when given too much potash.
Nearly all soils have large stores of plant foods, but most of these are held in the form of complex substances in the soil minerals and in the organic matter of the soil. They are released too slowly to sustain a rapid succession of crops or the yield and quality that you would really be proud of. Plant breeders are constantly raising higher yielding and bigger plants which need heavier feeding to do them justice.
So, we have to provide additional supplies of plant foods in the form of fertilizers but first one has to find out whether our plants are well fed, hungry or starving.
How much fertilizer, what kind, when to apply and where to apply are the questions we now have to answer in order to get the best results from our fertilizers. But this is no easy matter to solve; there is no indicator that you can push into the soil or fasten on to the plant that will tell you whether it is hungry or not. If only plants squealed like pigs when they are hungry it would be a much easier task for us.
The feeding of plants is still a matter of intelligent guesswork with the aid of a little science.
The plant reflects the level of nutrition in many ways. The type of growth, the color of the leaves and yield are obvious indications of soil productivity. Plants develop characteristic symptoms when they are poorly nourished. Many of these deficiency symptoms have been recognized and described for all the essential elements for many plants. Deficiencies are easy to see, but not so simple to identify since several may show the same symptoms. There are many other things that can cause poor plant growth; the weather has a great deal to do with it; if the weather is cold and wet plants will make little progress. Poor drainage can produce symptoms like those of nitrogen deficiency; wind scorch often looks like potash deficiency.
By the time symptoms are shown the plant is suffering from acute hunger and it is often too late to correct the trouble.
Prevention is always better than cure and you should always make sure that deficiencies are corrected before sowing by having your soil tested. Then fertilizers can be used to compensate for the missing nutrients (see Soil testing).
Sometimes, even though a plant may look well, it can still be suffering slightly from a lack of certain nutrients; this is known as ‘hidden hunger’ and it will respond greatly to a fertilizer dressing that contains the food of which it makes extensive use. For instance your cabbage may be doing nicely, but they would do much better if nitrogen were added.
Three things must be done should your plants be suffering from the lack of certain nutrients. First, find out what ails the plant, ie diagnose the deficiency. Second, try to find out how serious the deficiency is. Third, determine as nearly as you can, how much of the missing nutrient is needed to bring the plant back to health.
Chemical soil tests may help to pin point the trouble. Another way is to diagnose the trouble as closely as possible. then spray the foliage with a foliar feed (see Foliar feeding); if the plants show marked improvement within two weeks you are on the right track. Continue to apply fertilizer every two weeks until the plant resumes vigorous growth. It is possible to let the plants speak for themselves by carrying out a chemical test on the leaf; this is known as tissue testing—a valuable tool in the hands of one who understands soils, fertilizers and plant growth but one which can lead you to apply fertilizer that is not needed and will not help your crop. A more refined method known as leaf analysis is used for guiding fertilizer programs in fruit and other perennial plants.
Methods of feeding with fertilizers Before planting
Broadcasting If new beds or borders are being prepared fertilizers are best scattered evenly all over the ground after digging. Then mix with the top few inches of soil by raking, lightly forking or with a rotary cultivator.
This is best suited to the crops that you sow in shallow rows or plant closely.
Banding along the row For peas, beans and other vegetables that are sown in widely spaced rows the best place to put the fertilizer is in a band 2.5cm (1-2in) to the side and 2.5-5cm (1-2in) below the depth of the seed by hoeing out a trench. The roots of the young plant contact the fertilizer within two or three days after the seed germinates and get a big enough ration of food that will last annual plants for quite a long time. But never scatter fertilizers down the seed drill because they will injure most vegetable and flower seeds.
Starter solutions Dilute liquid feeds may be watered on to drills, seed trays of plants or potted plants before planting to give a rapid start.
Top dressing Putting fertilizer along the rows after the plants have started growing is often done to add some extra nitrogen. You are most likely to need to top dress under these conditions:
1 On sandy soils and thin chalky soils because leaching is common.
2 After heavy rains have washed out nitrates.
3 On soils that are low in organic matter.
4 Where you have not given enough nitrogen when you planted.
This is the best way of feeding perennial plants growing in borders, established lawns, fruit trees, roses and shrubs. The fertilizer should be spread as far as the branches reach to contact the actively absorbing root hairs.
To apply fertilizer in perennial borders, spread a small handful in a wide ring around each plant and mix carefully with soil. Either solid or liquid feeds are suitable for these dressings.
Liquid feeding Liquid feeds which are merely fertilizers in solution, are often more convenient to apply, particularly in crowded borders where it is difficult to spread a solid fertilizer without getting it on the foliage.
The foliage of many potted plants completely covers the surface of the pot and feeding with solid fertilizers can be hazardous. With liquid fertilizers feeding and watering is done in one operation—a distinct advantage when you have large batches of pots to look after.
Liquid feeding is essential if you intend to use drip irrigation in which the water issuing from nozzles gently trickles into the soil and spreads sideways underneath, leaving the surface dry.
Ring culture Ring culture in which plants are grown in bottomless containers or in ring pots filled with compost and stood on an aggregate layer is another variation in feeding technique. After planting, the rings and the aggregate layer, which consists of gravel or a sand-vermiculite mixture, are both watered until the roots penetrate the aggregate. Then the aggregate only is watered daily with a liquid feed to keep it thoroughly damp, so that the nutrients are taken up by capillary action.
Soilless cultivation It is quite easy to grow plants without using soil at all and excellent specimens can be raised in plastic troughs or plastic-lined wood filled with sand, fine gravel, mixtures of peat and vermiculite or several other materials.
Nutrient solutions made with special fertilizers are either applied on the surface or are pumped up from below through the aggregate which in this case must be in waterproof troughs. This is known as soilless culture (hydroponics).
Feeding through the leaf It is not only roots which can absorb plant foods but the foliage also can absorb nutrients very quickly thus short circuiting the long journeys from the root. Although it is unlikely that this method will supersede normal soil applications, it has advantages (see Foliar feeding).
Obviously the above comments are very general; soil conditions and the needs of the individual plants will determine which and how much fertilizer to use (see also Fertilizers).
It must be stressed, however, that fertilizers are not a cure-all for all garden troubles—they merely supplement the soil’s store of nutrients. And not only must the plant be well fed, the roots must have a good home to live in, otherwise fertilizers can never produce their best results.
For most species there is a fairly definite relation between root and top growth and so it follows that a well developed, healthy root system is essential for the production of a vigorous plant. The soil condition needed for normal root development can be described as good depth and friability. Such an ideal soil provides a well-distributed system of water reservoirs, a good ventilation system and plenty of large spaces into which roots can grow and develop freely. Such a condition is brought about by draining waterlogged soils, adding humus-forming manures, avoiding cultivating soils when they are too wet and watering during dry periods.