The soil is every garden’s most valuable asset. Not only is it a medium in which to anchor the roots of plants but it is the provider of an essential duo – that of food and water.
Soil can hold its moisture and the nutrients it accumulates in two ways: in its organic content, which acts like a sponge, holding as much as five times its own weight in water, and as a thin film on each of its mineral particles.
No local gardener needs to be reminded that unamended clay ground is difficult to cultivate. Particles of clay are extremely small and adhere tightly to each other so that efficient water drainage along with air penetration is hard to achieve. Clay becomes sticky when wet, dries out to a brick-like consistency, is slow to warm up in spring, and is generally described as heavy.
Difficult as they may be initially, because clay soils hold on to their nutrients in the same way they hold on to water, garden beds which have been derived from clay can become some of the richest and most productive of all.
Chemical breakers such as Agrosol, gypsum, lime or dolomite can reduce some of the heavy labor of spadework alone by flocculating or lumping together some of the finer particles of clay into crumbs of a larger structure.
The addition of coarse sand and generous helpings of organic matter are the next steps that will help turn the previously intractable ground into a fertile and friable loam. Organic matter, whether it is in the form of compost, animal manure or a green manure crop grown specifically to turn into the ground as built, is an essential component of the ideal soil.
The value of a well-made compost heap formulated from plant waste, lawn clippings, leafy fall and manure, all sandwiched between thin layers of soil, can never be overestimated. Not only will it add to the soil structure but also its chemistry – returning to the soil what most ornamental plants have taken out.
Composted plant and animal remains can be considered the original slow-release fertilizer. The nutrients that have been contained in living plant material are gradually broken down and recycled by microorganisms and earthworm activity in the soil.
The more or less stable material that remains after the bulk of the organic matter is decomposed is known as humus. Regular applications of compost turned into the vacant ground in both spring and autumn or used as mulches will help to retain the desired level of humus and keep the ground in good heart.
Inorganic fertilizers as opposed to compost and manures add nothing to the soil structure but provide a useful source of either quick-to-act or long-term nutrients as and when required. Balanced fertilizers such as Multigro contain a mixture of the three essential elements needed for growth, the proportions of which are stated on the pack as NPK – nitrogen, phosphate, and potassium.
Selected proprietary mixtures with either more or less of one of the three elements are readily available for specific plant needs and whether they are grown for their foliage, the fruit or the flowers.
Nitrogen provides the stimulus for new shoot growth and healthy green leaves. It is an essential part of plant proteins and, in its early stage, a plant needs a plentiful supply. Adequate nitrogen is a must for leafy vegetables like lettuce and spinach and for quick green growth of lawns.
The effects of too little nitrogen are usually quite notable. Plants are invariably stunted and seedlings, in particular, develop pale green or yellow leaves with narrow proportions. On the other hand, the effects of too much nitrogen become evident in soft, sappy, often over-large leaf growth at the expense of fruit and flowers.
As nitrogen is quickly leached from the soil by rainfall or irrigation it needs to be added to fast-growing crops on a regular basis. A combination of longer-lasting but slower-acting composts and manures, together with a high nitrogen liquid plant food, should take care of a leafy crop’s nitrogen requirements throughout the growing season.
Phosphorus is needed by plants in all the crucial processes of growth but in particular to foster root development and to ripen seeds or fruits. Soils that are deficient in phosphorus often give rise to plants with purplish (rather than normally green) leaves and produce reduced yields of seeds and fruit as well as stunted growth.
Though phosphate is available in varying quantities in animal manures, the main inorganic source is from superphosphate. Unlike nitrogen, phosphorus is not readily leached from the soil. Australian native plants that have learned to adapt to normally phosphorus-deficient soils are invariably intolerant of the phosphorus in proprietary fertilizers and will quickly show symptoms of toxicity.
Of the three major elements required by plants, potassium is less frequently deficient in the new ground than cither nitrogen or phosphorus. It is more likely to be lacking in sandy soils than it is in heavy clay loam. Like phosphorus, potassium is not easily leached but if supplies depleted by plant growth are not replaced the lack of it will be displayed in reduced yields of fruit and flowers, spotted or curling older leaf growth, burnt leaf margins, weak stems and an increased susceptibility to frost damage on normally hardy plants.
Potassium can be supplied via a balanced fertilizer or separately as either potassium chloride or sulfate of potash. Wood ashes that have not been exposed to the weather and thus had the nutrients washed away contain a variable but useful form of potassium and so do banana skins, which can be added to the soil mix of potted plants.
The advent of slow-release granulated fertilizer has helped to take the guesswork out of recalling just what plant was last given a feed, particularly for gardeners who grow a variety of potted plants.