Healthy plants are much less likely to be affected by pests and diseases than ones that are not thriving.
Gardeners often assume that poor growth is related to insufficient soil nutrients, and apply fertilisers. In fact, results of soil analyses carried out by the RHS Soil Analysis service find that shortage of phosphorus and potassium are seldom encountered. As fertiliser is relatively cheap, gardeners usually add plenty of nutrients to their soil.
Soil pH or acidity is also usually satisfactory, but should be checked before considering other potential causes of poor plant growth and health.
This suggests that plants fail to thrive because of other factors. In investigating members’ problems the Advisory Service finds evidence that in many cases, poor soil conditions, lack of moisture and insufficient nitrogen limit the plant’s ability to use the adequate levels of fertility present in the soil.
Nitrogen supply
Nitrogen promotes soft, leafy growth and its availability primarily determines the amount of growth made by plants, given suitable soil and weather conditions. Nitrogen is easily lost from the soil. Levels are low in March and April after winter rains have washed - or leached - soluble nitrogen compounds out of the soil, but usually there is plentiful soil nitrogen materials in September as availability of organic nitrogen increases with rising soil temperatures and associated increasing bacterial activity.
It is important to remember that while the use of nitrogen dressings (sulphate of ammonia for example) alone can substantially increase the yield of leafy food and forage crops, most plants need to make balanced, and therefore healthy, growth. For this they also need adequate phosphorus for healthy root growth and potassium to help promote flower and fruit development. Compound fertilisers such as Growmore and blood, fish and bone are balanced, in contrast to single nutrient fertilises, such as superphosphate (phosphorus) and sulphate of potash (potassium).
Potassium, in contrast, promotes harder’ more disease-resistant growth. However, merely adding more potassium-rich fertiliser such as sulphate of potash, is not a remedy as plants can only use a certain amount, and also an excess of potassium interferes with magnesium uptake causing magnesium deficiency.
A crop will remove potassium continuously during its growth whereas for nitrogen it is important to provide enough during periods of rapid growth and it is wasted at other times of the year.
Soil structure
For plants to access soil nutrients their roots must be able to explore the soil, as relatively little nutrient flows through the soil in the soil moisture. Root spread depends on sufficient air in the soil, which in turn requires adequate channels and spaces that allow water to drain away and air to flow into the soil. Poor soil, especially that which is compacted say after building activities for example, is not readily penetrable by roots.
Water supply
Where roots cannot penetrate the soil moisture supply is also likely to be inadequate. Without water plants cannot grow to their full potential and remain vulnerable to pests and diseases.
Each factor (nitrogen, water and soil structure) can limit plant growth and ameliorating one without dealing with other limiting factors, is unlikely to be effective.
Organic matter
Fortunately, organic matter, such as garden compost, manure or composted bark, can alleviate all these areas. Mixed into the soil it improves structure, increase water-holding capacity of the soil and slowly releases nitrogen to feed plants. For permanent plants - such as trees, shrubs and fruit - organic matter laid on the surface as a mulch is highly effective. This also promotes high populations of soil organisms such as earthworms that tunnel into the soil and carry the surface organic matter downwards. Worms increase the permeability of the soil to water and air, and improve the soil structure. A soil devoid of worm activity is likely to be poorly drained and inclined to ‘slump’.
Preparation before planting
Cultivation before planting also improves soil structure, especially if the improvement is stabilised with bulky organic matter. A bucketful every square metre is a useful rule-of-thumb on how much organic matter to use.
However, in most soils organic matter alone is unlikely to produce optimum growth of demanding plants such as fruit, vegetables, bedding plants, plums and blackcurrants. Also obtaining sufficient organic matter can be expensive, time consuming and impractical for many gardeners. Here fertilisers are likely to be very helpful, ideally as a supplement rather than a replacement for organic matter, raked around plants in spring. In the longer term building up organic matter levels, by frequent manuring and annual mulching will greatly reduce the need for fertilisers. Many mature trees, shrubs, climbers and perennial ornamental plants have such a limited requirement for nutrients that they often get by on bulky organic matter alone.
Also where root run is restricted, by growing in pots for example, extra fertiliser will be needed.
Other properties
Organic matter, such as compost, has other amazing properties in helping to promote healthy plants. The increased humus in the soil increases microbial activity reducing the activity of plant pathogens (they are busy fighting each other; the manure supplies their food), and reduces soil ‘tiredness’. This can sometimes reduce the incidence of root disease. Although variable in effect, applying organic matter should be a gardener’s first line of defence.
Guy Barter