What is a ‘Good’ Soil?
A good soil is one which has a high water holding capacity, but drains freely leaving air space.
Water and nutrients in such a soil, will be easily available to plants.
A good soil will be slightly acid (pH 6 – 6.8) at which level the nutrients required by plants are most freely available.
A good soil will have all the nutrients required for growth in the correct balance
Nature delivers us soils with varying physical characteristics – clay, gravel or silts, volcanic loams or ash, sand, and peat.
- Clay soils – have very small particles which pack tightly together limiting drainage and air space. The water and nutrients are held tightly making it difficult for plants to take up.
On the positive side clay soils hold moisture well and are usually high in nutrients except perhaps nitrogen.
- Silt soils – are usually alluvial flood plains or similar, formed by repeated layers of parent rock which has been broken down by nature. The water and nutrients are usually reasonably freely available, but occasionally the silt is of a clay nature. Silt soils are usually high in nutrients.
- Volcanic loams – formed from ash showers of varying ages which determines soil type, but they are generally free draining, have a good water holding capacity, and are high in nutrients.
- Sandy soils – usually in some coastal areas are the opposite to clay – large particles, very free draining, very low water holding capacity, and low in nutrients because of the leaching effect of very free drainage.
- Peat soils – are in many ways a perfect soil, as they are both free draining and have a high water holding capacity. The water and nutrients are easily available to plants. However peat soils are very acid, requiring major adjustment, and low in nutrients compared to other soils.
Improving the physical characteristics of your soil
Fortunately all soils except peat, can best be improved for plant growth by adding organic matter – humus, compost, green-crops, straw or similar materials. These materials will improve drainage, water holding capacity, and the easy availability of water and nutrients. Large quantities must be added to make a difference – 5 to 10 cm thick and worked into the natural soil. At least 5 cm should be added each year until the soil reaches the standard of the introductory sentence above.
In clay soils which have poor drainage, it will also be necessary to raise the garden or planting area, above the surrounding soil to ensure drainage in the root zone of your garden plants.
Lime and its Functions
Lime including, dolomite is used for a number of modification purposes.
1. To assist in conditioning of clay soils. Adding lime to clay soils causes a flocculation of the very small particles into larger particles which in turn creates larger air spaces and improved drainage. This improvement however is small and lime is not an alternative to addition of organic matter.
2. To reduce acidity. Lime is added where soils are acid (peat soils) or where large amounts of compost are added to improve soils. Organic matter such as humus, compost, green crops etc. all create acidity which needs to be countered by adding lime. Sometimes lime is added in the composting proceeds which reduces the need somewhat.
3. To supply the element calcium. Lime contains the element calcium which is required for plant growth especially for flowering and fruiting function. As calcium is a minor element the required amount is usually available naturally in most soils. Added lime will be necessary in peat soils possibly in sandy soils. It is also added into potting mixes. If potting mixes are reused it will be necessary to add lime each year.
4. To improve the availability of phosphate. Phosphorus is an important element required for plant growth. It is taken up by the plant as Phosphorus pentoxide (P2O5) Phosphate in the various forms of fertiliser is released into the soil where it often combines with other elements which ‘lock’ it up. By maintaining a good level of calcium in the soil, the ‘free’ phosphate will more likely combine with calcium in which form it is still available to plants, thus ensuring the phosphate fertilisers added are used not wasted.
Forms of lime commonly available are-
1. Garden Lime – calcium carbonate (CaCO3)
2. Dolomite Lime – a mix of calcium carbonate (CaCO3) and magnesium carbonate (MgCO3)
3. Gypsum - calcium sulphate (CaSO4)
Although dolomite is a mix of both calcium and magnesium carbonate, it is usually safer to use a mix of Dolomite and Garden Lime in order to maintain a balance between calcium, magnesium and potassium.
When these three are out of balance nutrition problems occur.
Gypsum performs three of the four functions of lime. It conditions clay soils, supplies calcium and facilitates phosphate availability. It does not reduce acidity as it is neutral. Gypsum has a place as a soil conditioner, and for supplying calcium to acid loving plants and lawns.
Nutrients required for plant growth
The elements required for plant growth fall into three groups.
Major elements – required in significant quantities – nitrogen (N) phosphorus (P) potassium (K)
Minor elements – calcium (Ca) magnesium (Mg), sulphur (S), iron (Fe)
Trace elements – manganese (Mn), zinc (Zn) boron (B), molybdenum (Mo), copper (Cu), Aluminium (Al), Selenium (Se).
Whilst all these elements are usually present in most soils, several are often present in insufficient amounts to enable a satisfactory yield or performance.
Most modern compound / mixed fertilisers contain levels of most or all of the above in a balance that suits growth of most plants. Hence these compound fertilisers are the preferred nutrient source. There may be a few plants and a few situations or growth stages where extra amounts of a particular element may need to be supplemented, eg. – extra potassium to initiate flowering or fruiting, or to provide disease resistance; chelated iron to correct deficiencies in certain plants; chelated aluminium sulphate to ensure blue hydrangeas.
Which Fertiliser to Use?
General Compound Fertilisers
These are fertilisers that contain nitrogen, phosphorus and potassium - the three major elements required for plant growth. In addition many of them contain minor and trace elements.
There are five types of compound fertiliser –
1. General basic granular based on a mix of blood & bone, sulphate of ammonia, super phosphate, and potassium sulphate. These fertilisers are moderately fast acting and will provide nutrients for 3 – 4 months.
2. Hi tech, high concentration mineral fertilisers with nitrogen inhibitors to slow the release and minimise leaching. This fertiliser provides nutrients moderately fast and over a period of 6 months with generally the best results
3. Organic & organic based – these are based on either chicken manure or sheep manure which have been processed and pelletised. In some cases additional mineral fertilisers are added.
These animal manure based products are lower in nutrient, NPK varies around 3/2.5/1, but ican Organic Vegetable Food is the best organic option with NPK at 5/2.5/2.5.
Take care not to over use organic products in pots, as these products go through a breakdown process to convert to the elemental form which plants require, and over use can lead to a build- up of toxic ammonia.
4. Controlled Release fertilisers are compressed or resin coated to slow the release rate of the nutrients. The release rate generally varies from 3 to 12 months, although there are one or two that release over 2 years. These products are useful when planting long term plants in large pots or planting trees and shrubs in the garden. Used at planting time in the soil around the roots they ensure plants get a good start.
5. Soluble and Liquid Fertilisers are excellent for providing food quickly for sick or unthrifty plants, or to give a boost to fruit, vegetables and roses. The two types available are mineral or organic.
The mineral soluble type have a much higher analysis typically NPK 25:5:10 for vegetables and 15:5:25 for fruit and flowers.
The liquid mineral type are typically NPK 10:2:8 but do vary.
The organic type have an NPK typically NPK 4/1.5/2
Organic fertilisers have a lower level of nutrient and should be applied at a higher rate, which means they may cost a little more, however there is a beneficial effect on soil through promoting microbial activity, and it makes sense to recycle animal or fish waste, rather than dump it into landfill.
All the above products should be applied to the soil as major elements cannot be absorbed by foliage in sufficient quantities to make a difference.
Seaweed is not a fertiliser but best described as a tonic. It contains very little of the major elements – NPK.
(One exception is Grosafe Bio Power which is made from a distinctive seaweed which is high in potassium)
Seaweed products contain trace elements and the plant hormones – auxins, gibberellins and cytokinins.
Unlike the fertilisers above, seaweed is effective applied to foliage where the plant hormones are readily absorbed. Additional uses for seaweed are –
• soaking the roots of new plants in a seaweed solution prior to planting
• standing cuttings in a seaweed solution for 24 hours prior to applying a rooting hormone and setting them into a propagation mix
• adding to fungicide sprays to enhance performance.
• adding to a liquid fertiliser to and applying to newly planted seedlings
Garden information supplied by Bill Brett, horticulturist, Garden Retail Success Ltd