Lesson 2 culture of native plants

Infiltration and Water retention

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Infiltration and Water retention 

Infiltration refers to process of water entry into the soil and is influenced by: 

  Soil type and soil texture.  Sandy soils generally have higher long term water penetration rates than 

clay soils. 

  The condition of the surface soil.  Water will enter faster if the soil surface is friable and open or is 

extensively and deeply cracked.  Compacted or crusted soil with few cracks reduces infiltration. 

  The stability of the surface soil.  Low water stability means that the soil crumbs do not stay together 

when wetted.  Low water stability results in slow water penetration unless the soil is sandy.  Also, it 

often results in the formation of a surface crust as the soil dries which will reduce infiltration at the 

next irrigation. 

  Depth of soil above an impermeable layer.  For example, if a soil consists of light loamy topsoil over a 

clayey subsoil or parent material, any water up over the impermeable layer reduces water 



Infiltration and retention of water into the soil can be improved by adding organic matter – it stabilises and 

strengthens aggregates improving soil structure, increases cohesion of sandy soils and decreases 

cohesion of clays. In both cases it creates a better range of pore sizes (tilth) for optimum root growth. 

Organic matter also decreases adhesion in most soils. 


Suitable organic materials include: 









  Aged sawdust  


When to water 

Knowing when to water is extremely important for healthy plant growth. Over and under watering can be 

equally detrimental.  


Under-watered plants will exhibit the following symptoms: 

  Leaves wilt - especially new growth. 

  Leaves turn yellow. 

  Leaves burn and sometimes drop off the plant. 

  Stunted growth, poor flower and fruit set – typical symptom of long-tem water deprivation.   


Over watered plants will appear: 

  Leggy, brittle stemmed and have lush, or even rank, new growth.   

  Flowering will be reduced at the expense of leaf and stem growth.  

  General plant health will be reduced as soil nutrients are leached from the soil  

  Disease problems will increase, particularly root and leaf fungus.  


Observing stress symptoms should be a last resort in deciding when to water. 

A simple test that involves simply feeling a sample of soil can indicate soil moisture levels. The sample 

should be taken from the root zone of the plant: 


Degree of moisture 

Feel of soil 

Amount of moisture 

Dry Powdery 



Crumbly, does not stick together 

25% or less 


Crumbly, but does hold together 



Will form a ball with some  pressure 



Pliable ball which sticks together readily; 

some clear water can be squeezed from it 


Too wet 

Sticky ball which water can easily be 

squeezed from. 



Period of watering 

  In sandy soils you can apply a lot of water quickly and it will be absorbed. 

  In heavy clay soils you must water slowly over a long period (Heavy applications will not soak in, and 

a lot will be lost as run off). 

  Deep rooted plants such as trees should be watered slowly over a long period, so as to wet the soil to 

a great depth. 

  Deep rooted plants can be watered less often. 

  Shallow rooted plants such as annual flowers and vegetables need frequent watering, but of a shorter 

duration at each watering. 



  Wise usage of our water resources is very important. How often you water will depend on a 

variety of things including the types of plants you are growing; local climatic conditions such as 

the amount, frequency and timing of rainfall, winds, sunshine, etc.; and soil conditions. The 

following points should be considered when deciding when and how to water your natives. 

  Minimise your water requirements by selecting plants to grow that require little or no watering. 

  A good deep watering is more effective than a lot of shallow watering. It encourages roots to 

travel deeper into the soil, and reduces the development of extensive surface roots that require 

frequent watering. This is very important when establishing young plants. 

  Water only when you need to. If there is moisture in the soil within 2- 3cm of the surface (poke 

your finger into the soil to check) then there is generally no need to water. If new growth on your 

plants appears healthy and there is no evidence of wilting, burning, etc. during the warmer times 

of the day, this also indicates that there is adequate moisture present in the soil. 

  Try and group plants with similar watering requirements together. This helps reduce the 

likelihood of under watering or over watering particular plants. 

  Avoid watering plants from areas of low rainfall during hot weather. Wait till conditions cool 

down. In particular avoid watering the foliage of these plants as high humidity levels can help the 

spread of disease. 





Problem Solution 

Poor drainage 

Lay drainage pipes 

Plant in raised garden beds 

Treat clay soil with soil conditioners such as organic matter, 

gypsum, or lime. 

pH too low 

Add lime or dolomite 

pH too high 

Add powdered sulphur 

Dig in manure or compost 

Moss or algae growing on pots 

Re-pot plants into better draining soil 

Don't fertilise so often 

Reduce watering 

Sprinkle a layer of coarse sand on the surface of the soil. 

Soil getting too hot or cold 



Soil drying out too quickly 

Mulch surface 

Water more regularly (eg. trickle systems) 

Salt problems in pot plants 

(white cake on surface of the 


Leach out by ensuring good drainage and heavy watering. 




Creating raised beds 

Raised garden beds will enable many plants to be grown in areas where they would normally be difficult 

to grow.  Beds which are raised to a height of around 0.5 m or more will have a significant effect on 

drainage. Raised beds and mounds will also add interest to an otherwise flat garden. 

There are two common ways of building raised beds: 


1. Shaping the earth 

Raised mounds are created by moving soil (preferably with a machine) from other parts of the property 

and pushed into mounds. Mounded areas created this way are preferred because it tends to have less 

impact on the soil structure. 

2. Building a wall  

(Perhaps with railway sleepers or rocks), and filling in the area enclosed by the wall with imported soil (be 

sure to allow for drainage holes in the bottom of the wall). 


Installing drains 

Types of drains 

An isolated patch of dampness or soggy soil which will suffocate plant roots can probably be corrected by 

making a simple soak way.  Dig a hole about 1 metre wide and 1.2 metres deep in the damp spot - or just 

below it.  Fill with coarse rubble (ie. rock, bricks, broken tiles etc. anything which will drain well) and 

replace the former topsoil.  Drain holes such as this should be kept well away from buildings - they can 

damage foundations! 


Another method of drainage is sand slitting.  This involves digging a narrow trench through the area to be 

drained to a point at which water can be disposed of (eg. to a soak pit such as above - or to the storm-

water drainage system).  There should be a drop along the length of the drain (as with any drainage line).  

Water needs a fall of at least 2.5cm (1 inch) every 6m (20 feet) in length if it is to fall.  Once the trench 

has been dug it is then filled with coarse sand.  A thin layer of topsoil can be placed on top of the sand.  

The trench should usually be about 30-60cm (1-2ft) deep. 


The most permanent type of drain consists of clay agricultural drainage pipes; or PVC drainage pipe laid 

underground with an outlet into a storm water drain, large soak way pit or sump pit.  Trenches should be 

dug through the topsoil layer into the harder subsoil layer (often clay).  There should be a reasonable 

gradient in the trench sloping towards the outlet. This should be at least 1 in 100, or in other words for 

every metre of pipe there should be a fall of 1 centimetre, so for a six metre length of pipe there would 

need to be a minimum fall of 6 centimetres. Trenches may penetrate the hard subsoil layers in places to 

achieve the required depth and slope.  Trenches should be deep enough to allow a 10 cm cover over the 

pipes at least - they can be much deeper if you wish! 


Once the trench is dug, lay a very thin layer of porous aggregate (eg. 1-2cm stones) in the bottom. This is 

to keep the pipes of the subsoil so that slot/holes that allow water entry into the pipe are not blocked by 

loose soil. Pipes can then be laid. These should then be covered to a depth at least equal to their 

diameter with coarse aggregate (ie. half or three quarter inch screenings). A layer of newspaper or 

cardboard can be placed on top of the screenings and top soil or sand placed on top of this layer. This 

will allow good water penetration, and the layer of newspaper/cardboard will prevent silt being washed 

into and blocking the pipe. The drain may be alternatively covered with coarse mulch (for aesthetic 



Limestone under-lay technique  

This method was developed after observations showed that many difficult to cultivate plants occurred 

naturally in soils which have calcium or lime rich layers below the surface.  


The method involves laying about a 15cm thickness of crushed limestone or limestone chips below about 

30cm of topsoil.  

It is particularly useful for growing Banksias, Dryandras and other plants from Western Australia which 

have proven difficult to cultivate outside that state. 


Experiments at the Australian National Botanic Gardens (Canberra) have also shown that the technique 

may be useful in overcoming problems caused by Phytophthora cinnamommi, as the presence of 

calcium is thought to inhibit the growth of the fungus.  At this stage the experiments have been 

inconclusive, but the indication is that it can be very helpful in growing such things as members of the 

Proteaceae family, the Darwinias and Verticordias. 




Compost is indeed the powerhouse in any growing system, but is particularly valuable in organic 

growing where chemical fertilisers are not used. Its benefits cannot be underestimated.  


  It improves soil structure in all types of soil.  

  It provides slow release nutrients for plants to use when required. 

  It increases the level of soil micro organisms beyond measure. 

  Composted soils produce plants more resistant to pest and disease attack 

  Compost making is a environmentally sustainable method of recycling ‘waste’ material,  

  Garden produced compost is effectively cost – free. 


Compost bins 

There is a wide range of compost containers now available from garden centres, local councils, hardware 

shops and via direct mail. They are usually made of plastic (often recycled), or wood.  These are useful 

for composting small amounts of waste and for making compost in small gardens where you don’t want 

to look out at an open heap of rotting waste.  Fixed bins can be difficult to aerate, and are prone to 

becoming too dry or too wet. Rotating bins are more expensive but provide quicker, more reliable results.  


It is equally possible to produce compost successfully in a heap without any container. Systems where 

large quantities of compost are made will often use this method. Always have the heap covered to 

prevent the material becoming either too dry or too wet. 


Make sure that, wherever you have your compost area, it is accessible for all the equipment you’re likely 

to be using. If you propose to turn the material using machinery, allow enough room to do so. 


What can be composted? 

Any organic material, if left long enough, will eventually rot down due to the action of micro-organisms.  

Composting is simply a way of harnessing and maximising this process. It speeds up the rate of 

decomposition, and minimizes nutrient losses.  


The raw material for successful compost making is a mixture of organic materials, such as: 

  lawn clippings  

















Ideally, the mixture should contain around 25 times woodier, carbon-rich material, than moist, nitrogen-

rich material (grass clippings, kitchen scraps, green plants).  This gives the best C/N ratio and results in 

effective composting. 


What is the C/N ratio? 

The micro-organisms that break down plant materials require food in the form of nitrogen, phosphorous 

and potassium. The most important requirement is the ratio of the percent carbon (C) in the materials, 

to the percent Nitrogen (N). This is called the carbon/nitrogen ratio.  

Woody waste has 25 times as much carbon as it has nitrogen, so its C/N ratio is simply expressed as 

the number 25.  A C/N ratio of around 30 is required for compost activity to take place at an optimum 

rate. To get a suitable C/N ratio it is necessary to mix materials with a high C/N ratio, such as wood 

shavings, with materials that have a low C/N ratio, such as green plant waste. 



Materials to avoid in a compost heap: 

  Protein-rich kitchen waste, such as meat or fish. It can attract vermin, and become putrid in hot 


  Roots of perennial weeds (unless the compost will heat up sufficiently – see below) 

  Seed heads of annual weeds 

  Too much of any one material in one layer. A large quantity of grass clippings should be added in 

layers not more than 4 – 6 cm deep.  

  Layers of evergreen pruning material. These need to be managed differently (see below) 

  Diseased plant material – particularly those diseases that are soil-borne, such as club-root 

(brassicas), white rot (onions), potato cyst eelworm.  

  Material that has been sprayed with herbicides (such as lawn clippings). 

  Thorny or spiky plant material. The thorns will not decompose sufficiently and can cause injury once 

the compost is spread on the soil. 


Animal manures are a good addition to a compost heap. The most commonly used are sheep, cattle, 

poultry, horse and pig. Animal manures should be composted for a minimum of six weeks to prevent 

problems such as burning of leaves and roots from the presence of high levels of ammonium ions in the 

fresh manure. The ammonium ions are rapidly lost during composting. Large quantities of manure are 

best covered and composted separately. 

If manure from an inorganic source is brought onto an organic system, check with your certification body 

to verify how long it must be composted before it can be used.  


The basic conditions needed in a compost heap:  

  A good mixture of materials, as described above. Too much dry material will slow the process down, 

and too wet a mix will become smelly and slimy.  

  Moisture - take a handful of the material from about 15 or 20cm deep in the heap, and squeeze it. It 

should be about as moist as a moderately squeezed wet sponge. If it is too dry add water, or plenty of 

fresh green waste (grass clippings are ideal). If it is too wet, drag out the material and mix with plenty 

of carbon-rich material. Junk mail and cardboard works well for this purpose. Put everything back into 

the container once it has been mixed. If mixing is not possible, use a spade to make slits or holes in 

the wet pile and push dry material into these gaps.  

  Oxygen - this is incorporated by turning the mix occasionally. Decomposing micro-organisms require 

oxygen to survive. If turning the heap is not possible, then make sure that there is a good mixture of 

fine and coarse materials. This will create air spaces. 

  Warmth – keep the heap covered at all times. The sides should not be slatted. Wind blowing through 

slats will dry out a heap. Rain sluicing through will cool materials down and wash nutrients away. In 

hot countries, composting is often done in pits or trenches in the soil, to prevent material from drying 


  Temperature - if the temperature drops below 40C the rate of decomposition decreases, if it goes 

over 60C many of the micro-organisms causing decomposition will die.  Temperature conditions will 

always vary from one part of a compost heap to another.  Usually the centre of the heap is the 

warmest and, for this reason, decomposition is usually faster in the centre of the heap.  So it is 

advisable to mix up the contents of a heap from time to time. 


Hot heaps versus cold heaps 

Much has been written about ‘the hot heap’. This is where a large quantity of compostable materials is 

piled together – a minimum of one cubic metre is the recommendation – and mixed well. Organisms in 

the materials become very volatile and active, which creates heat. Temperatures in a hot heap can soar 

to 80C and above. After a few days, the organisms start to die off, and the heap starts to cool. At this 

point, the heap is turned in order to incorporate more oxygen, and mix un-composted material from the 

sides, to the middle. The temperature will rise again. This turning process can be done several times, 

resulting in finished compost within a few weeks, even in winter. However, this is a time consuming 

process, and not always possible or practical. 

Many compost heaps remain cool, as their volume is small, and material is added ‘little and often’.  

If this is the case, compost will take much longer to mature – usually around 12 months. It is most 

important that this type of compost heap should not be overloaded with large quantities of one type of 

material, such as a huge pile of hedge clippings, or a thick layer of grass clippings. Always make sure 

that the balance of wet to dry material is correct. Remember, weed seeds in a cold heap will not be killed. 

Although a cold heap will take much longer to produce finished compost, both methods will produce an 

excellent product, rich in fertility for your farm or garden. 


The benefits of a hot heap are: 

  Compost produced quickly 

  Weed seeds killed in the heat 


The benefits of a cold heap are: 

  No time spent turning the heap 


How to build a compost heap 

  The easiest way to build a compost heap is simply to pile materials in a heap, or in a container of 

some sort.   

  Use a good mix of organic materials. Wet material should be in thin layers (no more than 3cm thick) 

covered by dry organic material, such as dry straw or shredded paper.   

  If using a large amount of dry material such as straw, wood shavings or paper, add some manure 

to boost the levels of nitrogen in the composting material. To prevent sawdust from packing down in 

a solid layer, mix well with coarse material, such as chopped stalks, and plenty of green waste to 

add nitrogen. 

  If possible, turn the heap with a garden fork weekly. Remember to keep the heap covered, 

especially in wet weather. In hot countries, or long spells of dry weather, it may be necessary to 

water the heap occasionally. 

  In warm conditions, a heap that is regularly turned and aerated can be ready in around six weeks

in colder weather it can take several months.  


A compost heap should be made on bare soil. If piled onto a solid base, such as concrete, liquid will 

soon start to seep from the waste material. This can cause contamination of drains and water courses. 

If you intend to produce large quantities of compost, make sure that you comply with your local 

environmental regulations covering management of leachate.  


The Finished product 

Compost is ready to use when: 

  It is crumbly and generally an even texture. (Material such as straw, or flower stems might be still 


  It should drain well, but still have good moisture holding capacity. 

  It should be dark in colour. 

  It should smell earthy and sweet, not of rotten eggs. 

  Temperature should be air temperature. All heating/cooling should have finished if the hot heap 

method has been used. 


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