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Archive for November, 2010

Watering Part 2

Just thought I’d let you know that the evaporation this last two days in Perth has been over 11 mm.  Not your average 8-10 mm. So if things are looking a bit stressed you know why!

On that note I thought I’d better quickly convey a couple of concepts.

Commercial growers water for maximum production and growth.  Their aim is to never stress their plants or make them work to get water – or food. Except in certain special circumstances.  As home gardeners, under the current imposed watering regimes we are being asked to water for survival at best.  So unless we can get additional water to our plants somehow, they are going to look yukky.  Of course many WA natives do go dormant and look pretty awful in summer anyway – its only us who try to entice them into growing when they normally wouldn’t.

So between the commercial type regime and our currently imposed survival regime there is a continuum of watering.  And a continuum of what your plants might look like over summer.  Just something to bear in mind.  If you have fruit trees, though, or a vege patch, you may want something a bit better than survival.  At the moment my citrus have fruit on, they are sizing up.  If my trees get stressed they will drop their fruit in response.  So they get extra.  Citrus are surface rooting and so they stress easily because the roots are in the top 15-30 cm of soil that has the most fluctuation in water content.  A reasonable size citrus tree can easily require 30 L water a day in summer.

Just one more concept for today.  The wetted zone.  No, not that one :-).  Get your minds out of the gutter!

If you are using drip irrigation – or even subsurface (more on the problems of subsurface in another blog) , you need to think about how much of the root zone of your plant is wetted.  Since water doesn’t move laterally in sand, the chances are that if you have, say 2-3 drippers around a decent sized plant, you may be only watering a fraction of the root zone.  Maybe half or a third.  Now that is sort of OK.  Plants adapt and most of the roots will head for that wetted area.  BUT it doesn’t alter the water requirements of your plant.  So if your canopy area is 4 square metres the water requirement for that whole 4 square metres has to be delivered to the area that the drippers are wetting.

In the case of my hypothetical lemon tree, that mean my 30 L of water has to be delivered through the 3 emitters around it.  That means 10 L per emitter.  And I’d prefer to see that in at least 2 doses of 5 L each – morning and evening.

If your emitters are drippers that give out 4 L an hour then you are going to have to have them on for over an hour twice a day to put out that amount of water since each dripper will need to put out 5 L of water.

Needless to say drippers aren’t the best way of watering citrus – low level (under tree) sprinklers are much better.  They wet the whole area and they put out much more water so they don’t need to be on as long.

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Watering – Part 1

Since summer is upon us it’s probably a good time to talk about watering.

Watering is one of the hardest things to get your head around.  And on un-amended sand its hard to get right without constantly either under or overdoing it.  There are so many variables to think about.

I’m going to start with un-amended sand which is the worst case scenario.  Then we can talk about what happens when you add clays and/or compost.

The first thing to forget is that phrase “infrequent, long deep watering”.  If you are growing in sand, it’s the worst thing you can do.  The best way to water sand is little and often.  If you have a reasonable amount of clay and compost in your sand then you might get away with longer deeper waterings but only if your amended sand is a lot more like loam than sand.

I’ll try and introduce concepts bit by bit.

The first is mm of irrigation.  We horticulturalists talk in mm.  One mm of irrigation is equal to one litre of water poured over one square metre of soil.

The second is water holding capacity (of soil).  This is expressed in %.  The water holding capacity of Perth sands is only 8%.  This means that in a cubic metre of sand ie one metre square by one metre deep, only 8 litres of water can be held. That isn’t very much.  And just to complicate things, plants can’t access all of that water, they can only use about half of it – depending on what sort of plant they are.  Some drought tolerant plants can extract water from soil more efficiently.  Other plants aren’t very good at extracting water and will wilt well before some of the hardier plants.  You may not know that getting water out of soil requires energy on the part of the plant.  The less water in the soil, the more energy it takes to extract that water and that hampers plant growth – plants don’t grow as well.  There are lots of other complicating factors too, however – for example plants can only take up nutrients in solution so when the soil is dry they can’t feed themselves so the problem is compounded.

The other problem with Perth sands is that they are made up of coarse particles with big air gaps in between – we call this low hydraulic conductivity.  For plants to take up water there needs to be a continuous film of moisture present over the particle of soil.  Soils made up of smaller particles or a mix of small and large ones can maintain a continuous film of water on the surface of their particles for much longer than a sand ie the hydraulic conductivity of the soil is higher/better.

Where is this taking us?

Well what it means is that un-amended sands don’t hold much water and just to add to our woes, plants can’t easily access much of what’s there anyway.  There is a highly technical term for the water they can get – easily available water!

OK, the only other term I will introduce in this post is mm of evaporation.  One of the ways we can get an idea of how much water to put on is by looking at how much evaporation there is in any one day. Evaporation data is posted on some websites.  A typical hot summers day in Perth has 8-10 mm of evaporation.  Spring and autumn are often 3-4 mm.  This means that on a hot summers day you are losing 8-10 litres of water from every square metre of soil surface.

Does this mean this is how much water we have to put back every day?  Not necessarily.  It depends on a whole host of things.  What sort of plant for a start.  As you can imagine many WA native plants like banksias are can get along quite nicely for some time without water.  In summer, their deep tap roots can access water from well down in the soil profile.  But not all plants have tap roots.  Many garden plants are grown from cuttings and may only have roots down to 30-45 or 60 cm.  Plant some petunias in your garden – how much water do you think they can access in the first week?  Only that which is stored in about the top 2 cm!  And because water doesn’t move laterally in sand their whole access to water is only as far out as the roots go – again only 1-2 cm in that first week.

So your plants access to water not only depends on what sort of plant it is but also how big its root ball is and how deep its roots go.

Think about sand again.  Think about that 8 litres of water in that metre depth of soil.  Then halve it for the amount of available water.  Then think about how deep your plants roots are – for many plants that will only be 30-45 cm.  So you have to cut that 4 litres back yet again by half to a third (roughly speaking). So now your hypothetical plant can only get to about 2 litres of water.  And we are losing 8-10 litres from a square metre of soil on a hot summers day!

Are we in trouble or what! 🙂

OK, I think that’s enough for today.  Like I said this is a complicated business.  But until you understand all the components that you have to think about it will be hard to understand if that plant you planted last week may have simply died from lack of water or whether it was something more insidious.

In my experience it pays to look at the simple things first!  And actually when it comes to dead plants the best thing you can often do is dig it up and have a good look at the root ball.  More on that later too.

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The three main nutrients in all plants are nitrogen (N), potassium (K) and phosphorus (P).  There are a heap of others too – some required in relatively large amounts and others, crucial for plant development but only needed in minute quantities eg iron, copper, zinc etc).

You hear most about phosphorus.  Phosphorus is over-rated in Perth.  Why?  Because we have sands.  Our biggest problem nutrient is nitrogen – in terms of quantity required and ability to maintain a good steady supply to the plant.  But all you hear about is phosphorus.  Why is this?  Well its because most people come from Europe or other countries which have soil, not sand.  Perth is a very special case.  We have unique soil (sorry, sand).  Wherever you have clay, you have the ability to adsorb and hold onto P.  And in soils with large amount of clay you often have to put on up to TEN times the amount required by the plant because the soil hangs onto added phosphorus and doesn’t give it back to the plant.

Because we don’t have clay, you don’t need to do that here in Perth, you only need to add what the plant actually needs.  And that is only about one tenth (10%) of the amount of nitrogen and potassium and for some native plants it is even less.  More on this in another blog.

Even a crop like potatoes which is commonly touted to require heaps of phosphorus only needs 10% as much phosphorus as nitrogen and potassium on Perth sands.  The most phosphorus any other crop needs for maximum growth is one fifth or 20% of the amount of N and K.

The other misconception is that if you add clay to soil, or even in soils with a lot of clay such as those in the hills or many areas of regional Western Australia, that clay will hang onto any amount of added phosphorus.  That is simply not true.  Imagine the capacity of soil to hold phosphorus is like a bucket.  That bucket can fill up and spill over.  When that happens phosphorus will leach out into the groundwater just like any other nutrient.

The problem with animal manures and many popular products like bonemeal and blood and bone is that they have almost as much phosphorus in them as nitrogen.  Of course animal manures, like all organic products are quite variable in composition but you seldom see many with less than 30% phosphorus relative to nitrogen.

What happens as a result of this, is that often when people apply manures and don’t see sufficient growth, they apply more.  This drastically overloads the system with phosphorus when the plant really needs nitrogen.  ONE application of 40 mm (less than 2 inches) of compost or manure can apply enough phosphorus for up to THREE years growth whereas the nitrogen (and potassium) may only be enough for 6 months to one year.  Sure, many people get fantastic growth using manures and other “organic” products but that growth is often only achieved by grossly oversupplying phosphorus simply to get enough nitrogen.

To get around this problem you need to have another source of nitrogen.  If you’re not into chemical fertilisers and don’t wish to use urea or ammonium nitrate then you’re stuck with bloodmeal which is expensive and pretty hideous to use, or you can use more plant based products in your garden such as greenwaste compost.  Some fish based products have reasonable nutrient ratios to but many are supplemented with chemical fertilisers to boost the nitrogen.

So, if you want to be a responsible gardener, get the best plant growth and look after the environment at the same time WATCH your N:P ratios and keep them to between 5 and 10:1 by minimising your use of animal manures.

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OK, I’m being a bit loose with my terms here. I’m really meaning the risk of groundwater pollution associated with organic growing practices on sandy soils. Getting that perfect balance on sands is hard. It requires a balance between the water you apply, the fertiliser you use, and plant growth. Each one of these could be the subject of many blogs! No one said growing plants was easy. Despite what you may think! Sands are unforgiving, they have no buffering capacity (ie ability to soak up any extra you apply and hang on to it for a later time) unless you amend them by adding clay and organic matter. The ‘ideal’ soil or loam is a mixture of sand, silt and clay. Adding organic matter or compost to sand only completes half the picture. To make real gains in holding water and nutrients you need clay. In fact, our sands have a major problem with wettability. You know it well I’m sure. You pour the water on yet the soil below the surface remains dry because it all runs off the top. Actually you’ll get a gold star from me if you have ever dug down to see what’s going on down below! So very basic and almost never done.

It happens because the waxes that coat organic matter particles repel water. We use special wetting agents to overcome the problem. Not detergent – that makes it worse in the long run.

Using clay as well as organic matter or compost will help overcome that problem and also help with water and nutrient retention. And what’s more clay is permanent. A once only solution. Do be careful where you get your clay from though as you may import diseases such as jarrah dieback or even nematodes. Make sure its from a good hygienic source. If you’re not sure, get it tested at a reputable laboratory.

How much to add? Well that’s probably a rhetorical question, its hard work carting clay and digging it in! You probably need to incorporate it throughout the top 30 cm minimum, 45 cm is better. This is the active rooting depth of many plants – 85% of the roots will be there. Between 10 – 20% clay is about right. That’s a 3-6 cm layer if you’re digging it down to 30 cm.

There is a special way of making compost called the Luebke method which uses 5-10% clay in the mix to encourage the formation of a clay-humus crumb. The use of clay in composts is not common but should be, as it provides a much better product.

In my next blog I will talk about balancing nitrogen and potassium and why animal manures (and that includes blood and bone and bonemeal) need to be used with a lot of restraint.

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Welcome to gardening WA style!

I guess I should introduce myself.  Why should you read this and take notice of what I say as opposed to the hundreds of other experts out there?

Well, I’m a horticulturalist – originally trained in NZ, four year degree.  Not many of us around, we’re a threatened species these days.  I’ve been a practising horticulturalist for almost 30 years.    Yes I am old!  All but the first three of those years have been in Perth doing research and advisory work for growers here.  A lot of it with ornamentals and much of that with WA native flora.

Over the past few years I’ve become increasingly concerned that a lot of good information never seems to make it to the public arena.  Consequently there is a lot of  misinformation and misunderstanding about gardening in WA.  As if it isn’t bad enough that block sizes are becoming so small as to virtually preclude the need for a garden, in these times of pressures on water resources and the environment we need to know we are getting best use out of these resources with minimum impact on the world around us.

And above all gardening should be a pleasure!  It is good  for you in so many ways.  It can provide a source of physical activity or be meditative as you enjoy the fruits of your labour.  Fresh air and sunshine abounds in WA – make the most of it!  And I am here to help you have a better garden.

Gardening in Western Australia, or more specifically the Swan Coastal Plain is unique.  We have beachsand.  The ideal hydroponic medium.  Pretty much inert, a support medium for plants but little else.  Really good for weeding.  Doesn’t turn to concrete in summer.  But it has its own set of problems.  Don’t think you can come here and have a successful flourishing garden by applying the principles used everywhere else without setting in train some pretty bad side effects.  Pollution!  Anything you apply to our sands, runs straight out the bottom.  Water, fertiliser, pesticides.  And where does it run to?  Right into our aquifers, our drinking water supplies and our rivers.

Over the next few months I will show you how to make the most of your garden in WA.  Believe me your thinking will be challenged!  What you’ve come to accept as gospel will be no longer.  Much advice so readily dispensed through the media is downright wrong.  What is done over east,  in Europe, England or the US doesn’t work here – and if it does you will be not only wasting water and fertiliser but polluting your environment as well.  One of my biggest annoyances of late is the emphasis on organic gardening and permaculture.  Wonderful systems with a lot of benefits but in WA they would have to be one of the biggest sources of nutrient pollution that exists.

I guess I’m going to have to expand on that aren’t I!  I will be back soon to explain why.

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