The Case of Phosphorus Deficient Soil

Many of us have at least some soil in our gardens that isn’t suitable for growing certain plants. Perhaps you wish to grow proteas but your soil has too much phosphorus in it because previous owners recycled their grey water but didn’t use environmentally friendly detergents. Or maybe you want to grow lots of vegetables but your property used to be an apple orchard and so the soil has lots of arsenic in it from the pesticides that were commonly used in many apple plantations. Perhaps you want to plant a herb garden near your back door but your house was once painted with lead-containing paint so the soil immediately surrounding your house contains elevated levels of lead. Maybe you’ve just built a new house and all the top soil was stripped from your property leaving bare clay, which isn’t great for any plant really.

There are usually ways to improve your soil but this can take quite some time so an alternative may be to build a few raised beds and buy some soil to fill them with. This enables you to get growing quickly and, if you wish and it’s practical, you can improve the other areas of your soil while you’re establishing the raised beds.

Well this is exactly what we did in our garden. We purchased 8 cubic metres of soil to fill the new raised beds we built for our vegetables and herbs. We had previously purchased nutrient rich soil that did ok but which had very little life in it. In fact, two years down the track we still don’t even have many worms in it despite adding lots of organic matter. So when we were at the rural supply shop and saw some soil that was full of organic matter and which was growing mushrooms (so had quite a bit of life in it), we thought it would be a great alternative. Unfortunately, we soon found that this soil was nutrient deficient. Specifically, it didn’t have enough phosphorus in it.

It took a little while for us to figure out what the problem was. Initially, seeds germinated ok and seedlings grew fairly well. Soon though, our seedlings started to show signs of distress.

Our corn seedlings grew very slowly and were well behind corn plants we were growing in our front garden (in different soil) which we had actually sown a few weeks later. After a while, the affected corn leaves also started to turn yellow in the centre and purple along the edges. 

phosphorus deficient corn - leaves are pale yellow with purple edges
Phosphorus deficient – so small and pale compared to healthy corn
Phosphorus deficient corn. A close up showing purple discolouration along the edges of pale yellow leaves
Phosphorus deficient corn – see the purple discolouration at the edge of the leaves
Healthy corn sown three weeks after the phosphorus deficient corn - seedlings are much bigger and are a deeper green
Healthy corn sown 3 weeks later

The leaves of our strawberry plants started to go very red. One was a new plant and I initially thought it had a disease but the other was an existing plant that I had transplanted from different soil and it had been fine until it went into the new soil (and was not close enough to the new strawberry that it was likely to have caught any kind of disease).

Red-brown discolouration of old strawberry leaves
A phosphorus deficient strawberry with red-brown discolouration of the older leaves

The blueberries looked pretty sad too.

Phosphorus deficient blueberry leaves with red discolouration
Phosphorus deficient blueberry leaves with red discolouration
A phosphorus deficient blueberry plant - stunted with red leaves
What a sad looking blueberry plant

Our poor tomato seedlings did even worse than they normally do (our climate isn’t really suitable for tomatoes but we love them so much that we try to grow them every year anyway). They grew even slower than normal (I didn’t think such a thing was possible!) and the leaves turned purple in places.

Spindly, discoloured tomato plant
Phosphorus deficient tomato plant
Photo - tomato leaf chlorosis
Tomato leaf chlorosis
Photo - purple undersides of phosphorus deficient tomato leaves
Purple undersides of phosphorus deficient tomato leaves

Many other vegetable seedlings also struggled. They grew slowly and the leaves of many of them became chlorotic or developed necrotic spots.

Photo - Necrosis of bean leaves
Necrosis of bean leaves
Photo - discolouration of bean leaves
Discolouration of bean leaves
Photo - discolouration of pea leaves
Discolouration of pea leaves

Interestingly, some of our potatoes suffered but others didn’t. The ones that were ok, had had lots of ‘chicken straw’ incorporated into the soil as they were hilled up. (Chicken straw is my term for the bedding straw that we collect from our chickens’ house. It has lots of chicken manure in it, which is very high in phosphorus).

Normally nutrient imbalances can be difficult to diagnose visually (usually you need a tissue and/or soil test to confirm a suspected imbalance) but luckily in this case, so many different species were affected and the chicken manure made such a big difference to the health of the potatoes that I could conclusively determine that the problem was phosphorus deficiency. So I set about correcting the problem. I added ‘chicken straw’ to all of the beds as it became available. I also regularly watered the plants with a combined fish emulsion and seaweed solution to get some phosphorus into the plants quickly and to help keep them strong until the soil could be improved. For long term improvement, we also put lots of an organic, pelletised fertiliser in the beds.

Over time, the pH of the soil started to rise slightly so I also added some sulfur to the soil in which the berries are growing. Berries are very sensitive to alkaline soil so that was an important step in helping them recover from their phosphorus deficiency.

In some beds, I also added some superphosphate. While this isn’t approved for organic gardens, we decided it was better to take this step than to have to purchase lots of organic additives and because there isn’t really an organic solution that has just phosphorus in it (so it would be difficult to boost the phosphorus levels long term without also boosting the levels of other nutrients in the soil). I did however make sure that the product we selected did not contain cadmium in it – many synthetic phosphorus fertilisers do contain cadmium as a result of the industrial processes used to produce the product.

So while I’m disappointed that we haven’t been able to stick to completely organic principles, I am confident that we have chosen the best course of action in our situation and we have at least not taken any action that is likely to kill of all the fauna in our garden. In fact, our butterfly population has taken off this year!

So the moral of this story for us has been to always ask for soil test results before purchasing soil. And while this has cost us quite a bit of time in our garden it is a lesson well learned and this experience has at least prompted me to write about different nutrient deficiencies and toxicities and how they can be treated. So if any of you have some suffering plants, why not check out the new nutrient deficiencies and toxicities section of plantprobs.net so you can determine whether the problem is a nutrient imbalance and then start to treat the problem.

International Compost Awareness Week

It was International Compost Awareness Week (ICAW) this week so in celebration I thought I’d do a post all about compost.

In some areas of the world, substantial amounts of the waste deposited in household mixed-waste garbage bins is organic matter that could be used as mulch or to make compost. In 2011 in Australia for instance (according to EC Sustainable) approximately 1/3 of the rubbish in mixed-waste systems was food scraps and 1/10 was garden waste. That means that close to half of the waste going to landfill could have been composted! tweet this

When you consider just how easy it is to compost garden waste, it is surprising that so many people do not compost their organic waste. When you realise the benefits of composting through, it is downright astounding.

Compost improves water retention in sandy soils and also improves drainage in clay soils. It provides nutrients for your plants and helps improve soil structure. Aaaand, did you know that research has shown that plants are less likely to succumb to disease if they are grown in soil treated with compost! tweet this

So if you’re keen to start composting in your garden here’s what you should know about composting:

  • composting is for organic matter – anything that was once living – so don’t add material in plastic bags
  • composting works best when it’s aerated so turn your compost regularly and choose a container that allows plenty of air circulation
  • don’t compost diseased material, the roots of weeds that spread by runners or weed seeds
  • compost should be made up of brown material and green material
  • brown material is usually older organic matter such as straw and autumn leaves
  • green material is fresher organic matter such as lawn clippings and fresh food scraps
  • for every 1 part brown material added to your compost, add about 2 parts green material – this will give you the fastest compost system

If you want to know more about compost and how to make your own, check out the Garden Realm compost page.