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Archive for September, 2011

Phosphorus(P) is an essential component of cell membranes, plant genetic material (DNA, RNA) and plant energy systems. Early plant growth is particularly dependent on P because that is when there is a lot of cell division and expansion as stems, buds, shoots and roots form. So phosphorus is needed all over the plant, not just for roots.

Plants can only take up P when it is dissolved in the soil solution. Phosphorus can either move towards the roots in the soil solution or plants roots will intercept it as they grow through the soil. Some plants have special ways of accessing phosphorus. Our Proteaceous plants (such as banksias, grevilleas and hakeas) secrete acids (such as citric) which will dissolve phosphorus held in the soil. Other plants have associations with mycorrhizal fungi that help them to access phosphorus.

Phosphorus is held (adsorbed) by clay minerals in the soil or by organic matter. On the Swan Coastal Plain where we have very sandy coarse soils, neither of these are very significant and P levels are very low unless it is added as fertiliser or in manures or organic matter. Because sands are just that – sands, their ability to hang on to added P is minimal and so if you add more than the plant needs it will leach out into the groundwater very quickly. Animal manures such as cow, pig and horse are quite high in P relative to nitrogen and potassium and so P leaches from them very easily. Despite what most people think, most phosphorus in animal manures and compost is easily water soluble.

Table 1. Readily leachable P and organic P as a proportion of total P in various animal manures

Manure type % of total P that is water soluble
(readily leachable)
% of total P that is organic P
(more slowly available)
Cow 39.2 11.3
Pig 29.9 6.4
Layer 16.3 20.1
Broiler 8.7 38.1

Ref: Koimiyama T, Ito K and Saigusa M, 2010. Water solubility of phosphorus in animal manure compost, 19th World Congress of Soil Science, Brisbane

As you can see from Table 2 (courtesy Katrina Walton ChemCentre) below the ratio of nitrogen to phosphorus fits best to plant needs (5:1 to 10:1) in the greenwaste compost and while the P content of each product is low it is because it is high relative to the nitrogen content, PLUS its solubility than means much of it will be leached when applied.

Table 2. Typical values for important parameters of conditioners and two types of compost

  Analyte  Soil conditioner  Premium compost

 Greenwaste compost

  Nitrogen (%) 1.8

2.1

0.8

  C:N ratio

7.6

20.7

14.2

  Phosphorus (%)

0.99

0.89

0.22

  Ratio of N:P

1.8

2.4

3.8

How best to apply phosphorus.

In sands, little and often. And NEVER up front. The notion of pre-plant superphosphate on sands is not a good idea because it will simply leach. Blood and bone on sands is no better. All fertiliser should be applied as plants need it. Small plants need a lot less. And try to always apply N, P and K together. Controlled release fertilisers are good. Compound fertilisers such as NPK Blue are also good though they need applying more often. And most fertilisers still have too much P relative to N FOR OUR SANDS.

If you want to use composts try to use greenwaste compost as its nutrient ratios are closest to plant needs and bear in mind that even a 25 cm layer of manure based compost may supply about 150 kg/ha of nitrogen (enough for 6-12 months) and 80 kg/ha of phosphorus (enough for 18 months to 3 years) provided there is no leaching.

What do plants look like with too little or too much P?

Symptoms of P deficiency are simply generally stunted growth, and in cold weather, purpling of leaves. However that is often due to low root activity rather than actual deficiency and when the weather warms up the symptoms will disappear. Invariably, I find that plants deficient in P are deficient in everything ie they haven’t been fed at all! Unless of course you are in the hills or out in the regions on clay soil and your soil that adsorbs P – that is quite a different situation.

If you apply P to Proteaceous plants they will show signs of toxicity. That can range from yellowing of young growth (induced iron deficiency), blackening of leaf tips or death. Sometimes it can be helped by adding more nitrogen and potassium to dilute the P but some of these species don’t like high fertiliser levels at all and so that may not work. And if this is happening in a clay soil (as in the hills), trying to leach the P out by watering won’t work because the clay likely has a good supply of it that will release slowly over time of its own accord and will not respond greatly to watering.

This has been a long post but I hope it has helped your understanding of phosphorus a little.  Plant nutrition is not simple.

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