W7.2, Phosphorus (P) nutrition

Question 1

True or False?

What is a typical rate of P recovery in the year of application?

Answer 1

< 20%

Cropping systems generally recover more P than pasture systems (42-53%).

Question 2

True or False?

We’re not applying enough P to meet crop demand.

Answer 2

False. In SE Australia, most wheat-growing soils exceeded the critical value by 1.5x.

(Simpson et al (2011) Plant & Soil 349:89)

Question 3

True or False?

Why is the ability of roots to continue exploring soil advantageous for P uptake?

Answer 3

Because P moves by diffusion (i.e. slowly, and over short distances).

Question 4

True or False?

What are some typical responses to delayed P supply in cereals?

Answer 4

• Smaller leaves
• Fewer tillers
• Darker leaf colour
• Higher root:shoot ratio (lower shoot wt., higher root wt.)
• Increased root length, decreased root radius
• Delayed maturity

Question 5

True or False?

Rhizosheath volume is related to P requirement

Answer 5

True. The larger the rhizosheath, the lower the critical value ([P] required for 90% potential yield) (generally).

Rhizosheath size is related to the density and length of root hairs, so more hairs = greater capacity to acquire P = reduced requirement to apply it.

Question 6

True or False?

A soil low in P will benefit sub clover over ryegrass because sub clover is better able to take up P effectively.

Answer 6

False; it’s the other way around.

Question 7

True or False?

What are some root adaptions to low P availability?

Answer 7

• Larger rhizosheath volume
  
  • Increased root hair density and length
• Cluster/proteoid roots
  
  • look like a bottle brush
  • increased surface area
  • can exude organic acids to acidify the rhizosphere and make P more available
• Exudation of organic acids and enzymes
  
  • rhizosphere acidification
  • phytase and phosphatase
• Form mycorrhizal associations
  
  • not really as much a root adaption as a plant adaption, but it’s worth mentioning because AM fungi can be extremely good at extracting phosphorus in low P environments.

Question 8

True or False?

How does N nutrition affect P nutrition?

Answer 8

There’s quite good experimental evidence to indicate that you’ll only get the maximum uptake of P when N nutrition is adequate (and vice versa)

Question 9

True or False?

White lupins are the only agricultural plant with the ability to grow cluster (proteoid) roots.

Answer 9

True.

Question 10

True or False?

Growing legumes is likely to improve the P uptake in the following crop.

Answer 10

True.

Question 11

True or False?

How can the organic acid-producing benefit of legumes be utilised?

Answer 11

By incorporating them into the rotation.

By growing them as an intercrop.

Question 12

True or False?

The earlier the sowing, the lower the yield penalty for under-applying P fertiliser (i.e. the less fertiliser you need to add to get the same yield).

Answer 12

True.

Question 13

True or False?

How can soil biology improve phosphorus availability?

Answer 13

• Mycorrhizae
  
  • important in low P environments
  • can assist with uptake of other nutrients too (e.g. Zn)
• Phosphate solubilising microbes
  
  • free-living bacteria and fungi
  • e.g. Aspergillus sp., Pseudomonas sp.
• They can be applied as seed dressing and innoculants, but experimental results on their success shows variable results.