Farmnote 93/96 [Reviewed August 2006]
By Blakely Paynter, Plant Research and Development Services, Northam and Mike Bolland, Plant Research and Development Services, Bunbury
Discusses the effect of phosphorus (P) deficiency in annual legume pastures burr medic and serradella, including P requirements and treatment.
Phosphorus (P) is essential for plant growth and reproduction and is found in every living plant cell. It is involved in several key plant functions, including energy transfer, photosynthesis, the breakdown of sugars and starches, transport of nutrients within the plant, and inheritance.
Subterranean clover has been the most widely sown annual legume pasture on the marginally acidic soils in the wheatbelt of Western Australia. As it does not persist in intensive cropping systems and on hard-setting soils, burr medic and yellow serradella have been introduced for some soils.
Serena, Santiago and Circle Valley burr medics have been introduced for the marginally acidic to neutral, medium textured soils (where pH in calcium chloride is more than 5.2). The earlier maturing, yellow serradella varieties Madeira and Paros have been introduced as persistent legumes on well-drained, light soils in low to medium rainfall zones. The later maturing varieties Tauro, Pitman and Avila are more suited to the high and very high rainfall areas of Western Australia.
Phosphorus deficiency in burr medic
Phosphorus deficiency in burr medic plants is shown by small, dark green leaves. Healthy, non-deficient leaves of burr medic are pale green to deep green, depending on the variety. The cotyledons in deficient plants usually turn yellow and die earlier. Often the plant stem below the cotyledons becomes a brighter red and their petioles redden. Burr medic is more affected by low soil phosphorus than subterranean clover and yellow serradella.
Acutely deficient plants fail to show much vigour during the growing season and set little seed. Deficiency symptoms are similar to those observed in subterranean clover - see the illustration in Farmnote No. 42/94, 'Phosphorus deficiency in subterranean clover' (Agdex 137/632).
Phosphorus deficiency in yellow serradella
Visual symptoms of phosphorus deficiency in yellow serradella plants are rarer because of this apparent ability to thrive when phosphorus supply is low. Also, yellow serradella does not appear to show clear visual symptoms of phosphorus deficiency unless growth is severely impaired. This highlights the need for tissue testing to confirm the plant status.
Moderate deficiency reduces plant size. With severe phosphorus deficiency, the uppermost sides of the cotyledons redden. This is associated with reddening of the petioles of the old leaves, yellowing of their leaflets and reddening of the leaflet margins.
Figure 1. Differences in phosphorus supply required for 90 per cent maximum yield in three annual pasture legume species. Yellow serradella (YS), Subterranean clover (SC) and burr medic (BM).
Yellow serradella's longer, thinner root system explores more of the soil volume and places more phosphorus into shoot tissue. This enables it to survive better under conditions of low soil phosphorus.
Some of the visual symptoms of severe phosphorus deficiency are similar to the symptoms of nitrogen deficiency, because of a reduction in energy supply to the legume nodules.
Different phosphorus requirements
Yellow serradella, subterranean clover and burr medic differ in their requirements for phosphorus. Differences in the requirements of these three species are illustrated in Figure 1. At 90 per cent maximum yield, burr medic pastures require more phosphorus than subterranean clover, while yellow serradella pastures require less.
These differences in response at low soil phosphorus supply are due to differences in their root morphology and their capacity to supply phosphorus to above-ground, growing shoot tissue.
Burr medic has a shorter, thicker root system than yellow serradella and subterranean clover, and delivers less phosphorus to the shoot tissue. Hence, burr medic has a higher requirement for phosphorus to maintain growth than the other two species.
Correcting the deficiency
The best time to apply phosphate fertiliser to the pasture is just before the break of the season, although in higher rainfall areas, later applications may also be justified on the leaching sands. An adequate supply of phosphorus will promote early vigour of the pasture and maximise the growth potential through the season, provided there are no other nutrients limiting plant growth.
The optimum phosphorus fertiliser rate is determined by knowing the value of the product (in this case, dry matter), the cost of the fertiliser, the background soil phosphorus level and the shape of the response curve to increasing phosphorus application.
For subterranean clover, the most profitable rate of phosphorus fertiliser can be calculated using fertiliser prediction models, such as Decide®, PhosulK® and WoolModel®, produced by Agriculture Western Australia.
Burr medic has a higher optimum phosphorus fertiliser requirement than subterranean clover and responds very well to extra phosphorus. For the same soil test results, it is suggested that burr medic pastures receive 2 kg P/ha more than that recommended for subterranean clover pastures.
This recommendation will be influenced by soil pH. On the more alkaline soils (where pH in calcium chloride is more than 6.0), differences in root morphology between burr medic and subterranean clover are less important. Phosphorus fertiliser recommendations for burr medic on these soils will be similar to those for subterranean clover.
In rotations with wheat, the productivity of burr medic pastures is severely reduced when the medic is forced to rely on residual phosphorus from the wheat phase. These pastures must be top-dressed to realise their full nitrogen benefit. Similarly, seed production in burr medic pastures has a high requirement for phosphorus. Regeneration following the cropping phase relies on adequate fertiliser application during the pasture phase.
It is suggested that yellow serradella pastures receive the same rate of phosphorus fertiliser as subterranean clover pastures, because of their large dry matter response to low rates of applied phosphorus. This recommendation appears to be unchanged by soil type or soil pH.
Other medics and serradellas
Medic species are available for soil types not suited to burr medics. Barrel medic varieties Cyprus, Caliph, Parabinga and Paraggio are well-suited to neutral or alkaline clay loams (where pH in calcium chloride is more than 5.8). The sphere medic variety Orion and the murex medic variety Zodiac grow on more acid soil types than the burr medics. The phosphorus requirements of these medic pastures are likely to be similar to burr medic pastures. It is suggested that barrel, sphere and murex medic pastures receive more phosphorus fertiliser than subterranean clover pastures for a given soil test.
The only other serradella suggested for Western Australia is the slender serradella variety Jebala. Slender serradella is well-suited to acid, winter waterlogged soils in high rainfall areas. Its applied phosphorus requirement is similar to yellow serradella. It is suggested that slender serradella pastures receive an amount of phosphorus fertiliser similar to that recommended for subterranean clover pastures.
Diagnostic analysis of phosphorus status
Plants are often very deficient in phosphorus before visual symptoms appear. Soil testing and tissue testing are the two main tools we can use to decide whether a legume pasture is likely to be phosphorus deficient.
Although the phosphorus requirements of yellow serradella and burr medic over a range of soils are not as well quantified as those of subterranean clover, soil testing can determine whether the soil has enough phosphorus for plant growth.
The Colwell test, which measures bicarbonate extractable phosphorus, is used in Western Australia. Wheatbelt soils that are less than 10 parts per million (ppm) Colwell phosphorus will be acutely deficient, while soils that have soil tests between 10 and 25 ppm are likely to restrict maximum plant growth. These figures do not apply to leached, grey and yellow sands in medium to high rainfall areas.
Soil testing is best used with other information such as soil properties, paddock history (fertiliser, cropping and grazing), tissue testing, previous soil test values and estimates of the amount of phosphorus removed in grain and animal products.
For more information on soil testing, refer to Farmnote No. 16/93 'Soil testing for phosphorus' (Agdex 533).
Plant tissue testing is used increasingly to determine when phosphorus fertiliser applications are required to avoid phosphorus deficiency. It cannot be used alone to accurately predict plant responses to fertiliser applications, but can be used with soil testing to design strategies to alleviate any deficiency.
For more information on tissue testing, refer to Farmnote No. 32/96 'Tissue testing for phosphorus' (Agdex 100/533).
Phosphorus analysis in plant tissue is best done on whole tops because phosphorus is a mobile plant nutrient. Plants are best sampled from six weeks after emergence until mid-flowering. The amount of phosphorus in the plant can be related to the level of productivity and this level can be used to define the status of the plant. The level of phosphorus at a sampling date can be related to one of three nutrient status levels of the plant: critical phosphorus concentration, moderate deficiency or severe deficiency.
The phosphorus concentration in whole tops that produces 90 per cent of the maximum yield is defined as the critical phosphorus concentration. Moderate deficiency is the phosphorus concentration in whole tops which relates to a reduction in yield of 30 per cent. Severe deficiency is the phosphorus concentration in plants which relates to a reduction in yield of 50 per cent.
The phosphorus status of a plant decreases with plant age. It differs between species and between environ-ments and soils.
Critical phosphorus concentration
In burr medic, the critical phosphorus concentration in whole tops decreases from around 0.62 per cent at 40 days after emergence (DAE), to 0.58 per cent for 41 to 60 DAE, 0.55 per cent for 61 to 90 DAE and 0.26 per cent for later than 90 DAE (Table 1). The critical phosphorus in seed is around 0.52 per cent.
Table 1. Average concentration of phosphorus in whole tops, dried tops and seed at various phosphorus (P) status levels in the plant at intervals after emergence for burr medic and yellow serradella
|Average P concentration (% P, dry basis)|
|Burr medic||0 to 40 days||41 to 60 days||61 to 90 days||90+ days||Seed|
|Critical P level||0.62||0.58||0.55||0.26||0.52|
|Moderate P deficiency||0.39||0.43||0.40||0.20||0.43|
|Severe P deficiency||0.29||0.34||0.30||0.17|
|Yellow serradella||0 to 40 days||41 to 60 days||61 to 90 days||90+ days||Seed|
|Critical P level||0.52||0.63||0.39||0.35||0.46|
|Moderate P deficiency||0.34||0.42||0.32||0.28||0.42|
|Severe P deficiency||0.27||0.30||0.26||0.22||0.38|
In yellow serradella, the critical P concentration in whole tops decreases from 0.52 per cent around 40 DAE, to 0.62 per cent for 41 to 60 DAE, 0.39 per cent for 61 to 90 DAE and 0.35 per cent for later than 90 DAE (Table 1). The critical phosphorus in seed is 0.46 per cent.
Moderate and severe phosphorus deficiency
Moderately deficient burr medic and yellow serradella plants contain less than 0.42 to 0.43 per cent phosphorus for 41 to 60 DAE, while severely deficient plants contain less than 0.30 to 0.34 per cent phosphorus at the same period.
In early to mid-flowering (90+ DAE), moderately deficient whole top samples contain less than 0.20 to 0.28 per cent phosphorus and severely deficient whole top samples contain less than 0.17 to 0.22 per cent phosphorus