If growers reduce P rates at seeding then, unless it replaces what is removed, soil test values will decline, according to IPNI’s Dr Rob Norton.
Getting phosphorus inputs right at seeding is critical to good early plant establishment and, according to International Plant Nutrition Institute’s Dr Rob Norton, the best way to get P inputs right is to get good, credible soil tests and interpret them properly.
Dr Norton says growers spend from $50 to $100 per hectare on P every year, which is a big investment when they do not know what the result is going to be at the end of the season. However, it is still an investment that needs to be made.
But after a dry finish to the 2014 season, many growers might be asking if they can lessen the amount P they put upfront. In many areas of south-east Australia, grain yields and P removal was low but even if the crop was salvaged for hay, P was still removed.
“Cereal hay removes about 1 kilogram of P per hectare per tonne, while on average wheat grain removes around 2.5kg P/ha/t, and canola hay removes around 3kg P/ha/t,” Dr Norton said. “In any case, knowing the P removal is important. Grain P can vary from 1.8-4.5kg/t, so getting grain tested will give a guide to the actual P removed.
“If you reduce your P rate at seeding then, unless it replaces what is removed, the soil test value will decline.”
After last year, much of the P applied on neutral to moderately acid soils is still likely to be available, so reduced rates are feasible in 2015. On calcareous or very acidic soils with a high phosphorus buffering index (PBI), P fixation occurs, even under dry conditions.
“In that case, using fluid P in-furrow might be a good option if good delivery equipment is available,” Dr Norton said. “Even so, MAP and DAP are still reliable resources in many situations.
“There is a positional effect of P in developing plants. After the 2006 drought, we applied P the following year and there was a benefit of having 5kg of P with the seed, even though it was a relatively high soil test. Between 3-5kg of P with or near the seed is an important management strategy and should be considered as the bottom line.
“I don’t think that current equipment can apply less than 2-3 kg P evenly. Even if you’re applying 2kg of P on 25 centimetre row spacings, that’s granules 8-12cm apart and narrower rows make this spacing more distant. This makes early engagement of roots with fertiliser a chancy business.”
But before growers make a decision on how much P they will apply, they need to know their starting point in P levels. The key to that is getting a good soil test, Dr Norton says.
“A key message is to take a good sample and for P, depth is critical as most P is in the topsoil,” he said. “Shallow samples over-estimate soil test values and deeper samples underestimate the true value, therefore representative 10cm deep samples are critical to interpret the current soil P tests.
“The Colwell soil test is still a reliable indicator of the potential response to added P, as is the DGT test. If you do get soil tests done, make sure it is through an Australian Soil & Plant Analysis Council (ASPAC) certified laboratory.”
Applied P will go to any three places. Some will go to the available P pool, which can raise soil test values, some will be fixed into less available pools such as organic, calcium, iron or aluminium P and will be tied up for a period of time. The rest will be taken up to plant roots from the available P pool, and most of that will be removed in the grain.
Different soil types and crops will have different critical values and ranges for soil P tests (table 1).
Table 1: Critical Colwell soil test values and ranges as taken from the Better Fertiliser Decisions for Crops database
Critical value (milligrams per kilogram)
|Critical range (mg/kg)
|Wheat and barley
|| All soils
| Field pea
“The values indicate that canola is less responsive than wheat or barley on many soils,” Dr Norton said. “However, there is a suggestion that wheat after canola is more P responsive than wheat after wheat, possibly because the canola is quite efficient at accessing soil P.”
Because both the soil and plant are competing for applied P, the amount of P that crops access from fertiliser is relatively low at about 25 percent, with the balance coming from less available pools supplying P to the more available pools. Soils with a high phosphorus buffering index (PBI) will demand a lot of the P applied.
“Where soil tests are at or above the critical values, a maintenance program can be implemented where application is equal to long-term removal plus a contribution to the fixed soil pool,” Dr Norton said. “Below the critical soil test value, yield are limited by their access to adequate P and so there is a need to build soil P levels, as well as accounting for P removal and fixation.”
P pools explained ...
- There are two major pools of phosphorus in soil – inorganic and organic.
- Not all inorganic P is plant available in soil solution, some P may become available but is temporarily stuck in low solubility compounds, whereas some P is more permanently unavailable (‘lost’) as it is fixed to other minerals in the soil.
- Unavailable inorganic P may be ‘released’ and become available P by the action of exudates from crop plant roots.
- The amount of plant available P relative to the total inorganic P pool is influenced by soil type, especially clay content and mineralogy.
- Soil organic phosphorus is not directly accessed by plants, it is readily converted to plant-available forms of P by soil microbes during a process called P mineralisation. P in the microbial pool is one stage closer to being available to plants.
Dr Rob Norton, firstname.lastname@example.org, 0428 877 119
View a GRDC phosphorus management fact sheet
Go to the International Plant Nutrition Institute website
Go to the making better fertiliser decisions for cropping systems in Australia website