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Soil phosphorus concentrated near the surface with zero-till

Yields are not affected but soil testing and P placement strategies may need altering.

November 15, 2007
By Bruce Barker


38aWhen moving from a conventional to zero-till system, questions often arise
regarding the impact on nutrient availability and placement. The concern is
especially justified for immobile nutrients like phosphorus (P), which risk
stratification near the soil surface. Without tillage, crop residues are not
mixed into the soil to the depth of tillage. Rather, immobile nutrients in the
residues can get left near the soil surface. Plus, banded P is not mixed around
in the soil with zero-till, leaving the P concentrated in bands where it was
placed. Are these issues cause for concern?

"There doesn't tend to be a yield effect when phosphorus is concentrated
near the soil surface under zero-till," says research scientist, Fernando
Selles with Agriculture and Agri-Food Canada's (AAFRD) Semiarid Prairie Agricultural
Research Centre at Swift Current, Saskatchewan.

When comparing a continuously cropped zero-till wheat rotation to a wheat-fallow
rotation grown under conventional tillage, 12 years after converting to zero-till,
available P accumulated in the top two inches of soil. This was not the case
for no-till fallow-wheat or conventional tillage continuous wheat. The difference
was attributed to the lack of soil disturbance, and accumulation of surface
residue and the slower decomposition of residues in no-till.

Adrian Johnston, Northern Great Plains director with the Potash and Phosphate
Institute at Saskatoon, says the studies have shown that if P is concentrated
near the soil surface, plant roots will compensate by proliferating in the highly
concentrated area.

"Nutrient uptake is limited by the amount of root surface in the soil,
but plants can increase their root hairs in the richly concentrated nutrient
area. In no-till, because there is generally better soil moisture at the soil
surface due to the increased crop residue, plants can increase their rooting
in this area. As a result, there likely won't be a problem with deficiencies
of immobile nutrients," explains Johnston.

At Swift Current, the higher levels of available P in the top two inches did
not result in any additional plant uptake under zero-till. This result does
not necessarily contradict Johnston's opinion, though. Rather, Selles says the
lack of additional P uptake from this area is possibly due to 15 pounds P2O5
fertilizer per acre placed with the seed annually. Plus, in cool spring soils,
soil P is less available anyway, which is why starter P fertilizer is recommended
in most situations. However, an increased soil supply of available P could be
beneficial to mid-season uptake of P by a high yielding crop.

In another study at Brandon, Manitoba, AAFRD research scientist Cynthia Grant
found that after four years of deep banding a N/P blend, the P had accumulated
at the four inch depth where it was banded. This occurred under both zero-till
and conventional tillage. The soil P concentration, though, was higher under
no-till than tilled systems, likely due to lack of soil disturbance and soil
mixing with tillage.

Pay more attention to P placement and soil testing
Johnston says that under conditions where the soil surface dries out, symptoms
of P deficiency may mean that plants cannot access these surface nutrients.
"This places a greater emphasis on in-soil band placement of P with no-till."

Application of P in the seedrow or side-banded at seeding increases early season
uptake by the crop and will help overcome low availability of soil P in cold
soils. When crop P requirements cannot be met with seed placement, Johnston
says that a low disturbance banding operation prior to seeding would be the
best alternative in no-till fields. Farmers should not allow seedrow application
limits of P to limit the yield potential of their crops. Care should be taken,
though, to minimize soil disturbance that compromises seedbed quality.

The other factor that needs further work is how these concentrated layers of
P affect soil test results. When P is banded in the soil, a practice most western
Canada growers use, and tillage is eliminated, the P remains concentrated in
these bands. Under conventional tilled systems, the bands get mixed around in
the soil and are not a concern.

But under no-till, if P is seed-placed in a narrow band on nine inch row spacings,
for example, that means there are all these little one inch diameter bands every
nine inches at the depth of seeding. In the next year of no-till, a second band
would be present; and so on down the years.

"The question is how long do those bands persist? After the second year,
there would be two bands. Maybe after 10 years, the bands wouldn't be noticeable
and the P would be uniformly distributed at the depth of banding," says
Selles. "We don't know."

Johnston says that under no-till, the answer to soil testing accuracy might
be found in more creative ways of soil testing. He says that sampling procedures
may have to be changed to accommodate the bands of P under no-till. Selles says
that in southern Brazil, research investigated how soil test procedures could
overcome these bands of P. He says a flat spade sampling across the seedrow
was more accurate in estimating available P than core sampling.

"For farmers, the immediate implications of moving to no-till are not
a concern since we see good response of crops to starter P. In the longer-term,
we need to understand how to better estimate available P so that we can accurately
determine whether we can cut back on starter P in some circumstances, as a short-term
risk management tool, for example," explains Selles. -30-