Features Agronomy Fertility and Nutrients
Phosphorus deficiency in seedling corn – crop rotation considerations

Silage corn (Zea mays L.) is grown on approximately 20,000 acres in
the coastal region of British Columbia as a forage for the local dairy industry.
While soil test P levels on dairy farms are generally medium to high in the
region due to the addition of manure, it is not uncommon for farmers to report
P deficiency symptoms at the three to six leaf growth stage of the corn crop.
Cool soil temperature is the main reason given for the purpling of corn seedlings,
with the high yielding corn hybrids considered most likely to display the colour
change because of high levels of the pigment, anthocyanin.

In an attempt to evaluate the impact of early season P deficiency on silage
corn production, research staff at the Pacific Agri-Food Research Centre in
Agassiz have been monitoring the fertilizer and crop management practices used
in research trials. They have regularly observed that the purple colouration
and stunting of well fertilized corn corresponds to cropping practices in the
previous year. Corn planted in the previous year's alleyways showed severe purpling
and stunting, whereas the corn planted in areas previously growing corn flourished.
Typically, these alleyways were fallowed using a rotovator. The most severe
stunting was observed on soil areas that were fallowed for several years. Having
P-starved corn plants growing next to P-sufficient plants on soils with abundant
levels of soil test P was a major problem for the corn hybrid testing program
being managed at the centre.

The phenomenon of P deficiency in young plants growing on previously fallowed
soils has been well documented for corn, wheat and other crops. There is considerable
information that points to inadequate populations of VAM in fallow soils as
the cause of this P deficiency. In fact, the current opinion of many researchers
is that young seedlings require an established network of VAM hyphae to enhance
early season P uptake. The spores of VAM, which have long-term persistence and
found almost everywhere in agricultural soils, require substantial time to develop
a sufficient network. Cool soils influence the activity of VAM, slowing their
early season development. As a result, farmers routinely use low rates of starter
P with the seed at planting of corn to try and overcome this early season deficiency.

At present, P recommendations in Canada do not take the VAM status of a soil
into consideration. Two reasons may be suggested:

1. There is no convenient test for VAM in the soil, especially one that can
   be used prior to planting.
2. Relatively few studies on VAM have evaluated crop yield.

No attempt has ever been made to systematically develop soil test correlations
for P in concert with assessment of VAM in Canada. This is despite clear indications
that common farming practices – such as summerfallow, crops planted following
'cruciferous' and other non-VAM crops, intensive tillage, and flooding –
may affect P nutrition in young crops. Today, farmers need to know not just
when starter P is required, but also when it is not required to achieve optimum
yield. In particular, the strategic use of P in heavily manured soils is critical.

To evaluate the effect of rotation on early season silage corn, P nutrition
research trials were established in the coastal region of British Columbia.
Data were collected on corn growth in 1995, 1997 and 1998, with the plot area
in the pre-seeding year managed with corn, summerfallow or canola.

Locally adapted silage corn hybrids were planted between early May and early
June at a rate of 30,000 to 32,000 seeds per acre using 30 inch row spacing.
Fertilizer, without P, was broadcast at recommended rates. Nitrogen (N) was
applied as ammonium nitrate at 180 to 225 pounds N per acre broadcast prior
to seeding. On some treatments, fertilizer P was side-banded at a rate of 60
pounds P2O5 per acre at seeding,
while on other treatments no P was applied. Plants were sampled at several growth
stages for tissue P concentration and VAM colonization at the three leaf stage.
Silage yield and percent dry matter were measured at the dent stage.

In all three years of the study, colonization of corn roots by VAM was significantly
lower after fallow than after corn (Table 1). The effect of planting corn after
canola on VAM was equivalent to planting corn after fallow. These results are
consistent with previous research on corn. However, in this study the application
of P fertilizer had little effect on VAM colonization, in contrast to many published
reports. There was no evidence of an interaction between the previous crop and
P application on VAM colonization. Corn seedling tissue P at the three leaf
stage showed a positive response to fertilizer P addition. However, it was not
influenced to any great extent by the previous crop (Table 1). At the six leaf
stage, the previous crop did affect tissue P content significantly. As the season
progressed, the influence of crop management or P addition had a minor impact
on plant tissue P concentration (data not shown). Growing silage corn on either
fallow or stubble of a non-VAM colonizing crop like canola can result in early
season phosphorus (P) deficiencies that limit harvested silage yield and reduce
dry matter at maturity.

At harvest of the corn silage, the effect of previous crop was generally greater
for the unfertilized than the fertilized treatments, with significant differences
in 1998 (Table 1). While the differences were small, the trend over all of the
trials was for increased corn yield and earlier maturity, as shown by lower
percent dry matter (DM), after corn than after fallow or canola. The trend was
observed even when adequate P was applied.

The results of this research confirm what previous studies have shown. That
is, the colonization of corn by VAM is influenced by the previous crop in rotation.
They also provide new information indicating that the colonization of corn roots
by VAM was not negatively influenced by side-banded P application, a treatment
that in most instances improved the final silage yield and DM percent.

Early season colonization of corn roots by VAM had a positive effect on seedling
tissue P concentration. Side-banding P fertilizer can correct low P uptake associated
with poor colonization of corn roots with VAM. However, this may not fully compensate
for low P when there is poor root colonization. -30-

*Dr. Bittman is forage agronomist,
Dr. Kowalenko is soil scientist and Derek Hunt is senior research technician
at the Pacific Agri-Food Research Centre in Agassiz, British Columbia. E-mail:
bittmans@em.agr.ca