Features Agronomy Fertility and Nutrients
Compost makes a good P fertilizer for potatoes

Research at the Crop Diversification Centre (CDC) South at Brooks, Alberta,
shows that potato growers do not need to shy away from using manure compost
on their potatoes. The phosphorus (P) is readily available to the crops in the
year of application.

"We were worried that the organic phosphorus in the compost would not
mineralize fast enough to meet early season phosphorus needs of the potato crop,"
says Shelley Woods, soil and water research scientist at CDC South, a division
of Alberta Agriculture, Food and Rural Development. "From the evidence
that we saw, you don't need to add fertilizer phosphorus to potatoes if you
are using compost."

Colin McKenzie, a CDC scientist who passed away several years ago, initiated
the research. Woods summarized the information and says the research was conducted
because potato growers were concerned about P availability with compost, and
wanted to know if disease pressure might also be higher when they use it. "Potatoes
are such a high value crop, growers don't take chances with their fertility
program," says Woods.

In the trials, conducted in growers' fields over several years at six sites
in southern Alberta, compost was compared to mineral phosphate fertilizer to
examine the effects on disease, yield and quality of potatoes. At the same time,
potato petiole tissue sampling for P was conducted to see if there was a correlation
between P applications, P uptake and eventual yield. Total plot sizes ranged
from 12 to 23 acres. Each year, on each field, petiole samples were collected
at three dates through the growing season: early July, late July and early to
mid-August.

The compost and phosphate fertilizer treatments were surface-applied and incorporated
in the spring. Fertilizer nitrogen was added according to recommendations.

Compost P available for crop growth
Results from the trials show there was no significant difference in yield, disease
or specific gravity, whether phosphate fertilizer or compost was used as a source
for P nutrition. A typical example from the 2000 Cranford, Alberta site shows
that similar rates of phosphate fertilizer and compost gave similar yields.
For example, phosphate fertilizer applied at a rate of 110 pounds per acre gave
a yield of 19.3 tons per acre, specific gravity of 1.091 and a rhizoctonia rating
of 0.945 percent of tuber surfaces. A similar P205
rate in the compost (91 pounds per acre) gave a yield of 19.7 tons per acre,
specific gravity of 1.091 and rhizoctonia rating of 1.105 percent. The initial
soil test at this site showed 108 pounds per acre P205
in the top six inch soil sample depth.

One exception was at the 2000 Fincastle, Alberta site, where yields for the
compost treatments were higher overall than for phosphate fertilizer treatments.
At this site, the compost treatments also received a phosphate fertilizer treatment.
The soil test levels at this site were 66 pounds per acre P205.
Whether the lower P levels in the soil, in combination with the added phosphate
fertilizer, contributed to the better yield is unclear, especially in light
of the compost-only treatment of 130 pounds per acre P205,
which yielded better than any of the phosphate fertilizer only treatments.

Looking at all of the results, though, compost seems like a good option for
potato growers. "Given the results, some of the P in the compost was immediately
plant-available, even in early July," says Woods.

Petiole P test may not be valid
An interesting side-note to the results was that although the petiole P tissue
test was strongly correlated to P application rates and uptake, the increased
P content in the tissue did not translate into higher yield.

"The results indicate that increased inputs of P gave increased levels
of petiole P for both mineral fertilizer and compost. The lowest application
rate used in the experiment (50 pounds per acre P205)
usually translated to a small increase in yield (less than one ton per acre),
but for the larger application rates, there were no discernible effects on yield.
That calls into question current fertilizer and petiole P recommendations,"
says Woods.

As a result, Woods is now working with the Potato Growers of Alberta (PGA)
to look at developing specific fertility recommendations for southern Alberta.
That PGA sponsored research is on-going. Many of the current recommendations
and grower practices are based on information from the US. As part of that study,
Woods hopes to be able to see if petiole P standards can be tailored to southern
Alberta conditions.

For potato growers developing a fertility plan based on composted manure, starting
with a soil test to understand the P levels in the soil is the first step. Another
critical part of the plan is a compost test to understand the nutrient composition
so that application recommendations can be developed based on crop needs. After
that, working with fertilizer dealers to develop an appropriate fertility plan
is the best way to proceed until new recommendations are developed. Given the
large amount of manure available in southern Alberta, composted manure on potatoes
may be a good option for utilizing this valuable resource. -30-