Features Agronomy Fertility and Nutrients
Up in smoke – nutrient loss with straw burning

In Manitoba, there is considerable controversy regarding the value and cost
of returning straw from crops to the soil. In much of the province, ample growing
season moisture produces high straw yields. This straw is sometimes burned in
the fall when it cannot be marketed for fibre, or when it impairs tillage and
seeding operations the next spring. The most obvious consequence of straw removal
or burning is the loss of plant nutrients.

Past work on straw management in this region has estimated that straw burning
produced total loss of N and S, with no loss of P and K. As a result, subsequent
guidelines have considered this the standard nutrient loss from burning. To
clarify this estimate, a study was carried out to evaluate the fertility value
of straw and the losses that occur during removal or burning.

Spring wheat, oats and flax straw samples were collected in three regions of
Manitoba with a portion retained for straw nutrient analysis and the remaining
portion burned on a steel grate to allow retention and collection of the resulting
ash. Ash weight from the burn was determined, and the resulting straw and ash
samples were submitted for analysis of total carbon (C), N, P, K and S.

Straw samples were between four to six percent moisture content and much of
the straw mass was lost during burning. The amount of straw weight lost through
burning varied greatly among sources, with flax burning more completely and
only four percent of this mass remained as ash versus eight percent for oats
and 13 percent for wheat.

The nutrient concentration in straw and resulting ash is presented in Table
1. The amount of C in straw varied little within straw types. The amount of
C remaining in the ash varied more as a result of the degree of combustion (where
less combustion, more C remained). The N content of straw generally varied more
than othernutrients. Variation in straw nutrient content is expected as it reflects
the differing management and fertility regimes the crop is grown under. Nitrogen
concentration of the ash is similar in magnitude to the concentration in straw.
Unlike N, the P, K and S tended to be concentrated two to 10 times more in ash
than in the original straw. This concentration of nutrients indicates increased
retention in the ash left after the burning was carried out.

Nutrient loss through burning is illustrated in Table 2, where the amount of
nutrients present in one tonne of straw is compared before and after burning.
Carbon and N loss due to burning was greater than 90 percent across all straw
types and sources. On average, 98 to 100 percent of the N, 24 percent of the
P, 35 percent of the K and 75 percent of the S were lost through burning.

While the loss of N and S with burning agrees with previous assumptions, the
primary question asked from these results is: Where did 24 percent of the P
and 35 percent of the K go? It is likely that most of the loss was smoke or
particulate matter that drifted away from the fire, since no attempt was made
to collect or retain it. There is some possibility that this particulate matter
may settle down over the field being burned – but this will depend on wind
and other smoke dispersion factors. Other factors like high temperature volatilization
of K may explain the loss, but are less likely.

Determining the economic impact of burning straw may be as difficult as when
straw is baled. A complete job of burning converts the vast majority of all
above-ground straw and chaff to ash, while baling removes only a portion of
the straw and usually no chaff. However, the usual objective is to burn only
that excess straw that is dropped in the swath, leaving stubble intact between
swaths. Such burning practices can also influence nutrient distribution in a
field, especially when straw is burned in rows dropped behind a combine. This
will cause nutrients to concentrate along this row position in the field. Soil
sampling should avoid any cores from these ash rows.

The variability in straw nutrient content observed in this study supports the
argument that straw nutrient content is largely influenced by the growerÕs fertility
management.

Caution when soil sampling burned fields
Burning crop residue to improve equipment operation is a common practice on
no-till fields in parts of the northern Great Plains. However, one must be careful
when soil sampling fields where crop residue has been burned in the windrows.
An agronomist working in northeast Saskatchewan reported that a composite soil
sample from a burned field gave a false reading on soil test K.

While the field composite reading was 223 parts per million (ppm) K, further
sampling found that 25 percent of the field where the windrows were burned was
325ppm, while the remaining 75 percent of the field was 114ppm. So, be cautious
of misleading results when sampling burned fields.

John Heard, CCA, is provincial soil fertility extension specialist;
Curtis Cavers, CCA, is provincial land management extension specialist;
Greg Adrian is a soil survey technician, all with Manitoba Agriculture,
Food and Rural Initiatives in Carman.

Reprinted from Better Crops with Plant Food with permission of
PPI/PPIC.