Top Crop Manager

Features Agronomy Fertility and Nutrients
Balancing nutrient inputs with crop removal:

Applying fertilizer to match crop removal can work, but soil testing still forms the basis of a good fertility program.

November 19, 2007
By Bruce Barker


6aThe harvest weather of 2004 chewed in to the time usually allocated to fall
soil testing. As a result, many farmers are going into the spring without soil
test recommendations to base their fertilizer programs on. While the temptation
is to do a seat-of-the-pants guess, there are other tools that can be used to
help refine fertilizer recommendations.

Adrian Johnston, Northern Great Plains director with the Potash and Phosphate
Institute (PPI), says that soil testing is the basis of nutrient management
because over time, it gives important insights into how the fertility program
is impacting soil nutrient supply levels. However, in the absence of soil tests,
Johnston says that balancing nutrient inputs with removal is a good alternative.

"There were some very high volume crops taken off in 2004, and after a
couple of dry years with lower fertilizer applications, some of the fields are
now running low on reserves," says Johnston. "A soil test is still
the ideal way to develop a nutrient management program, but if a grower can't
get one done before spring seeding, replacing the nutrients that were taken
off can help guide application rates."


Johnston recommends getting started by gathering all available information
including past soil tests, nutrient removal estimates based on crop yields and
yield goals for 2005. In looking at nutrient removal estimates, researchers
have developed rule-of-thumb calculations for various nutrients.

For nitrogen in spring wheat crops, several options are available to determine
crop removal. If just the seed is harvested and the straw is returned to the
field, grain protein gives a good indication of nitrogen removal. Wheat grain
protein content, divided by a factor of 5.7 will indicate the nitrogen content
of the grain. So for a 50 bushel wheat crop with 13.5 percent protein content,
the nitrogen removal calculation would be: 50 bushels x 60 pounds per bushel
x (13.5 percent/5.7/100) = 71 pounds per acre, or 1.4 pounds per bushel. The
factor of 5.7 is unique to wheat. For all other grains, forage and straw, the
factor is 6.25 and nitrogen removal can be determined in a similar manner.

For phosphorus removal, past research indicates that 0.5lb/ac P2O5
is removed with the harvested wheat seed. With a 50 bushel wheat crop, that
amounts to 25lb/ac of P2O5 removal.
When conducting a nutrient balance, Johnston says that it is important to account
for all removals. This includes the straw baled for livestock bedding, total
crop removal for silage and burning of crop residues, for example. Alternatively,
the Canadian Fertilizer Institute and PPI both have published general crop removal
estimates of various nutrients.

In the absence of a soil test, fertilizing to meet crop removal can work. Obviously,
the crop rotation does determine fertilizer rates. For the 50 bushel wheat crop
that removed 71 pounds of nitrogen per acre, if peas or other legumes that fix
most of the nitrogen were planned for 2005, nitrogen fertilizer would not be
applied. Similarly, on fields where legumes were grown that fixed all their
nitrogen, using crop removal estimates would not work. As an example, upwards
of 125 pounds of N are removed with a 50 bushel pea crop. You would not necessarily
apply 125 pounds of N for the subsequent wheat crop. Rather, you would target
the yield of the wheat and assess the amount of N required to grow that crop.
For a 50 bushel crop, you might want to aim at around 70 to 80 pounds of N.
That is where common sense comes in.

Another difficulty is found where soils have a history of manure application.
On these fields, estimating the amount of fertilizer that should be applied
becomes more difficult in the absence of a soil test.

New tool helps guide fertilizer recommendations
A new software program developed by Westco is helping to provide further guidance.
Called Virtual Soil Testing (VST), the program is helping Westco's member agronomists
to further refine fertilizer recommendations when soil tests are not available.

"VST is a tool that supplements conventional soil tests. It is based on
soil test information from the previous year and growing season characteristics,
yield, protein, rainfall, and so on, and helps make a qualified guess when a
soil test is not available," explains Rigas Karamanos, agronomy manager
with Westco at Calgary, Alberta.

As a crop removal estimating program, VST goes one step further by estimating
N mineralization, which is missing from a simple removal calculation. It includes
allowances for organic matter content of the soil and makes estimates of the
amount of nitrogen that will be mineralized (released) from the organic matter
in the soil. "No other estimating system is operating with organic matter
corrections. We had to calibrate the recommendations for nitrogen based on the
release of nitrogen from the organic matter."

VST generates soil test values for N, P, K and S, which are subsequently used
to derive fertilizer recommendations. Karamanos says that for farmers who have
had a good fertility program over the years with sufficient levels of nutrients,
basing fertilizer recommendations on crop removal can work very well. "You
have to start with satisfactory levels of nutrients in the soil and a history
of soil tests to guide the decisions," cautions Karamanos.

Karamanos ground-truthed VST by conducting research in Alberta, Saskatchewan
and Manitoba between 1999 and 2002. He also compared the recommendations generated
from VST to those by six different soil test laboratories operating in the region.
In 10 of 12 wheat and nine of 10 canola trials, the VST resulted in maximum
yield, and in 18 of 22 experiments in total, it generated the maximum net return
(revenue less fertilizer input expenses).

Another caution with using nutrient input and removal balances is ensuring
that yield accurately reflects crop removal. For example, fields badly frozen
in 2004 might have had yields half their normal, but using that yield would
not necessarily reflect the amount of nutrients removed from the field, or help
guide 2005 fertilizer rates.

At PPI, the organization has also developed an Excel spreadsheet program called
PKalc that was developed to help crop advisors and farmers balance P and K (potassium)
removal from individual fields. It calculates an estimate of net change in soil
P and K as a result of management practices. That program is valuable for helping
to understand P and K management under local conditions, and impacts on crop
production and the environment.

Karamanos says that in reality, farmers have been doing their own version of
nutrient removal and input balancing, and that VST is just an additional tool
to more accurately consider N mineralization. "It's like a cheque book
where you balance income and expenditures. We like to see soil testing as part
of the fertility program. VST is a supplement, not a replacement." -30-

Table 1. Nutrient requirements of crops (pounds per acre).

Crop N P2O5 K2O S
Wheat 40bu/ac Uptake 76 to 93 29 to 35 65 to 80 8 to 10
Removal 54 to 66 21 to 26 16 to 19 4 to 5
lb/ac removal 1.1 to 1.5 0.50 0.35 n/a
Barley 80bu/ac Uptake 100 to 122 40 to 49 96 to 117 12 to 14
Removal 70 to 85 30 to 37 23 to 28 6 to 8
lb/ac removal 1.10 0.40 0.35 n/a
Oats 100bu/ac Uptake 96 to 117 36 to 45 131 to 160 12 to 14
Removal 55 to 68 23 to 28 17 to 20 4 to 5
lb/ac removal 0.80 0.25 0.20 n/a
Canola 35bu/ac Uptake 100 to 123 46 to 57 73 to 89 17 to 21
Removal 61 to 74 33 to 40 16 to 20 10 to 12
lb/ac removal 1.88 0.91 0.46 n/a
Flax 24bu/ac Uptake 62 to 76 18 to 22 39 to 48 12 to 15
Removal 46 to 56 14 to 17 13 to 16 5 to 6
lb/ac removal 2.00 1.10 0.65 n/a
Pea 50bu/ac Uptake 138 to 168 38 to 46 123 to 150 11 to 14
Removal 105 to 129 31 to 38 32 to 39 6 to 7
lb/ac removal 2.40 1.20 0.71 n/a
Lentils 30bu/ac Uptake 82 to 101 22 to 27 69 to 84 8 to 10
Removal 55 to 67 17 to 20 29 to 36 4 to 5
lb/ac removal 2.00 0.62 1.10 n/a
Soybean 50bu/ac Uptake 230 to 290 40 to 50 120 to 220 17
Removal 187 to 200 40 to 44 69 to 70 5
lb/ac removal 4.00 0.80 1.40 n/a
Sources: Canadian Fertilizer Institute and PPI. Note:
Legumes fix most of their nitrogen from the air.