Precision Agriculture

What terraces have to do with variability of improved grazed pastures?

Jesús Santillano-Cázares, Spring 2006.

Among the causes of variability in pastures that have received the most attention are:Patterns of deposition of animal excretaGrazing methods and stocking ratesTopographyAspectWeatherLong-term fertilizer management.

The many sources that cause variability to increase in pastures seem to be almost case specific, or at least, region specific.

In eastern Oklahoma the existence of man-made microreliefs (terraces)

that were built to control soil erosion and water conservation around 1930 and afterwards (Carberry, 1934; Cox et al., 1951); may be contributing to the, already, typically high variability of pastures. We believe close attention needs to be paid to identify local or regional causes that may be further increasing the naturally high variability of grazed pastures.

One objective (of several in the study)

To determine how fertilizer applications and microrelief points affect soil nutrients in terraced pastures.

Research is being conducted at the Eastern Research Station located near Haskell OK. The station consists of 298 acres and is located 14 miles west of Muskogee at the intersection of Highways 16 and 72.

Materials and MethodsLocation

Data collection site description

Four exclosures (30 X 100 m) located in mixed grass-legume pastures were constructed to help control grazing and to fix our sites of data collection (plots).

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

0 20 40 60 80 100 120

Distance from reference point (m)

Dep

th in

rel

atio

n t

o r

efer

ence

po

int

(m)

Exclosure No. 1

Exclosure No. 4

Exclosure No. 3

Exclosure No. 2Slopes of exclosures

No. 1 = 1.5 % No. 2 = 1.8 %No. 3 = 2.8 %No. 4 = 3.7 %

Slope

Site DescriptionEach exclosure was oriented to include most of the

variability of the pastures. Each exclosure was relatively long and laid out parallel with the natural slope of the land and perpendicular to water retention terraces.

Two way factorial in a RCBD with four reps.

10 treatments and 2 sampling seasons/year repeated in 4 pastures (during 2 years).

Treatments: Fertilizer applications of: N, P, K, NPK + lime, and a control (no fertilizer applied). Microrelief points: top of terrace on the channel side, top of terrace opposite to the channel, back of terrace, channel of terrace, and flat interval between terraces.

Sampling seasons: Winter and Spring.

Treatments & Design Structure

N NPK+L Control K P

FlatTops of terraceChannel Back side of terrace

Microrelief profile

Patterns of soil sampling to test soil fertility parameters among fertilizer treatments (left) and microrelief points (right)

Results of two years of data collection

Response Variable Source DF SignificanceTreatment 9 NSSeason 1 ***Treat. X Season 9 NSTreatment 9 ***Season 1 ***Treat. X Season 9 NSTreatment 9 ***Season 1 ***Treat. X Season 9 NS

NS, Not significant at P < 0.05.* Significant at P < 0.05.** Significant at P < 0.01.*** Significant at P < 0.001.

Soil N

Soil P

Soil K

Neither fertilizer treatments nor microrelief points influence soil N. Because plant beneficiated from N-fertilizer to increase forage yield and a redistribution effect of animal deposition across the exclosures, no differences from N-fertilized to no N-fertilized treatments were found at the end of the grazing season in soil N content.

Season of sampling was significant; more soil N in winter samples that in early spring.

Soil Nitrogen

Soil Phosphorus

89 Winter

69 Spring

Mean (P Index)

Season

89 Winter

69 Spring

Mean (P Index)

Season

125P69Top Terr. “a”75Top Terr. “b”78Back Terr.

105Channel Terr.

52Control

69Flat interval

104NPK + L54N58 K

Mean Treatment

125P69Top Terr. “a”75Top Terr. “b”78Back Terr.

105Channel Terr.

52Control

69Flat interval

104NPK + L54N58 K

Mean Treatment

Again Winter higher than Spring.

Terrace channels have the same high soil P concentration as P- fertilized treatments!

Soil phosphorus (P) concentration as function of microrelief and time

0

20

40

60

80

100

120

140

160

Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06

P in

de

x (

lb/a

c)

top terrace a top terrace b tip terrace

channel of terrace flat undisturbed soil

Soil Potassium

And again, Winter higher than Spring.

Terrace channels have more soil K than K- fertilized treatments!

387 Winter

326 Spring

Mean (K Index)

Season

387 Winter

326 Spring

Mean (K Index)

Season

306P265Top Terr. “a”

271Top Terr. “b”

325Back Terr.

545Channel Terr.

331Control

349Flat interval

382NPK + L

303N

485 K

Mean Treatment

306P265Top Terr. “a”

271Top Terr. “b”

325Back Terr.

545Channel Terr.

331Control

349Flat interval

382NPK + L

303N

485 K

Mean Treatment

Soil potassium (K) concentration as function of microrelief and time

200

300

400

500

600

700

800

Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06

K i

nd

ex (

lb/a

c)

top terrace a top terrace b tip terrace

channel of terrace flat undisturbed soil

As additional information…Forage yield

Fertilizer treatments and microrelief highly significant (P<0.0001). Interaction, not significant.

2679 Control

2506 P

2746K

4199 NPK+L

4104 N

Mean (kg/ha)

Treatment

2679 Control

2506 P

2746K

4199 NPK+L

4104 N

Mean (kg/ha)

Treatment

3568 Channel Terr.

2348 Flat interval

3229Back Terr.

3765 Top Terr. “b”

3326 Top Terr. “a”

Mean (kg/ha)

Treatment

3568 Channel Terr.

2348 Flat interval

3229Back Terr.

3765 Top Terr. “b”

3326 Top Terr. “a”

Mean (kg/ha)

Treatment

1. Season of sampling was significant for all three soil nutrients analyzed. Soil nutrients contents (N, P , or K), were consistently higher in samples collected during the winter; although, we need to check the results of this spring to make sure this is consistent.

If consistent, this may be explained by a possible dilution of nutrients from dung and urine spots caused by an extended time of weathering in the spring soil samples, compared to the ones collected during the winter; the latter possibly including highly nutrient concentrated soil cores from relatively fresh urine spots.

Conclusions

2. More reliable soil samples would be obtained if avoided collecting soil cores from the terrace channels. Terrace channels appear to catch nutrients from higher elevations of the microrelief. Being this pastures densely covered with grass, this accumulation in the terrace channels is suggested to occur as a result of runoff water enriched with nutrients from dung and diluted applied fertilizers over a long period of time.

Conclusions…

In general: For more reliable soil test results in terraced pastures, some considerations seems to be important to take into consideration:

1. Sample in a fixed area of the pasture throughout the years, as we have done using our exclosures, would help reduce variability.

2. Sample during a time where enough time and rain has occurred after cattle has been removed from the pasture to allow dung and urine patches to disperse.

3. If decided to fertilize only terraces, don’t sample flats (fertilizing flats doesn’t pay!), and sample anywhere in the terraces, except the channels.

Take home message

Thank you