Managing Soil Phosphorous Accumulation From Poultry Litter Application Through Vegetable/Legume Rotations

Managing Soil Phosphorous Accumulation From Poultry Litter Application Through Vegetable/Legume Rotations

Summary

Applying poultry litter at rates sufficient to meet crop needs for N results in P accumulation that can lead to non-point source pollution of surface waters. Legumes are able to use significant amounts of P and K. An advantage of using legumes for removing excess P is that no additional N fertilizer has to be applied since legumes can obtain N from the atmosphere through N2 fixation. Including warm- and cool-season legumes for hay or silage may be one way to reduce excess soil P.

Objectives
1.) Investigate the use of warm- and cool-season legumes and legume-grass mixes in
rotational cropping systems to remove excess P supplied by poultry litter;

2.) Evaluate cool-season legumes for P uptake efficiency following litter application rates on spring vegetables

3.) Monitor P, and K accumulation, N leaching, and P runoff in a vegetable-forage legume rotation system

4.) Demonstrate use of annual legumes in cropping systems, utilizing poultry litter as a nutrient source, on grower-owned land under grower conditions.

Approach
Litter rates for all objectives were based on soil test nitrogen (N) requirement of the vegetable crop and percent N content of the litter. Litter was applied to the vegetable crop only. Treatments were incorporated immediately after application by power tilling.

Cumulative 1X litter rates over the study period were: obj. 1.) spring-11.3 tons/ac, fall-5.9 tons/ac; obj. 2.) 13.6 tons/ac; obj. 3.) spring-6.8 tons/ac, fall-5.9 tons/ac: obj. 4.) 4.0 tons/ac.

In objective 1 (Texas) the vegetable crops were: watermelons – spring 1995; broccoli – fall 1995; tomato – spring 1996; collards – fall 1996; squash – spring 1997; turnips – fall 1997. The spring legume crop was ‘Iron and Clay’ cowpeas and the fall crop was crimson clover. In Oklahoma the vegetable crops were: fall – broccoli, turnip, spinach: spring – sweet corn, muskmelon. The fall cover crop was hairy vetch and the spring crop was southern cowpea.

Dry matter yields of Iron and Clay cowpeas and crimson clover were not significantly affected by fertilizer treatment during the three year study period (1995-97).

Mean percent P increased over time for both legumes as rate increased (Iron and Clay cowpeas: control – .34%, 1X – .44%, 2X – .47%, 4X – .52%, commercial blend – .40%; crimson clover: control – .34%, 1X – .42%, 2X – .49%, 4X – .62%, commercial blend – .43%). Pounds per acre of P removed by both legumes also increased as rate increased (Iron and Clay cowpeas: control – 7.0 lbs, 1X – 9.0 lbs, 2X – 9.3 lbs, 4X – 11.4 lbs, commercial blend – 8.0 lbs. Crimson clover: control – 7.7 lbs, 1X-12.9 lbs, 2X – 13.2 lbs, 4X – 16.5 lbs, commercial blend – 11.3 lbs).

Average P accumulation in the 0-6 in. soil level over six seasons was less at the 1X (57 ppm) level of application than the 2X (112 ppm) and 4X (195 ppm). Phosphorus levels for the commercial blend (23 ppm) were equal to the control (21 ppm).

Utilizing a cropping system approach to reduce soil P accumulation proved to be effective. Mean data indicated that a system of spring vegetable-fall legume reduced P concentrations in the surface 0-6 in. of soil significantly to 48 ppm. Greater concentrations were found with systems of fall legume-spring vegetable (90 ppm) and spring vegetable- fall vegetable (96 ppm).

In Oklahoma under a cool-season vegetable rotation, cowpeas effectively lowered soil N levels but not soil P levels. In a warm-season vegetable rotation, hairy vetch appeared to raise soil N levels, but showed some evidence of controlling soil P levels. There was no buildup of soil P after two litter applications, even at the 2X rate.

In objective 2 the crops were: watermelon – 1995; sweet corn – 1996; tomato – 1997. In fall 1995, cool-season legumes consisting of crimson clover, berseem clover, hairy vetch, and red clover were seeded. Due to loss of stand of berseem clover because of freezing weather, a crimson clover-ryegrass mix was substituted in 1996.

Poultry litter rate showed no significant effect on mean dry matter yield of the four legumes. There was a significant effect by legume species on mean dry matter yield over time. Crimson clover-ryegrass mix produced 3,066 lbs/acre followed by hairy vetch with 2,012 lbs/acre. Crimson clover produced 1,361 lbs/acre and red clover 832 lbs/acre.

Mean plant P concentration in the 0-6 in soil depth increased as litter rate increased (control – .49%, 1X – .54%, 2X – .60%, 4X – .70%, commercial blend – .50%). Phosphorus uptake also increased as rate increased (control – 5.8 lbs/ac, 1X – 7.4 lbs/ac, 2X – 7.9 lbs/ac, 4X – 11.4 lbs/ac, commercial blend – 6.8 lbs/ac). Hairy vetch contained a mean percent plant P of .63% followed by crimson clover (.57%), crimson clover-ryegrass (.54%) and red clover (.48%). Phosphorus removal was greatest with a crimson clover-ryegrass mix (12.9 lbs/ac) followed by hairy vetch (10.2 lbs/ac), crimson clover (7.8 lbs/ac) and red clover (4.2 lbs/ac). Mean concentration of P in the 0-6 in. soil depth was reduced over time by hairy vetch to 77 ppm, followed by crimson clover (87 ppm), crimson clover-ryegrass (95 ppm) and red clover (103 ppm).

In objective 3 the vegetable crops were: turnip – fall 1995; sweet corn – spring 1996; turnip – fall 1996; watermelon – spring 1997. The cover crops were crimson clover and Iron and Clay cowpeas.

Due to a lengthy dry spell, there was not enough precipitation to collect run-off in either fall 1995 or spring 1996. In fall 1996 two major rainfall events occurred. Concentrations of P in the runoff were very low (< 0.8 ppm). No differences were found regardless of poultry litter rate or cropping system. This could be attributed to soil incorporation of the treatments. Mean P accumulation in the 0-6 in. soil depth increased from 70 ppm to 185 ppm when litter rate was increased from 1X to 4X. Phosphorus levels from the commercial blend remained close to that of the control (17.5 ppm and 27.0 ppm respectively). The least amount of residual P in the surface 0-6 in. soil depth was from a system of spring vegetable-fall legume (57 ppm) followed by spring vegetable-fall fallow (76 ppm) and spring legume-fall vegetable (92 ppm). Objective 4 was implemented in spring 1996 with the establishment of two demonstration plots. Litter at the rate of 4 tons/ac was applied. Tomato plants grown on plots with litter produced an average of 28 lbs of fruit per plant. Yield was not obtained for sweet corn but was reported that more ears were harvested from the poultry litter plots than the commercial fertilizer plots. The plot area that received litter produced 1,356 lbs/ac more vetch than that receiving commercial fertilizer. Results
Results have identified strategies that reduce non-point source pollution and soil imbalances and offer an opportunity for adoption of improved, environmentally sound management practices. Due to demonstrations of litter use in vegetable production programs, grower interest and awareness of the nutrient value of poultry litter has been increased.

Continued demonstrations will help show growers how a cropping system approach can be used to alleviate problems associated with litter use, especially P accumulation. Also, through outreach programs, we will continue to educate growers on nutrient management strategies through environmentally sound best management practices.

Contributions
Benefits to sustainable agriculture derived from this study include: 1.) Identifying new vegetable farming systems that are productive; 2.) Identification of strategies which reduce non-point source pollution and soil nutrient imbalances; 3.) Offer an opportunity for vegetable and other crop producers to adopt improved, environmentally sound management practices that do not sacrifice profitability; and 4.) Increase the data base that can be used by regulatory agencies to enact policies that are environmentally and producer friendly. These practices will help to conserve natural resources as well as condition, maintain, and conserve our most important natural resources, soil and water. Poultry litter’s value as a fertilizer will be enhanced instead of becoming an increasing environmental problem.

December 1998.

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