Options for Reducing Erosion and Phosphorus Losses

in Potato Systems

Final Report to Maine Potato Board

Tim Griffin

Wayne Honeycutt, and Gordon Starr

USDA-ARS New England Plant Soil and Water Laboratory

Matt Williams

University of Maine Cooperative Extension

Executive Summary:

Soil erosion and phosphorus (P) runoff can be severe in potato production systems in the Northeast U.S., which are characterized by intensive tillage, minimal ground cover, low crop residue return, and steep slopes. We used rainfall simulators in the greenhouse and field to assess sediment and P movement associated with three conservation practices: straw mulching, interseeding barley before potato harvest, and application of polyacrylamide (PAM). In the greenhouse, a sandy loam soil was packed boxes, and subjected to multiple rainfall events at intensity of 3 inches per hr. Runoff amount, sediment concentration, and inorganic and sediment-bound P were measured. Increasing straw mulch biomass (up to 3000 lb/acre) resulted in decreases in sediment and P loss. Mulch applied at rates as low as 750 lb/acre provided nearly 50% ground cover and reduced sediment movement by more than 50%. As expected, higher application rates reduced sediment loss by up to 95%. Results in the field, with mulch applied at 1500 lb/acre, were very consistent with greenhouse results. Increasing the application rate of PAM ( ranging from 0 to 20 lb/acre) also reduced sediment loss. The efficacy of this practice decreases slightly with successive rainfall events, but still had significant benefit through four simulated rainfalls. In the field, most of the benefit from PAM (at 15 lb/acre) was during the first rainfall event. Sunlight, fluctuating temperature and moisture, and surface roughness probably all contribute to the reduced effectiveness of PAM. Live barley biomass was also effective at reducing sediment and P movement, although high seeding rates are needed to overcome the lack of tillering in fall-planted barley. In general, runoff volume was not strongly influenced by any of these practices and most of the P loss was comprised of sediment-bound P. All three conservation practices are effective at reducing soil and nutrient loss in intensive potato systems.

Report of Results:

Growing long-season potatoes in a short-season environment makes soil conservation a challenge. Potatoes leave little residue after harvest, and this leaves soil uncovered from October until May or June of the following year. Several management options are either currently in use or being considered that could substantially improve soil conservation during these periods, including: interseeding small grains prior to potato harvest, application of mulch (straw or hay), and application of polyacrylamide (PAM), a polymer that binds soil particles together. We used rainfall simulators to establish the relative effectiveness and efficacy of these management options, directly measuring both sediment loads and phosphorus (P) movement. This included experiments both in the greenhouse and in the field.

The objective of the greenhouse experiments, which used field soil packed into boxes and exposed to rainfall, was to look a different rates of mulch, barley interseeding, and PAM application. Straw mulch to soil at rates equivalent to 0, 600, 1200, 1800, 2400 and 3000 lb/acre. Barley was planted at rates from 0 to 300 lb/acre, and PAM was applied at rates from 0 to 20 lb/acre. Rainfall was then applied to the boxes at a rate of 3 inches/hour (a very hard rain) until 30 minutes of continuous runoff occurred. This repeated three or four times for each set of treatments.

For applied mulch or growing barley, the key to reducing erosion is ground cover. The amount of ground cover in these experiments is shown in Figures 1 and 2 (below). The barley grew during the experiment, but higher seeding rates resulted in smaller plants. Even at the higher rates, ground cover from barley was less than 40%.

The amount of P contained in runoff water was higher in the mulch treatment (compared to bare soil), indicating that P was being leached from the mulch. However, most of the P lost during erosion and runoff is attached to soil particles. The movement of sediment is directly controlled by ground cover, as shown below. Higher mulch rates reduced soil movement by up to 90% (this is total soil loss for four rainfall events; results for individual rainfall events was similar). Similar simulations in the field, using mulch at 1500 lb/acre, had similar results. Growing barley as a cover crop also reduced erosion, but to a lesser extent because there was less ground cover. In addition, the lack of tillering in fall seeded barley (compared to spring seeded) requires that seeding rates be considerably higher – in the range of 200-300 lb/acre.

Polyacrylamide, or PAM, has been used to reduce irrigation-induced erosion in potato systems in the western US. Our interest was to see how well, and for how long, it reduced erosion on bare soil. In the greenhouse, PAM applied at 15 lb/acre reduced erosion by 85% during the first rainfall; it reduced erosion by more than 50% during the fourth rainfall (about 12 days later). In the field, however, most of the benefit was during the first rainfall after application; by the third rainfall, erosion from PAM-treated and untreated soils were similar. This was expected, as the degradation or breakdown of the PAM would be faster in field, as a result of sunlight and fluctuating moisture and temperature.

All three practices discussed here can reduce the movement of soil and P. Mulch is particularly effective, but can be costly and time-consuming to apply. The interseeding of barley has the lowest cost, is relatively easy (it can be seeded using a spin-spreader driving on spray rows), but probably has the highest risk. It also results in a non-uniform stand. It is not likely that PAM would be applied to entire fields, but it could be used for areas that are especially vulnerable to erosion, like steep slopes or field roads.