Reducing Sediment and Nutrient Runoff: Problems

soil water

In 2019, Lake Erie had the 5th largest Harmful Algae Bloom (HAB) due to record water runoff and 2-3 feet higher water levels than normal. Lake Erie also had the highest total sediment, total nitrogen, and total phosphorus loads ever recorded, but the extra water diluted the sediment and nutrient concentrations, possibly reducing the impact. Examining the facts and problems may offer some possible solutions.

About 60% of phosphorus (P) nutrient loads come from the Maumee River watershed (Johnson, 2018). Roughly 30% of P comes from surface runoff and 70% from subsurface (tile) runoff through preferential flow (soil cracks, crevices) with about 90% of the losses occurring during the most intense rainfall events (Watters & Hoorman, 2018, Fussell et al. 2017). Depending on the soil and landscape, 60 -90% of P come from 10-40% of land, generally on soils located close to or with access for transportation to ditches and streams. Average Ohio P loss is 1-1.2# P/acre with a goal of losing less than .4# P/acre. Farmers are roughly 97% efficient at keeping P fertilizer on the land (assumes 1.2# of P lost for 35# of P fertilizer applied). As much as 50-70% of soluble soil nutrients (N & P) may be lost in late winter (snow melt) & early spring (heavy rains) according to Owen et al. 1995. These facts vary by soil type and local conditions. The following factors are important:

Weather: Rainfall events are more numerous, larger intensity and longer duration since 1970’s. The soluble reactive phosphorus (SRP) flows with the water but Particulate P (soil attached P) also erodes from the soil and is 30% bioavailable in surface runoff. Soils lose most SRP during the most intense high rainfall events.

Tillage: Average Ohio soil erosion rates are 2.61 tons per acre or 1.7 pounds of topsoil lost for every pound of soybean produced based on 50-bushel average yield (USDA-NRI, 2015). Tillage increases soil erosion by sealing off the soil surface. Vertically tillage on conventional fields have about five times more sediment losses and three times more SRP losses than long-term no-till fields (Smith et al. 2015). Tillage also causes poor soil structure (compaction) and the soil to shrink and swell, leading to higher preferential flow. Vertical tillage (2 to 4 inches deep) popularity has increased since 1995, causing new shallow plow pans to form and increased surface runoff.

Stratification/Placement of P at Soil Surface: Due to larger farms, most P fertilizer is broadcast on the soil surface, causing P to runoff during high rainfall events from soil sealing from tillage. It is not uncommon to see standing water in fields now after light rains. The form of P fertilizer has changed too with DAP and MAP being more soluble than the old triple super phosphate fertilizer. Some researchers advocate incorporation (heavy tillage) to mix and dilute SRP in soil surface. Tillage tries to fix the SRP problem but may cause excessive soil erosion and then seals the soil surface, breaking down soil structure, and decreasing water infiltration.

How is P stored in the soil? About 50-70% of soil P and 85-90% of N is stored organically in soil organic matter and this form is plant available. Aluminum (AL), Iron (Fe), Calcium and Magnesium (Ca/Mg) store P inorganically, but NONE is directly plant available. Al ties P up tightly, Ca/Mg less tightly. Fe has two forms: Ferric P (Fe3+) and Ferrous P (Fe2+) which are unstable. Ohio soils have about 1.43% iron content. Ferric-P (soil looks red/pink) and becomes Ferrous-P (blue gray) under saturated (wet) soil conditions, releasing SRP which may be lost to surface water or through preferential flow to tile water. However, when the soil dries, iron can steal P from Ca/Mg, which means it can be lost again with another rainfall event. This explains why Ohio SRP concentrations remain steady and high even after many rainfall events.

Rainfall pH: Another confounding factor, the 1995 Clean air act changed rainfall water pH from 4.2 (acid rain) to 6.2 (more alkaline) today. The main effect is that iron (Fe) in the soil now holds on even less tightly to SRP when it rains due to changes in rainfall pH (Smith et al. 2017)! This may explain why even forested land is losing SRP to surface water. While we may have cleaner air, the unintended consequence may be more SRP in surface water. We’ll look at possible solutions next time.

Part 2

Dealing with sediment and nutrient runoff issues is complicated. The 4R’s (Right Source, Rate, Place and Time) are15-30% effective at keeping soluble nutrients out of surface water and fails to address how soil nutrients are stored. Our carbon (C), nitrogen(N), and phosphorus (P) cycles may be damaged, so water quality improvements may be difficult until these biological cycles are restored.

Soil organic matter (SOM or carbon) is a warehouse for holding water and soluble nutrients and buffers soil pH. The cation exchange capacity (ability to hold soluble positive soil nutrients) is 9- 10X higher in SOM than in clay soil particles. As our soils lose their SOM (50-80% loss due to tillage), soil function declines and our soils become leaky. If this is really a biological problem, then maybe the 4R’s and technology might not be the best overall solution. What evidence exists to show that biological solutions may solve this problem?

First, Brady and Weil, (Nature and Properties of Soils) show that conventional tilled soils with low SOM had 8X higher soil erosion and soluble nutrient loss than undisturbed natural soils. Both had high P soil test, but the natural undisturbed soil had more organic P (P tied up by carbon) while the tilled soil was mostly inorganic P. Disturbed tilled soils are leaky and promote poor soil structure and higher sediment and soluble nutrient runoff.

Second, is there anywhere in the USA where biological solutions have been successful? The Chesapeake Bay region has promoted cover crops and long-term no-till for 15 years, resulting in a 42 45% improvement in water quality. The Chesapeake Bay is a huge body of water and large watershed; so it took 10-15 years for the soluble nutrients and sediment to move to the bay. The good news for Lake Erie (and possibly other smaller watersheds), is that the water retention time is shorter (two years for Lake Erie), so significant change may occur quickly.

Third, Smith et al. 2015 showed that long-term no-till fields had 3x less SRP, 3x less total P, and 5x less sediment loss than conventional tilled soil. Most research shows that using biological solutions and plants (buffers, filter strips, wetlands, cover crops, prairie strips) are effective, but large areas need to be covered to see positive results.

USDA-NRCS S has promoted Four Major Soil Health Principles that addresses soil erosion and nutrient runoff and restores soil function while producing economical high yields. First Principle: Maximize soil cover to reduce soil erosion and to reduce rain drop impact. A raindrop on bare soil may move soil 2 feet high and 4 feet out, displacing both soil and soluble soil nutrients.

Second Principle: Minimize soil disturbance to maintain soil structure and water infiltration AND soil connectivity to subsoil. Long-term no-till soils allow water to flow slowly and evenly into soil so that nutrients are absorbed by either plant roots, soil microbes, or soil minerology. Tillage ruins soil structure (causing compaction) and decreases water infiltration, causing water to runoff the soil surface OR flow by preferential flow (PF) to surface water via tile discharge.

Third Principle: Maximize live roots to 1) increase soil pore space, water infiltration, and water storage; 2) to fill soil cracks, earthworm holes, and large voids to prevent PF, 3) to allow live roots to absorb soluble N & P and 4) to increase SOM. Most soluble nutrients are absorbed and recycled by the soil microbes and live roots which together build SOM. There are 1000-2000X more soil microbes near live roots and each microbe is just a soluble bag of fertilizer feeding the plant.

Fourth Principle: Maximize Biodiversity with multispecies cover crops and diverse crop rotations. Grasses have fibrous roots and broadleaves have taproots which move water into the soil profile. Soluble nutrients are stored in living plant tissue during the fall, winter, and early spring months and released in the summer to fertilize our grain crops.

In stable undisturbed soils, arbuscular mycorrhizae fungi (AMF) flourish, forming a soil network that transports nutrients and water to the plant in exchange for sugars. AMF supply the plant with 6X more P than the plant can obtain by itself, greatly reducing the need for P fertilizer. Tillage, bare fallow soils, fungicides, and over P fertilization cause AMF levels to decline in the soil. The Four Major Soil Health Principles allow AMF and biological diversity to flourish, decreasing the overall need for fertilizer and restores our broken C-N-P cycles.