by Deborah Jeanne Sergeant
How can your farm better manage drought years? By improving soil health. A recent eOrganics webinar, “Preparing for Drought: The Role of Soil Health in Water Management in Organic Production,” addressed the topic.
Marck Schonbeck, Ph.D., with Organic Farming Research Foundation, was joined by Scott Park of Park Farming Organics to present research-based tips.
Soil health represents a big priority to many farmers, as well as irrigation and drought issues. Schonbeck said to combat water shortages, it’s important to manage soil health by designing drought-resistant systems, including cover cropping to improve water retention.
Production affects water quality and water quality affects production. Though it sounds paradoxical, Schonbeck explained that production methods can result in nitrate leaching, nutrient runoff, pathogen proliferation and soil erosion. Water quality can also affect crops through its salinity, alkalinity, sodium and pathogens.
“Organic farmers depend on healthy soil to protect water quality,” Schonbeck said. Soil properties affect plant-available water.
Schonbeck explained that during rainfall or irrigation, “water fills soil pore space from the surface downward. Excess water drains from larger pores. Plant-available capillary water remains in smaller pores.”
Schonbeck said soil texture, soil depth and profile, drainage and permeability and depth to restrictive layer all affect the plant-available water holding capacity (WHC).
Soil types make a big difference in WHC. Sandy soil has a lower WHC than clay soil, for example. Improving the quality of soil can help improve its WHC. Healthy soil allows rapid infiltration, retains moisture for unrestricted root growth and drains out excess water. Schonbeck explained how.
Each 1% of soil organic matter (SOM) adds a 4% increase of WHC.
“A network of pores open to the surface,” Schonbeck said. “Rainfall and irrigation infiltrate easily.”
This type of soil is well aggregated and has low bulk density so it drains well and facilitates root growth. Its high biological activity and biodiversity maintains SOM, soil structure and pore space. The entire soil profile is open to root growth so crops access deep moisture reserves that shallow roots can’t reach.
In contrast, compacted soil’s plant-available water is limited because of the crust on the soil. Less water is stored under the crust and the hardpan stops roots from extending.
“Rain enters the soil, but less is retained because of low SOM,” Schonbeck said. “Moisture and nutrients leach out below the root zone.”
Too much heavy rain can also negatively affect crops, including clogging soil pores, sealing the surface and causing soil erosion.
“Ponding or waterlogging damages plant roots, kills aerobic soil microbes, increases risk of crop disease, promotes certain weeds and forms greenhouse gases,” Schonbeck said.
Prolonged drought can cause long-lasting effects on soil health. Schonbeck said it causes soil life to go dormant. “Plant growth slows or stops,” he said. “Organic inputs diminish. Risks of wind erosion and fire increase. If a drought follows a wet spell, compaction may be severe and crops may be less resilient. Leaving residues in place during dry spells prevents wind erosion and protects soil health.”
That’s because the plants’ root systems hold onto the soil.
The NRCS soil health and water management principles include keeping soil covered to prevent crusting and to enhance infiltration; planting diverse crops, to build SOM and use water and nutrients efficiently; maintaining living roots to build SOM and WHC to create pore space; and minimizing soil disturbance to prevent compaction, crusting and erosion.
Composting and mulching can also add stable SOM and WHC. They “work with cover crops to build SOM and WHC and provide slow-release nutrients,” Schonbeck said.
While it may seem a great idea to pile on this beneficial material, more isn’t always better.
“Excess phosphorus inhibits mycorrhizal fungi, which may reduce crop drought resistance,” Schonbeck said. “Nutrient surpluses may run off to surface water and nitrogen may leach to groundwater.”
Schonbeck added that organic mulch, such as straw, can prevent crusting, improve infiltration, feed soil and save soil moisture by slowing evaporation and weed emergence, unlike plastic weed mats, which don’t offer these benefits, though it does block weeds and helps maintain moisture. Using plastic is allowed with organic management, as long as it’s removed by the end of the season. That can be labor intensive.
“Plastic saves a lot of labor with weed management and to help with precise water management,” he said. However, non-permeable plastic mulch can cause ponding.
Reducing tillage can also improve soil health. Schonbeck said mulch-tilling leaves, using ridge tilling or strip tilling and employing a rotary spader, rototiller or sweep plowing undercutter all leave more residue on the surface, which conserves moisture.
Rotationally grazing livestock can also help improve soil moisture.
“There’s nothing like management-intensive rotational grazing,” Schonebeck said. It maintains extensive, deep roots; builds SOM and WHC; yields drought resilient, high quality forage; and distributes manure and protects water quality. Rotational grazing systems that move herds to fresh grass daily help livestock operations withstand drought.
Weed control represents another important aspect of preparing for drought, since they steal away moisture from desired plants.
Schonbeck said shallow cultivation can prevent weed infestations and soil crusting. Organic weed integrated pest management can involve crop rotation, cover crops, preventing seed set, mulching, mowing, grazing and flame weeding.
Irrigation methods affect soil health. Overhead systems “tend to lose water to evaporation and can cause surface crusting,” Schonbeck said. “In-row drip irrigation delivers water more efficiently and gently to crops and reduces between-row weeds, but can also limit soil biological activity between rows.”
Many farmers tend to think of cover crops as more for grains than vegetables; however, they can offer many benefits for the latter, including nitrogen recovery, SOM and higher yield, explained Schonbeck.
But in dry areas or times of drought, cover cropping can produce less biomass, suffer from weed pressure and take moisture from cash crops. Plus, during fallow years, soil can lose SOM, WHC and fertility. Wind erosion increases.
The blade plow may prove helpful in both weed suppression and retaining moisture. Since it cuts just below the surface, it leaves residue on the surface, leaves soil profile undisturbed, save moisture, reduces wind erosion and improves crop yields over other tillage methods.
Park said he has saved on irrigation costs by eliminating the first and last 40 days of irrigating without compromising his crops. His produce quality is better and his farm’s yield is comparable to conventional methods of irrigation. He saves water by adding 15 to 20 tons of biomass every year to every field by using cover crops, compost and – depending on what’s following – crop residue.
“Building up soil health helps with quality, fertility and most pest issues,” he said.