Dec 17, 2020
Keeping soils fit for food farming
Soil health sounds simple but it’s based on soil organic matter and the processes affecting soil organic matter are complicated with many interrelationships.
That’s the message of Laurie Drinkwater, professor in Cornell University’s Horticulture Section in the School of Integrative Plant Science. Drinkwater spoke on soil health at the Great Lakes Fruit, Vegetable & Farm Market EXPO in Grand Rapids, Michigan last December.
The four Natural Resources Conservation Service soil health management goals are a good starting point, Drinkwater said, and all four affect soil organic matter.
The goals are to keep the soil covered as much as possible; disturb the soil as little as possible; keep plants growing throughout the year to feed the soil; and diversify as much as possible using crop rotation and cover crops. “How do we fine-tune our management practices to achieve these goals?” Drinkwater asked.
Soil organic matter consists of organic materials at differing stages of decay.
Typically, one-third to one-half of the organic material in the soil is stabilized organic matter in soil aggregates. Another one-third to one-half is actively decomposing organic matter. Undecomposed crop residue plus a few soil microorganisms make up most of the rest.
Understanding the rhizosphere is a key concept in soil health.
The rhizosphere is the plant-soil interface where roots meet the soil. The roots secrete carbon, called exudates – which contain sugars, acids and enzymes. Microbes and other soil organisms are abundant in the rhizosphere and the rate of microbial activity rate is 10 times higher than in the rest of the soil profile.
The organic matter added to the soil by plant roots is protected through two paths – physical and chemical. Physical protection results from the formation of aggregates which is promoted by sticky gels released by plant roots and microbes. Chemical protection results from chemical bonds that are difficult to break and are resistant to decomposition because few microbes are able to produce enzymes that break these bonds.
The concentrated microbial activity in the rhizosphere decomposes organic matter and also stabilizes it into aggregates. This trapping of plant organic matter into aggregates is an active process and temporarily protects the organic matter from further decomposition. These soil aggregates improve soil structure creating pore space in the soil and improving soil tilth, which improves aeration and helps the soil hold water.
“You’re constantly having aggregates forming and biodegrading in your soil,” Drinkwater said. Aggregate breakdown and decomposition of the organic matter that was inside of them releases nutrients which stimulate plant growth, which, in turn, adds more organic material to the soil.
The question then becomes, “what can growers do to foster these processes?”
Cover crops offer potential and Cornell is researching different types. “Can we target our cover crops to target specific types of microbial aggregation, and pathways of stabilization?” Drinkwater asked. “Is one cover crop better for the physical release of nutrients? Are other ones better for chemical recalcitrance on sandy soils where you do not have enough clay to form aggregates to protect organic matter?”
Cornell researchers have found, in general, perennial legumes are better at building soil organic matter than perennial grasses. “The amount of physically protected soil organic matter is greatly improved with legumes,” Drinkwater said. “All cover crop species do not have this positive benefit, but most legumes, and most grass and legume mixes, do.”
Legumes have deep taproots and roots are better at building soil organic matter than incorporated shoots.
Minimizing soil disturbance to protect soil aggregates is a good goal until you need to till to prepare a seedbed for planting vegetables.
Looking at strip tillage
Research on no-till vegetables has shown the practice often has lower yields, said Dan Brainard, Michigan State University (MSU) associate professor in the Horticulture Department. Brainard also spoke on soil health at the Great Lakes EXPO. No-till vegetables also often have the problems of poor stand establishment, delayed maturity, and pest and weed issues. The only major exceptions to the no-till disadvantage are asparagus and pumpkins.
MSU researchers are conducting a long-term study on planting vegetables using strip tillage. Strip tillage involves a tilled in-row zone and untilled area between the rows and is a compromise between no-till and conventional tillage.
The tilled in-row zone is warmer, has higher nitrogen mineralization, has a good seed bed and allows fertilizer incorporation. The untilled between-row zone retains soil moisture, protects the soil from erosion, provides habitat for beneficials and improves soil health.
The long-term study is a rotation of sweet corn, snap beans and cucurbits. The study compares conventional tillage with strip tillage with and without a rye cover crop. The cover crop is established in September.
The long-term research is showing an increase in soil organic matter, but it is a slow process. Six years of strip tillage plus a rye cover crop increased the soil organic matter at the soil surface by about one percent.
The system has reduced tillage costs, improved water infiltration, and reduced risk of drought stress, but it also has cooler soils and makes weed control more complicated.
The yields of plowing compared to strip tillage, understandably, vary with the crop and the year. “We live in a variable climate, so we get variable results,” Brainard said.
The greatest benefits to strip tillage with a rye cover crop have been in cool season crops in hot, dry years. The greatest challenges to the system are in nitrogen-loving crops that have limited weed management options. “You’re removing tillage which is a main method of weed control and you need to compensate for that somehow,” Brainard said.
The rye cover crop poses challenges – mainly the crop interface. Thick cover crop residue can delay planting and reduce crop stand establishment and yield.
Less nitrogen availability is an issue in the short-term because the large amounts of mulch sometimes result in high rates of nitrogen immobilization. The rye cover crop may also be an alternate host for pests such as nematodes in sweet corn.
The strip tillage system has shown the benefit of reduced soil splash during rain or irrigation which has reduced fruit staining and disease.
Organic systems’ concerns
The strive for soil quality is especially important for organic producers, said Doug Collins, Washington State University (WSU) Extension specialist. Collins also spoke on soil health at the Great Lakes EXPO. Collins has worked extensively with organic growers in the Pacific Northwest.
“Tillage is important, especially in organic systems,” Collins said. Washington’s northern location means a shorter growing season and the west part of the state has the rains rolling in from the Pacific. Reduced tillage in that environment means significantly cooler soils. “Anything we do to reduce soil temperature will be problematic,” Collins said.
Terminating cover crops is a big concern for organic growers and WSU is evaluating a number of systems for improved termination.
One project is evaluating the use of flail mowing. Crimping is an effective way for organic growers to terminate rye, but must be done at a specific growth stage. “With flail mowing, can we broaden that termination window?” Collins asked. Initial results have been promising.
Many organic producers trying to limit tillage utilize rotational, reduced tillage. This is a system of minimal tillage in the spring followed by crimping or flail mowing to terminate the cover crop. The vegetable crop is then planted with a reduced amount of tillage. This is followed by conventional tillage late in the season to get ready for the fall seeded cover crop.
“With flail mowing or roller crimping to terminate in the spring, organic growers will have some weeds by fall and need tillage to control them,” Collins said. “How do we decrease the soil disturbance in this system and not need full tillage in the fall? Getting good weed control is part of that answer.”
“There’s been a new understanding of soil organic matter,” Collins said. “We know the location of the soil organic matter and we know it’s stable while in the aggregates. The question is, ‘how do we manage weeds and still have a healthy soil?’”