Improving high clay soils

By James Hoorman, Hoorman Soil Health Services, Adapted from Dale Strickland article, Green Cover Seeds.

Working with clay soils can be difficult when trying to grow crops.  Sticky when wet and rock hard when dry, a high clay soil can drive you crazy!  However, clay soils have many great qualities.  Compared to sand and silt, clay has higher water holding capacity and greater cation exchange capacity (CEC).  CEC means the clay has a negative charge and can hold many positively charged soil nutrients. Water and soil nutrients are needed for plants to optimize yield.  Yet, clay has several problems.

First, even though a soil has plenty of water, plant roots have to access that water.  Roots need oxygen to grow and tight clay soils that are saturated have limited oxygen for roots to grow. The tight pore spaces in clay soils limits root growth and does not allow atmospheric oxygen from getting into the soil.  Limited roots and oxygen lead to poor plant growth.  It also tends to facilitate conditions for microbes that cause many diseases.  Plants growing on saturated clay soils or compacted clay soils tend to have shallow roots and tend to lodge more often. 

The second problem on tight clay soils; water infiltration is reduced because water cannot enter the pore spaces easily.  Water tends to bead and has high surface tension, so even if a soil is dry; sometimes the water cannot enter the soil until the surface tension is reduced.  Clay soils tend to crack, shrink and swell, so moisture is often lost from these soils when temperatures are hot and the soil dry out.  Often clay soils cling to the water that is in the soil, so clay soils can become droughty because the plants cannot get access to soil water.  With reduced infiltration, more water runs off, taking nutrients with it as surface runoff or if it does infiltrate through deep cracks, it often flows with the nutrients down to our tile lines. Even though clay soils have a high water-holding capacity and fertility; the lack of oxygen, shallow roots, lack of nutrient availability, and poor water infiltration means crops do not grow well.  

Here are some tips to improving clay productivity.  It starts with improving soil aggregation and increasing soil organic matter (SOM).  SOM has 10X higher CEC than clay, and together the two can hold even more water and nutrients.  Aggregation means that soil clumps together, to allow air and water to move into the soil easily where it can be stored.  Aggregation combines clay (the tiniest soil particles) with silt and sand into soil particles that crumble and are about 3/8 inch in diameter.  Soils with good aggregation have high calcium content, high SOM, lots of earthworm activity, and biological exudates from microbes and plant roots which glue the soil particles together.  Soil microbes call mycorrhizae fungi produce glomalin (a sugar nitrogen compound) reddish-brown in color that is the main ingredient for improving soil aggregation.  Soil aggregation is destroyed by tillage, keeping soils fallow and devoid of living plants after harvest, and high soil temperatures. 

Moldboard Plowed Field

Second tip: Grow live plants year-round and increase soil surface cover.  Plant leaves and surface residue absorb the impact of falling raindrops and improve water infiltration.  The surface residue is like a blanket in the winter, keeping soil temperatures warmer; and like an umbrella in the summer, keeping soil temperatures cooler.  It’s also habitat for beneficial soil organisms that recycle soil nutrients.  Soils with good cover have much less water and nutrient runoff  and less soil erosion.  Soils with good cover have less evaporation and less soil cracking than bare tilled soils. Better moisture and nutrients improve plant and soil health for higher yields.   

Third, increase your microbial and earthworm activity. Earthworms are the best indicator of high soil health. High earthworm soil activity leads to 13X higher aggregation,  and higher CEC (up to 4X), calcium (4X), potassium (3.5X), phosphorus (3X), and nitrogen (2.75X).  Earthworms need adequate calcium, neutral pH, and high amounts of surface residue to survive.  Tillage and bare (fallow soils) cause earthworm populations to decline.  Tillage dries out worm eggs and exposes adults to predators, reducing worm populations. 


Well aggregated clay soils that are high in SOM are quite productive.  Regular cover crops, hay crops, and even some manure tend to improve soil aggregation and soil health.  Tillage increases soil oxygen initially and may appear to improve crop productivity for a few short years; but in the long-run, it starts a downward spiral on soil productivity.  Maintaining soil health by improving soil aggregation is the key to keeping clay soils productive. 

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