Four Weeks No Roots Causes Compaction
Due to a lack of rain, the soil is getting hard. When soil gets dry and lacks adequate soil organic matter (SOM), clay particles set up like cement. Most farmers do a fall tillage, which will increase water infiltration short-term, but long-term the soil just gets denser as the soil compaction gets worse.
Brian Doughtery, a consultant for Understanding Ag., says soil compaction is caused by three things: 1) too much heavy equipment, 2) not enough biology, and 3) nutrients and/or a lack of nutrient balance. Soil compaction is common on most farms, but it is not a natural or inevitable problem.
On heavy equipment, when a piece of machinery crosses a field, the weight pushes the air or pore space out of soil. Without pore space, the soil compacts. If a field is wet, the soil particles get cemented together when the soil dries. Clay has a negative charge and when positive ions like magnesium (Mg) and potassium (K) get pushed together, the soil gets hard. Calcium (Ca) also has a positive charge but it tends to let soil flocculate, aggregate or crumble. High Mg and K levels tend to reduce aggregation. Ideally, Doughtery likes to see 60–70% base saturation for Ca on soil tests, 12–18% on Mg, and 3.5% K. For soil test ratios, he likes 3:1 Ca:Mg ratios on sandy soils (Ca ppm/Mg ppm), 5:1 Ca:Mg on loamy soils, and 7:1 Ca:Mg on clay soils. Calcium is good for getting oxygen back into the soil.
Soils need pore space as much as they need water and minerals for nutrition and also good soil structure for stability. Humans can live 30 days without food, 3 days without water, and about 3 minutes without oxygen, on average. Roots also need oxygen to decompose the sugars in the roots to release carbon dioxide back into the atmosphere. Gas exchange and water movement in the soil is critical for good crops. Without good pore space and good soil structure, crops grow poorly.
A natural glue called glomalin is produced by the biology, mainly fungus but also microbial by-products, to improve soil aggregation to allow air and gases to penetrate the soil to reach the growing roots. These glues and aggregates are constantly being destroyed, consumed, and then reforming by live roots and the soil biology. Soils that stay bare longer than 4 weeks will lose aggregation and soil porosity and start to compact. Cover crops, manure, or compost applications all improve soil aggregation.
Tillage is a short-term solution to compaction; it helps water infiltration but then the SOM burns up and the soil compaction gets worse long-term. Cover crops are slower; it may take 3–5 years, even 7–10 years in really poor soils that are deeply compacted, to make a difference. However, cover crops add roots which become the source of the carbon for the SOM and also aid the soil biology in forming the glues to keep soils well aggregated.
Tillage oxidizes (burns up) the SOM and soils start to compact. Surface compaction in the top 2–6 inches reduces yields up to 8% or more for 1–5 years. Subsoil compaction (7–15 inches deep) may reduce yields 7% or more for 10 years. Deep soil compaction (greater than 15 inches) can last decades and reduce yields 2% or more, especially in dry years.
Natural freeze/thaw cycles take out some soil compaction to mellow the topsoil 2–5 inches, but do nothing to glue the soil aggregates back together. Deep tillage may take out some compaction down to 15 inches, but again does nothing to improve soil aggregation. Actually, University research shows that deep tillage only improves crop yields about 25% of the time, and if done when the soil is too wet, it may increase sidewall compaction.
One of the best biological tools besides cover crops are earthworms. Earthworms have 5x more calcium and 3–5x more nutrients in their worm casts (worm poop). A good earthworm population can turn over the top 6 inches of topsoil every 5–20 years. Earthworms are good at alleviating nutrient stratification through earthworm burrows which allow water and nutrients to disperse. If you are worried about tile line losses, just add a root to clog up the burrow hole. Roots absorb soluble nutrients and move water and nutrients laterally throughout the soil. Most roots follow the path of least resistance, so they follow earthworm burrows.
There are three major types of earthworms: night crawlers go deep, red worms move laterally, and gray earthworms are more intermediate. Worm casts and worm slime are a good source of nutrients for plants—especially calcium.