Managing soil crusting

We've seen some very heavy rains followed by hot weather over the last couple of weeks. These conditions can cause crusting on the soil surface. In this brief overview, I'll discuss how / why crusts form, and how you can respond in the immediate and longer term.

At this point in the season, most crops are already in the field. However, many of you will continue to plant successions of brassicas, lettuce, carrots, and other crops. We tend to see some of our heaviest rains and thunderstorms in the summer, so keep the possibility of crusting in mind as you time your plantings.

Soil crusting: Why does it happen? 

Crusting often leads to uneven emergence Photo: Angie Peltier

Soils that have high aggregate stability (a function of soil texture, organic matter, and biological activity) are able to retain their structure in the face of heavy rainfall. A soil with lower aggregate stability is more likely to experience some disintegration of soil aggregates (clumps) during intense rain events. Heavy rains can separate soil into very small aggregates and particles; if the soil surface dries quickly after such an event, these small particles can bind together and form a cement-like surface over the top of your soil.

As such, the most risky weather conditions for crusting are heavy rainfall followed by hot, dry weather. Soils with a higher percentage of small particles (clay, silt, and loam soils) are more likely to experience crusting because the small particles are able to fill in the pore spaces in the soil.

So why is this a problem? There are a few implications. Soil that has crusted:

• is harder for seedlings to break through. This is especially true for very small seeded crops like carrots and beets (as opposed to beans, for example). 
• will not drain properly. The crust forms well... a crust, causing rain and irrigation water to move across the surface without penetrating through to the root zone. 

Managing crusting in an unseeded bed

This is by far the easier scenario. If you've prepared a bed to plant and you're utilizing a false seedbed (or holding back on planting for any number of reasons), and the soil crusts over, you can use any number of tools to break it up. The easiest solution is likely to use shallow cultivation, setting your sweeps / knives / etc. only as deep as the base of the crust. Rotary hoes or any other shallow tillage implement can also work.

Managing crusting after planting

Irrigation can soften the crust and help plants break through Image: Angie Peltier

If you've already seeded and the soil crusts over before your plants have germinated, a first step is to figure out how soon you expect your plants to emerge, and how deep the crust is relative to your seeds. . If you have a few days before you expect your seedlings to emerge and the crust is ends above the seed bed, a shallow cultivation pass will help to break up the soil surface without damaging seedlings too badly. If your seeds have germinated and the cotyledons are reaching the surface, cultivation could cause significant damage. So, if your plants are just about to emerge, consider opting for irrigation to soften the soil surface. Page 2 of this agricultural brief has a nice table showing days to emergence for different vegetable crops in various soil temperature.

Preventing future crusting

In the long-term: Improving the aggregate stability and organic matter content of your soil can help to prevent crusting. Investing in reduced tillage is one important way to do this. Adding more cover crops to your system is another.

A short-term solution is to over-seed. Seedlings will be better able to push through a crust if they are doing so in concert with other seedlings. You will inevitably lose some, either because they do not make it through the crust surface, or because they exert all of their stored energy in the process. The practicality of this approach depends on the cost of your seed and your seeding tools. If you do over-seed in anticipation of crusting and no crust forms, you'll need to thin your field.