The morning after the monsoon rains pass through, a shallow layer of muddy water sits in the furrows of the garden. The granules of side-dressing you scattered between the lettuce and the young radish greens the day before are nowhere to be seen on the soil. The water ran along the furrows and out the storm drain beyond the plot, and some of the fertilizer went with it.
Side-dressing doesn't stay in your garden
Nitrogen and phosphorus dissolve readily in water. When heavy rain follows right after you spread it, much of it washes away before the roots can take any of it up. City gardens have it worse. With concrete and asphalt all around, rainwater can't soak into the ground; it runs through the storm drains and reaches the nearest stream in no time.
When nitrogen and phosphorus build up in a stream, the makeup of its smallest organisms shifts. Plankton are the first link in the food chain that feeds the fish. When one group overruns the rest, the water clouds over green, and in drinking-water reservoirs that turns into taste-and-odor problems. Changes like these, though, don't show up in a year or two of data. The seasonal swings are wide enough that only years of consecutive measurements reveal a direction.
Eleven straight years of river readings
Water sampling data collected from 2012 to 2022 in the Houxi River basin, a subtropical region of southern China, has been made public. The survey stretches from upstream reaches where farmland remains down to a stretch lined with city blocks — a section where river and reservoir meet. Phytoplankton, which can be counted under a microscope, were checked twice a year, in summer and in winter. For the bacteria in the water and the slightly larger single-celled organisms, researchers read genes each season to sort out which types were present. Water temperature, turbidity, and nitrogen and phosphorus concentrations were logged the same day.
From this record, the researchers describe different patterns emerging by location and over the years — meaning the biological makeup at sites near the city diverged from that at the upstream sites. Records that measure a single river the same way for eleven years are rare. It's the kind of dataset that can serve as a baseline for stream restoration work down the road.
How to keep more of your fertilizer in the bed
One garden plot isn't going to change a river. But city gardens tend to sit close to drainage channels and streams, and plenty of people are fertilizing the same ground. Here are a few things you can start on today.
- Hold off on side-dressing on days when rain is in the forecast, and on the day before. Applying it when the soil is dry is better for the roots anyway.
- Rather than one generous application, split it into small doses every two to three weeks. That goes double for quick-growing leafy crops like lettuce and young radish greens.
- Cover the tops of your beds with straw or fallen leaves. Raindrops won't hit the soil directly, so less soil and fertilizer washes away.
- Work well-rotted compost into the soil instead of unfinished compost. Compost that hasn't broken down leaches its nutrients straight through, and it smells besides.
- Set saucers under your containers, and when water collects in them, pour it back onto the soil instead of dumping it out.
- Plant a row of perennials with roots that stay in the ground — garlic chives, or minari (water celery) — along the edge of the plot. Water drains more slowly and the soil holds better.
Start with the forecast today. If rain is coming, leave the side-dressing bin where it is, lay a layer of dry grass over the beds, and call it a day. Push this week's feeding to a day after the rain has stopped and the soil has dried to a crumble.
Source — an eleven-year river plankton monitoring dataset from the Houxi River basin in China, published in the journal Scientific Data
