Why Spinach Will Not Grow: Soil pH and How to Correct It

Spinach is the crop that stalls on acid soil

Other leafy crops grow perfectly well in the same bed while the spinach sits at the seed leaf stage, yellows and never sizes up. That is not a shortage of fertilizer, it is pH. Spinach wants 6.5 to 7.0, which makes it markedly less tolerant of acidity than almost anything else you grow.

As soil turns acid, aluminum and manganese become soluble around the roots and damage the growing tips. A plant that cannot extend its roots cannot take up water or nutrients, so feeding it produces no response at all. Rain leaches bases out of soil, so a bed left alone drifts steadily towards acid.

Do not judge by eye, measure. A cheap test kit is enough. Take soil from several points in the bed at about four inches (10 cm) deep and mix it before testing. Surface soil alone reads low because rain has washed it.

Liming after growth has stalled does not rescue that crop. Treat pH as something decided before sowing.

Which liming material to use

Choose by how fast it acts and whether it supplies magnesium. For spinach in a home garden, dolomitic lime is the default, quicklime only when you need a big correction quickly, and a slow material such as ground shell when you can only work on the bed just before sowing.

The practical advantage of a dolomitic lime is not only the pH. It also prevents the yellowing between the veins of older leaves that comes from magnesium shortage.

Work the rate out from your reading

There is no single rate: it depends on the gap between your reading and a target of about 6.5. Overshoot and you swing alkaline, and then the plant cannot take up iron and manganese and the new leaves come out pale. Spinach shows the damage from over-liming readily, so go light and top up next season if you need to.

Working back from the sowing date

If you have already mixed lime and fertilizer on the same day, push the sowing back a week and let the reaction settle.

1. Two to three weeks before: lime Spread it over the whole bed and dig it in to eight inches (20 cm). Spread on the surface it never reaches the layer the roots use. Water it and give it time to react.
2. One week before: compost and base fertilizer Work in compost and fertilizer and dig again. Mixing lime and nitrogen fertilizer together drives off ammonia and you lose fertilizer, so keep the two operations days apart.
3. Two or three days before: build the bed Make the bed and level the surface with a board. Sowing into freshly fluffed soil means it settles under watering and the sowing depth goes uneven.
4. On the day: firm the surface Firm the surface lightly just before sowing so the seed makes contact. Spinach is slow to take up water, and this shortens the time to germination.

Feed it well, and do not let it run out

Spinach crops in four to eight weeks and makes leaf continuously the whole time, so its demand is high for a leafy vegetable. Aim for about 3 to 5 oz per square yard of a balanced fertilizer worked in. A spring crop grows fast and runs out quickly; an autumn crop takes up nutrients slowly in the cold. Either way, base it well and top up.

Use only well-rotted compost, about 4 lb per square yard. Unrotted material feeds soil mites, which cause the growing point to stop. If you are adding organic matter, do it a month ahead at the latest, and preferably for the previous crop.

1. Mix through the whole depth Spread and dig in to eight inches (20 cm) before you build the bed. Spinach roots go down, so fertilizer only in the surface runs out late in the crop.
2. Compost a month ahead Well-rotted compost at about 4 lb per square yard. Later than that and it is still breaking down, which damages roots and feeds mites.
3. Do not run short of phosphorus In cold soil, phosphorus is harder to take up. For autumn and overwintering crops, a slightly higher phosphorus base dressing visibly improves early growth.

It is a crop that wants nitrogen, but excess makes dark, soft leaves that attract pests and disease. Very deep green is not the target.

Containers and rotation

In containers the pH problem is usually the opposite one: people add lime to compost that was already adjusted, and the new leaves come out pale.

1. Use a pH adjusted compost Most bought vegetable composts are already around pH 6 and can be used straight. Do not add lime, or you push it too high.
2. Eight inches of depth or more The taproot goes down, so in a shallow container the roots circle and the leaves stop small. Choose a deep trough.
3. Test reused compost Second season compost carries residual salts and the pH has moved. Test it, and mix in fresh material to restore the structure before using it.
4. Leave two years between crops Repeated cropping in one place brings wilt and root problems. Move the bed each year, or alternate spring and autumn positions in a small plot.

Working out why it is not growing

pH is the first suspect but not the only one. The same failure to grow can come from pH, running out of nitrogen, unrotted organic matter, or repeat cropping, and each needs a different response.

Check in this order: measure, dig, then look back at your records. Measure the pH, dig up a plant and look at how the roots have extended and how wet the soil is, and then work back to when you last added compost or lime.

1. Measure before you decide Adding fertilizer on a hunch does nothing if the cause is acidity. Take soil from several points at four inches deep, mix it, and test.
2. Fix it for the next crop You cannot correct pH mid crop. Take what you can from this one and plan to lime at least two weeks before the next sowing.

Both pH and organic matter are decided by when you added them. Rather than adding more once symptoms appear, look at your records and rearrange the next crop.