One common source of confusion in the veg patch is when plant leaves turn yellow. It often happens with beans, courgettes and squashes and can usually be rectified quite easily. Yellow leaves on brassicas are also common, but these tend to be older leaves dying and dropping off, so it is nothing to worry about if it is those older, lower leaves that are turning yellow.
As a very general rule, yellow leaves on veg plants are indicative of a nutrient deficiency. More often than not, we’d expect this to be a deficiency in iron, magnesium or nitrogen. This kind of nutrient deficiency can be a result of two things. Firstly, the soil itself may lack nutrients. Secondly, the plant may not be able to draw up enough nutrients from the soil due to a shortage of water.
If you find your squashes or beans looking somewhat yellow this summer, we’d be surprised if it wasn’t due to a lack of water, as it has been extraordinarily dry weather and even with regular watering you may have found it difficult to give these thirsty plants the amount of water they need!
With the intermittent rain that we have had in Cornwall this last few days, we’re hoping that the moisture levels in the soil will begin to balance out again, at least a little bit. If it does, then it should help the plants to draw up the nutrients that they need.
It may also be that the soil itself is a little deficient in nutrients, or that the compost/manure you have been using has not fully composted in which case the nutrients are not made available to the plants. In this instance, a liquid feed or some wormcast tea will really help. We’d recommend using a liquid feed once a month until the plants start to form fruits, and then increase to fortnightly.