Building soil fertility

Soil fertility is crucial for fruit and veg growing, so take time in winter to add compost and manure, writes RHS Chief Horticulturist Guy Barter

Dry weather sometimes hits yields on my allotment, so it's really important to add extra organic matter to boost the water-holding capacity of its light, sandy soil. Two bucketfuls of compost or manure per square metre should hold enough water for two weeks of summer growth, as well as adding nutrients and improving soil structure.

Organic matters...

Digging in rotted manureMy compost bins yield a useful but insufficient amount of decomposed organic matter (ie compost). A 7.5-tonne lorry load of composted stable manure for £160 every three years makes all the difference. 

Last year, the heavy autumn rains rendered the allotment road impassable for a lorry. After six inches (15cm) of October rain, the road is already dodgy. Plan B is to go to a local farmer who supplies 3-tonne trailerloads of cow manure at £60. His tractor has no problem with allotment roads. 

What's in my soil?

My soil analysis (last done in 2017) indicated a moderate need for potassium and magnesium. Manure or compost adds enough, but on un-manured areas, I spread fertiliser in February for the last of the winter rains to wash down into the root zone; sulphate of potash (potassium sulphate) and kieserite (magnesium sulphate) at 30g and 20g per square metre respectively. 

A soil analysis only needs repeating every 4–5 years. The last one showed such high phosphorus levels that I will not need phosphorus fertilisers for decades. This is very common in gardens where years of manure and fertiliser lead to accumulated phosphorus.

Spreading granular fertiliserMy soil test indicated mild soil acidity of pH 6.8 which is ideal for plant growth. To control clubroot disease of cabbage family crops, (which is rife on my plot), adding lime to raise the pH to 7.5 (very alkaline) would be ideal.

However, the spuds would not like it, and trace elements such as boron or zinc might be immobilised. I rely on using clubroot-resistant cultivars where available, plus a handful of lime in each planting hole and a spring dressing of Perlka (calcium cyanamide), a nitrogen fertiliser with a powerful anti-fungal effect.

Winter rains readily wash nitrogen from soils and soil analysis cannot easily measure levels. This is a pity as nitrogen is easily the most important nutrient. Happily, much will be supplied as the manure rots next year, topped up for greedy crops such as runner beans or beetroot with nitrogen-rich hoof and horn fertiliser applied before planting next spring.

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