British Cactus and Succulent Society

Highlands & Islands Branch

Plant Management


Soil Test Kit for NPK and pH


How can one decide what type of compost is most suitable for a particular plant. Fortunately the days have gone when some 'experts' advocated a special compost for every cactus.

Two approaches are outlined below. The first one is an amalgam of Branch committee members' views on composts for both cacti and succulents. The second is based on some elementary botanical reasoning which may also be helpful.

Almost all committee members base their composts on about 50% John Innes (JI) No 2 or 3. The other 50% is made up by chicken grit, small stones, or perlite, although one member said it sometimes looked like mealy bugs! None added any fertilisers because there is enough in the JI mixes. Fertilising is almost all done on roughly alternate waterings with Chempak plus occasional shots of normal fertilisers like Phostrogen and Baby Bio, especially at the beginning of growth. None use peat, but advise being careful about the content of JI because not all manufacturers follow the correct formulae. That formulation is given in the Appendix, John Innes.

The committee also urge caution in what is added as grit; be careful it is not crushed shells or other lime-rich stones. It may also be helpful to check the pH of your compost because the acidity of peat can vary. Another important consideration in any compost is the need for trace elements. It is not enough to see 'contains trace elements'; the actual elements should be named. Sometimes called 'micronutrients', one would expect boron(Bo), copper(Cu), iron(Fe), manganese(Mn), molydenum(Mb), zinc(Zn), to be actually specified because they help in a plants uptake of certain nutrients.

'Doing Your Own'
If you want to make up you own compost there are various alternatives. A loam or 'top soil' would comprise about 50% of it. An old gardeners' trick was to use soil from mole-hills. The reasoning being that it came from some depth down, not from the surface where all kind of things dwell. Then add a good portion of sand or gravel, and finally some (about 10-15%) of sphagnum peat. The critical gardener might then want to test the mixture for N, P and K with the kind of kit market gardeners sometimes use. These kits look similar to pH test kits. A good general fertiliser is 'Growmore' introduced during the last war, and still as good as any today. Alternately, a mix of bone meal, blood & fish, etc will be adequate to start with. It might be worth while testing pH. It is usually easier to make a soil more alkaline than more acid. If acid then hydrated lime would make it more alkaline. On a loamy garden soil 8oz (250g) of hydrated lime per square yard, would raise the pH by 1.0 point. If alkaline then sulphur, even some peat, would adjust that.

The second approach, referred to above, begins with a botanists' rule of thumb. The rule seems to be that wet areas are acid because there is more vegetation producing acids by respiration, decay, and excretion. Dry areas, on the other hand, have less vegetation and these processes do not occur because of less rainfall. There is then less washing out of substances which cause soil alkalinity to increase.

Following on from that it might be said that plant evolution has favoured mutations which enable dry area plants to cope with alkaline soils and wet area plants to cope with acid soils. Cacti and succulents seem to be nearly neutral (pH of 5.5-6.5) and not as sensitive as azaleas, camellias, and rhododendrons, for example, which need a pH of between 4.5-5.5 But another school of thought suggests that some of the more recently evolved cacti, such as mammillariae and its close relatives, might like something on the other side of neutral. pH values are not as close as one might think. See 'What is pH' in the Appendix for more details.

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