Amended planting holes dug out of damp clay may become collection points for water. Also organic amendments dug into soils used for long term plantings decompose and disappear over time, leaving the plant growing in the same soil that was there in the first place. This latter phenomenon will occur rapidly and markedly if you are in a hot and humid climate area. LIkewise anything like a potting mix piled on top of existing soil will melt away, leave the plants sittiing on the soil that was already there. So if you are certain what you have now is no good for citrus then you will have to buy an adequately large amount of an actual soil - that is one that is mostly mineral in makeup - and plant in that. By either digging out bed-sized sections of the clay soil and filling these excavations with the purchased soil. Or making berms or raised beds on top of the existing soil using the purchased soil, without mixing the two together. (A little clay goes a long way, so that mixing any soil you buy with the clay soil in an attempt to improve it is likely to result instead with the clay component of the existing soil cancelling out the purchased soil).
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