Compost is not only a soil amendment, it is the soil amendment. Soil with a serious pH imbalance should be treated with lime or sulfur or some other pH-specific amendment, but for most garden soil problems the amendment of choice is always compost. Assuming that you’ve got average soil with average problems and you can only add one thing. Compost would be the thing to choose. Other amendments will solve particular problems more quickly or completely, but compost is the best all-around soil conditioner available.
To stretch the conditioning metaphor to the breaking point, one could think of most soil amendments as an exercise for one body part — the biceps, or the hamstrings. But biceps curls are not a conditioning program. No one would do biceps curls and only biceps curls and expect the result to be a healthy mind in a healthy body.
Cross-country skiing, on the other hand, works just about everything — arms, legs, glutes, abs — all of it. If you’ve got a serious weakness or injury in one part of the body, you may need to work or cure it before setting out, but as a whole-body exercise that will benefit muscles, cardio-vascular health, and so on, nothing beats it.
If you want the whole works, you need to work the whole. This is what compost does for soil. It improves soil physically, biologically and chemically. (These roles actually overlap, but it’s helpful to consider them separately.)
Compost has the unique ability to affect all these factors positively. In increasing organic material, including humus, it raises the CEC index making nutrients less likely to leach away and stabilizing soil pH (chemical affects). It also adds micro-organisms which perform complex functions (biological affects) and it improves soil structure, reducing drainage problems in both clay and sandy soils (physical affects).
Compost therefore provides the optimal environment for fighting plant diseases as well for making nutrients available to plants.

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