Even if you have the perfect nutrient balance in your soil either too much or too little water can mean your plants cant get what they need.
Plant roots need water. Both all the chemical processes, and the transport of nutrients through the plant, take place in solutions made up mostly of water.
But plant roots need air too. If roots have no oxygen for a long time they will suffocate.
Ideally soil is about half solid matter and half porous areas filled with air and water. The moisture levels can vary from waterlogged - no air to speak of in the soil - to permanent wilting point - no water available for the plants.
Soils absorb and hold different amounts of water. Sandy soils have a tendency to drain too well - water and nutrients move too quickly through the soil not giving plants a chance to access them. Clay soils often have the opposite problem - water pools and stays in the soil crowding out all the air pockets and drowning plants.
Both problems are successfully treated by adding - you guessed it - compost. The organic matter in the compost acts as a sponge, holding water in the soil where the plants can reach it. For the clay soils it opens up the porous structure allowing water to drain through.
The first step for drainage testing is simple observation. Either when you have a good rain or when you do a deep watering look to see where water pools and puddles and where water runs off quickly.
Here is a test that will give you a good idea of how quickly your soil drains. You need a can, a coffee can is ideal.
• Remove both ends of the can.
• Dig a hole about 4 inches deep and big enough for the can.
• Put the can in the ground tightly so that the soil is all around the outside of the can.
• Fill the can to the top with water and leave it for an hour.
• Using a ruler measure how much the water level has dropped.
Two or more inches or 5 or more centimeters means your soil has good drainage.
If your water level drops less than 2 inches of 5cm you might want to try one of these strategies.
• Add organic matter to improve the soil structure and drainage
• Build a raised garden bed
• Create a bog garden with plants that love wet feet.
• Deeply dig your bed - maybe there is a hardpan trapping the water.
• Put in a small ditch to direct water to your bog or water garden.
If when you try to fill your coffee can you find the water level drops a lot and quickly you have drainage that's a little too good. Add organic matter to help the soil act as a sponge and keep adding organic matter every year.