Define field capacity in relation to soil moisture.

Field capacity refers to the condition of the soil when it retains moisture after gravitational water has drained away. This level of water retention occurs when the larger void spaces in the soil have emptied, but the soil is still able to hold onto water in the smaller pores through capillary action. At this point, the soil has the optimal moisture level to support plant growth, as it contains enough water without being overly saturated, which can lead to conditions that are detrimental to root health and plant viability.

The other options describe different concepts related to soil moisture. The maximum water a soil can hold is often referred to as saturation, which is distinct from field capacity. Saturation occurs when all pores in the soil are filled with water. The point at which soil becomes saturated is also not the same as field capacity, as it does not take into account drainage. The water content when soil is most productive relates more to factors like nutrient availability and soil aeration, which can be affected by moisture levels but is not strictly defined by field capacity alone. Therefore, defining field capacity as the amount of water held after excess has drained accurately captures its significance in soil moisture management and plant health.