Fertility Requirement and Response

All plants require certain basic nutrients for growth and survival, but differ in their requirement level and subsequent responses. 

Major nutrients for forage grasses are nitrogen (N), phosphorus (P) and potassium (K), while P and K are required by legumes.  Plants with a high nutrient requirement produce more on a fertile site or under a high level of fertilizer application than those with a medium or low requirement.  Plants with a low nutrient requirement are usually a better choice for sites low in fertility where, for various reasons, increasing the fertility may not be feasible or practical.  Under such conditions they produce at a higher sustained rate and are better able to maintain a stand against invasion, disease, etc. 
1 = Plants that require a high nutrient level for maximum produc­tion.
2 = Plants that grow well on moist soils, but respond less to fer­tilization than the first group. 
3 = Plants that grow under low levels of fertility, but they have a genetic limitation on growth.