The most important factor regarding organic matter is that it has very favorable effects on the physical properties of soil. Particularly, organic matter tends to counteract the unfavorable effects of exchangeable sodium on soils. Simply put, heavy applications of organic mulches will increase water holding capacity, infiltration rate, tilth and soil aggregation. Use caution when applying manures as a source of organic matter because they typically contain appreciable amounts of soluble salts and are usually somewhat high in pH as well. If the soil pH is too high, elemental sulfur or commercial gypsum can be added to the soil to rid the soil of excess sodium and indirectly reduce alkalinity.
Most ornamental plants require slightly to strongly acidic soil.
These species frequently develop iron chlorosis when grown in soils in the alkaline range. The two nutrient deficiencies can be distinguished by observing where on the plant the deficiency symptoms appear. With iron deficiency, the symptoms appear at the shoot tips and on the newest growth. With nitrogen deficiency it is the older, lower leaves that are most affected. Foliar sprays of iron can alleuicte iorn deficiency.