This is the second entry for the Understanding Soil Nutrients Series. Today we will focus our attention on a very important soil nutrient - phosphorus. If you missed the first entry to this series, here it is:

Understanding Soil Nutrients - Nitrogen

Why Is Phosphorus Important To My Vegetable Garden Plants?

Plants need phosphorus for strong root growth, fruit, stem and seed development, disease resistance, and general plant vigor. Phosphorus doesn't move in the soil as easily as nitrogen does so you don't have to add it as frequently. Depending on where you live in the country, your soil may have plenty of phosphorus, but it may be unavailable to plants. Phosphorus availability depends on warm soil temperatures, pH range, and the levels of other nutrients, such as calcium and potassium, in the soil. Deficiency symptoms include stunted plants with dark green foliage, reddish-purple stems or leaves, and fruits that drop early. Rock phosphate and bone meal are good sources of phosphorus.

Phosphorus (or P) is generally noted for its ability to promote a strong root system in plants. Most transplant or starter solutions contain higher levels of phosphorus to help the plant grow stronger root systems. Before adding any phosphorus, or phosphates, to your soil, make sure you complete a thorough soil test (For more on soil testing, please check out How To Properly Test Your Soil and How To Take A Soil Sample For Testing).

When using fertilizers, check the N-P-K ratings on the back of the container. This will tell you what percentage of the product contains phosphorus. Phosphorus is always the middle numeral in the N-P-K rating, demonstrated in the picture below:

The Middle Number Represents The Percentage of Phosphorus

The best times to add phosphorus to the soil is just before planting - especially with transplants. Use composts and well-aged manures as amendments to the soil, as these have good levels of available phosphorus for plants to use. Be sure to monitor phosphorus and pH levels in your soil to make sure you are not applying more than what is needed.