The Popular Fertilizer You Need To Avoid (And What Kind Your Garden Needs Instead)

Fertilizer plays a significant role in how to prep your soil for successful planting. People generally like the idea of balance, especially when it comes to nutrition, so a fertilizer that contains a balance of the nutrients plants most need — nitrogen, phosphorus, and potassium — seems appealing to gardeners. That balance is what 10-10-10 fertilizer represents, with its equal distribution of all three elements.

Most plants need and use more nitrogen than phosphorus or potassium. So applying that much phosphorus and potassium is wasteful at best and harmful at worst. Both phosphorus and potassium add salts to the soil, which can damage and appear to "burn" plant foliage. Excessive potassium can prevent water from penetrating the soil when clay particles jam up in soil pore spaces, stop uptake of certain nutrients, and cause nitrogen deficiency.

Phosphorus becomes a particular problem because of the way the three elements move through the soil. Phosphorus stays close to the soil surface, while nitrogen travels rapidly through the soil, and potassium stays in between. Too much phosphorus can result in lethal toxicity to plants or deprive plants of their ability to access iron and zinc. Toxicity becomes more likely as more fertilizer is added, and more phosphorus stays on top. Excessive phosphorus also minimizes root interactions that beneficial mycorrhizal fungi bring to most plants. This symbiotic action creates nutrient exchanges between fungi and plant that helps regulate plant and soil nutrients. When this action shuts down, plants are much more challenged to attain nutrients. Further, phosphorus runoff in water bodies damages water quality and threatens aquatic life.

What should you use instead of 10-10-10 balanced fertilizer? A 5-1-2 or 5-1-3 ratio (versus the equal ratio of 10-10-10 fertilizer) is more typical of the way plants use nitrogen, phosphorus, and potassium. But the true answer may be different for each gardener because soil varies from garden to garden. That's why failing to get your soil tested may be one of the biggest mistakes you're making when preparing soil for your garden. 

Your soil test results could indicate your soil needs no fertilizer at all or just needs organic material, such as compost or aged manure. Select fertilizer that provides nutrients the soil test shows are deficient in your soil. You can also choose fertilizer based on plant-specific needs. The popular 10-10-10 formulation isn't a complete waste. If you happen to have some of this balanced fertilizer in your garden shed, learn the best plants to use 10-10-10 fertilizer on (and ones to avoid).