The following Agriculture & Natural Resources article printed in the 2020 Summer edition of the Oldham County Extension Newsletter.

Soil Nutrients, Fertilizers, and Plant Growth

What Everyone Should Know

Nutrients and Fertilizer Know-How

The three major nutrients that plants need are nitrogen, phosphorous, and potassium. Phosphorous and potassium can be applied any time of the year because these generally stay where they are applied.

In general, nitrogen can be lost through volatilization, a process where it converts to gas and lifts into the atmosphere, and also to leaching, a process where nitrogen is lost as it moves downward through the soil with water. Because of these properties, nitrogen application should be made to coincide with key plant growth times in order to get the most benefit. For example, nitrogen should be applied close to planting time for vegetables and for field crops like corn. This allows it to be available to young, growing plants.

Some nutrients are naturally occurring, but most often additional nutrient sources are needed. Fertilizers or manures can provide these nutrients. Applying fertilizers without knowing what nutrients are already in the soil is guesswork at best. Even when estimating nutrient removal from soil by the previous crop, a soil test gives the most accurate picture of what nutrients may be needed for the specific crop to be grown.

Soil Testing Is a Foundation

Too often, folks overlook soil testing, and that’s a shame because it’s the foundation of good plant health. From vegetables to hay, all plants use nutrients. Fall is a good time to test soil for nutrient levels and pH. Testing soil now allows time to make any soil pH corrections necessary before next growing season. Soil testing also gives direction on what type and how much fertilizer should be applied to promote optimum health and production of gardens, field crops, pastures, and hay fields. Get details on how to sample and test soil from our website.

When using organic fertilizers, remember that plants can only utilize inorganic forms of fertilizers. This means, it can take 30 – 90 days for organic fertilizers to break down into an inorganic form that is available to plants’ roots. In the meantime, it is possible to see signs of nutrient deficiencies in whatever you’re growing. This can manifest itself as overall poor growth and reduced yields.

What About Fall Nitrogen on Grasses?

Cool season grass pastures can be rejuvenated with a fall nitrogen application. Many of our pastures take a beating, get overgrazed, and have their share of weeds. If we evaluated how much good grass was present versus weeds that livestock cannot or will not eat, we may be surprised.

Poor plant growth opens the door to weed competition. Weeds can thrive in nutrient deficient soils unlike most of our desired crops. Weeds often outgrow the desired crop and further reduce the desired crop’s growth and yield. It is a cycle that keeps on robbing you if not broken. Soil testing and managing fertility can help break this cycle.

A late fall nitrogen application can be beneficial in this situation. Nitrogen applied to grass in late fall does not trigger new top growth at that time, but it does initiate new tillers from the base of each grass plant, which helps thicken the stand. Thickening the stand means less weedy grasses next season. Late fall nitrogen application also help cool season grasses like fescue, orchardgrass, and ryegrass survive the winter better and green up sooner in the spring.

Does This Apply to Hay Crops?

Hay is different. Not only is fertilizer timing different, but nutrient removal is more intense and must be managed to ensure future productivity. Every time a cutting of hay is taken, a significant amount of nutrients is also removed from the field. In a pasture setting, some nutrients are returned to the soil through manure and urine. While this may not provide enough nutrients to maximize yields, at least it’s something.

We don’t have this luxury in hay fields, so fertilizers must be applied for the long-term health of the forage stand. Additionally, those applications should be timed to coincide with the greatest need of plants. An example is applying fertilizer after 1st and 2nd cuttings to optimize yields for the next harvests.

Does It Pencil Out?

Knowledge is your best decision aid, and Oldham County residents can get up to five free soil tests per year, thanks to a grant written by the Oldham County Conservation District. Take advantage of this – the soil test will give you knowledge of how to best use your fertilizer dollars.

As an example, when faced with a potentially large fertilizer bill, consider which of your fields are deficient in nutrients.  Fertilizer applications on these fields are going to give the biggest response, provided there’s still a good stand of desired forage there.

Another example – if you produce your own hay but think that fertilizer is going to be cost-prohibitive, consider buying hay instead. Price hay and then compare it to what you typically spend to make the same amount of hay. Of course, your strategy may be different if you sell hay.

There are many variables that have to be considered in input decisions, and these may be unique to each producer’s operation and management. No matter what type of operation you have, make sure you’re turning over all the rocks when it comes to these sometimes hard decisions.

Written by Traci Missun, Oldham County Extension Agriculture & Natural Resources Agent.