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Estimating turfgrass nutrient use

Does stressed turf require more nutrient supply, or less?

In a recent conversation, Carmen Magro and I showed a fundamental difference in thinking about turf nutrition.

My approach, which I explain in Everything you need to know about turfgrass nutrition in 1 lecture, is summarised by this statement:

If the grass is supplied with enough of an element, adding more of that element will provide no benefit.

Which is why I was so happy to answer when Magro asked "Why are some still saying NPK only for fertility in turf?" In fact, if there is enough P and K in the soil, I'll be happy to say N is the only element required!

As the discussion continued, it seems our point of difference is in whether the degree of stress on turf influences the nutrient requirement. The definition of plant stress that I like to use is Grime's. Stress is:

the external constraints which limit the rate of dry matter production of all or part of the vegetation.

One is always trying to produce good turf, grass that is as healthy as possible. That grass when healthy is going to have a certain concentration of mineral nutrients in it. Creeping bentgrass for example is going to have about 4% N, 2% K, 0.5% P, 0.5% Ca, 0.2% Mg, and so on. As the degree of outside stress increases, let's say from temperature, or drought, or close mowing, the rate of dry matter production will be reduced. There is a consequent decrease in the plant demand for nutrients. So if the quantity of nutrients supplied were sufficient at a lesser degree of outside stress, how can it be that the nutrient demand increases when stress increases?

If the stress is not caused by a nutrient deficiency, and is rather caused by something else, where is the connection between that outside stress and a grass requirement for increased supply of a particular element?

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