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Photo­syn­thetic ef­fi­cien­cy

Under the bonnet Episode 3 4 min read

The short version

Everything a crop gives you, it pays for with sugar its leaves make from sunlight. Photosynthetic efficiency is how much of that sunlight gets turned into sugar. Steve doesn’t ask growers to learn the chemistry. He asks them to learn what keeps the engine running, and to check the one part he singles out: manganese.

You know your tractor to the last horsepower. Know your crop’s engine the same way.

The engine that pays for everything

“The plants are put on this earth to do one thing,” Steve says: “to photosynthesise.”

Light, water and air go in, and sugar comes out. Everything a crop does is paid for with that sugar: new leaves, roots, fruit, its defences, and the sugar it feeds the life around its roots. Photosynthetic efficiency is how much of the light falling on the crop gets turned into sugar.

Sugar, made in the leaves. The soil’s life. A dotted line: only a trickle.

  • A bean plant at pod set, cut away below the ground: green leaves, a few white flowers and young pods above, and a pale taproot with fine roots spreading through red soil below.

    Running well

    a bean plant at pod set

    its leaves’ sugar pays for new leaves, roots, pods and its defences, and feeds the life round its roots

  • A bean plant at pod set, cut away below the ground: green leaves, a few white flowers and young pods above, and a pale taproot with fine roots spreading through red soil below.

    Running rough

    the same plant

    less of the same light turns into sugar, and every part gets less, often before a leaf looks wrong

Everything a crop does is paid for with the sugar its leaves make: new leaves, roots, fruit, its defences, and the sugar it feeds the life around its roots. When the engine runs rough, there’s less to pay with, often well before a leaf looks wrong. An illustration, simplified.

Under the bonnet

Steve’s picture: “You farmers are really good at understanding, when you go and buy something, all the bells and whistles.” When you buy a tractor, you know it to the last horsepower: what it can and can’t do, whether it’s got air conditioning or not. Then growers put a program on a crop without asking what’s under its bonnet. It’s no good admiring the Ferrari if you lift the hood and there’s no engine in it.

He’s clear about how deep to go: “We don’t have to go through the mechanics of what is photosynthesis.” You need enough of a look under the bonnet to know what keeps the engine running.

Water. Sugar. Air: carbon dioxide. Magnesium. Manganese. Nitrogen. Iron.

  • A broad, heart-shaped green leaf on its stalk, tilted up to the right, in soft sunlight.

    Under the bonnet

    the leaf’s engine, and its parts

    light, water and air in; sugar out. The minerals are the engine’s parts

Light, water and air in; sugar out. The minerals are the engine’s parts. An illustration, simplified.

What keeps it running

  • Light. A full, healthy canopy, catching as much light as it can for as long as it can.
  • Water. A plant short of water closes the pores in its leaves to save it. With them closed, little air gets in, and the engine slows right down.
  • Heat. A hot spell slows it down. As Steve says, the plant will tell you: this year I’m three degrees hotter, and I’m struggling. The Stress and salt page has more.
  • The parts. Magnesium at the heart of every chlorophyll molecule, manganese where the water gets split, and nitrogen and iron in the machinery.

Light. Sugar.

  • Three young bush bean plants in a row, cut away below the ground, small, with wide stretches of bare soil between them.

    Bare ground between

    a row of young beans

    much of the light lands on the soil, and makes nothing

  • The same three bean plants three weeks on, full and bushy, their leaves meeting over the row.

    A full canopy

    the same row, three weeks on

    nearly all of it lands on a leaf, where it can make sugar

A crop makes most of its sugar from the light that lands on a leaf. Light that falls on bare ground is lost. The sooner a healthy canopy fills the row, the more of it the crop turns into sugar. An illustration, simplified.

One thing to check

Of all the parts, Steve picks one for growers to take home: manganese. The number one thing for photosynthetic efficiency, he says, is making sure the manganese in your plant is where it should be. And it’s one you can check.

Don’t wait for symptoms

The engine runs rough well before a leaf looks wrong, and by the time a shortage shows, you’ve lost sugar you won’t get back. Sap tests taken ahead of the critical stages can pick it up in time to act.

“I’m going to give you one thing to take home today: photosynthetic efficiency and manganese.”

— Steve, on the hidden engine