Contrary to popular belief, legumes do not fix Nitrogen, and have never fixed Nitrogen. The microbes living in symbiosis on their roots do the Nitrogen fixing.

So the real question is, ‘what is it that legumes DO?’
What legumes do is they draw oxygen into the soil and unlock calcium. The pH along the roots of a robust legume will generally be below pH of 4, and if the legume has any depth of roots it draws this calcium up into the top layer of soil, enriching it with calcium. Of course, if you do like many legume hay farmers and haul this away, there is no gain. However, I have seen permanent pasture legumes such as lucerne (alfalfa) grow for many years as long as they are just grazed. Generally about the third or fourth year they stop nodulating-but that doesn’t mean nitrogen fixation ceases around their roots. It simply means that the topsoil has gained enough free, organic calcium than other nitrogen fixing organisms, that do not depend on being spoon fed calcium inside the legume nodules, take over the job and there is more nitrogen being fixed (rather than less).

In general a robust legume such as soybean, lab lab, sunn hemp, vetch, etc. will unlock 4 to 6 times as much calcium as is used in the nitrogen fixation processes going on in the nodules on its roots. If this calcium is returned to the soil from the digestion of the legume plant it has a follow on effect on the nitrogen level of the soil because it is then available to feed calcium to the non-rhizobia (non nodule) nitrogen fixers such as azotobacters, azospirilla, etc. that free fix nitrogen in the soil. Once this free calcium is built up (typically after 4 years of lucerne) you can plough it up and plant maize (corn) and get a crop with little or no nitrogen fertilizer because there will be nitrogen fixation occurring in the soil around the maize roots where root exudates feed the microbes abundant energy-but maize has alkaline root exudates and releases silica rather than calcium so you can’t keep this up without elaborating more calcium in biological form.

And sure, the folks that say the nodules break down in a couple weeks are able to show you the decay of the nodules and seemingly prove their point. But what actually is happening when the nodules break down? Do you imagine the nitrogen just evaporates in a poof at that point? By no means. It is taken up by various other micro-organisms and higher organisms in the soil, and although there is a wonderful turn over of soil biology going on, it takes months and even years to exhaust the boost legumes give to the soil biology. Only superficial observation, superficial thinking and a perverse desire to justify using heaps of nitrogen fertilizers can be so
scientifically perverse and dishonest as to propose that legumes only give about a 2 week boost to things.

What you’ve got to do to arrive at a clearer truth of these things is take a TOTAL test of the nitrogen in the soil. Let me give you an example. In my garden my soluble nitrogen tests (taken two months ago) showed 30.5 ppm nitrate N and 20.6 ppm ammonium N. My total N, however, was 3400 ppm (good enough to grow maize, okra, potatoes, pumpkins, etc. without nitrogen fertilizer seeing as that’s what I’m growing). Without a TOTAL test you wouldn’t know this. Why such a huge amount of nitrogen in the total test?
Because of previous soybean crops feeding nitrogen into the soil, and because of my humified compost (which must contain clay) with all its nitrogen fixers and co-factors. The nitrogen then is caught up in the biology of the soil in complex forms-which is what you want. You don’t want it as nitrates or ammonium. That’s hogwash. It won’t grow tasty, nutrient dense stuff that is immune to insects and diseases. But if you go to the universities and the extension stations that’s what they advise people and that’s what sort of stuff they grow-garbage. They wreck the soil, poison the water table, leach the boron and calcium, etc.

Of course, In my garden soil at present I’ve got 7.26% organic matter, 75% of my base saturation is calcium, 16% is magnesium and only 3% is hydrogen (Ph 6.8, CEC 34.11) It ha taken me 3 years to get there. I’ve fed this garden mostly gypsum, compost and mulch, with plenty of legumes and grassy paths-the soil is rarely bare and not for long because bare soil in the wet tropics goes downhill really fast. You’ve got a somewhat similar situation in Hawaii, only with much newer soils. This means you REALLY need to concentrate on legumes because they bring oxygen (the basis for acidity) into the soil and dissolve the rocks. Man, in Hawaii that’s what you’ve gotta have. Legumes don’t fix nitrogen. Get that out of your head. Legumes eat rocks, which then makes it possible for nitrogen fixation to occur.